Santa Monica City Council
November 18, 2025

2025-2030
Local Hazard Mitigation Plan
Prepared in collaboration with:
1642 E. Fourth Street,
Santa Ana, California
www.earthconsultants.com
2025-2030
Local Hazard Mitigation Plan
Volume II: Hazards
Local Hazard Mitigation Plan
City of Santa Monica, California
2025 Table of Contents – Volume II Page i

TABLE OF CONTENTS
SECTION ......................................................................................... PAGE # VOLUME I: MITIGATION ACTION PLAN
Executive Summary
Resumen Ejecutivo
Section 1: Introduction
Section 2: Community Profile
Section 3: Risk Assessment
Section 4: Goals and Mitigation Actions
Section 5: Plan Maintenance
VOLUME II: HAZARDS
Section 6: Earthquakes
WHY ARE EARTHQUAKES A THREAT TO THE CITY OF SANTA MONICA? ................................................................. 6-1 EARTHQUAKE BASICS – DEFINITIONS ................................................................................................................ 6-3 CAUSES OF EARTHQUAKE DAMAGE .................................................................................................................. 6-7 HISTORY OF EARTHQUAKE EVENTS IN SOUTHERN CALIFORNIA ............................................................................ 6-9 Unnamed Earthquake of 1769............................................................................................................ 6-10 Wrightwood Earthquake of December 12, 1812................................................................................. 6-10 Unnamed Earthquake of December 21, 1812..................................................................................... 6-13 Unnamed Earthquake of 1855............................................................................................................ 6-13 Santa Monica Earthquake of 1930 ...................................................................................................... 6-13 Long Beach Earthquake of 1933 ......................................................................................................... 6-13 Torrance-Gardena Earthquakes of 1941 ............................................................................................. 6-14 San Fernando (Sylmar) Earthquake of 1971 ........................................................................................ 6-14 Point Mugu Earthquake of 1973 ......................................................................................................... 6-15 Malibu Earthquake of 1979 ................................................................................................................ 6-15 Whittier Narrows Earthquake of 1987................................................................................................. 6-15 Malibu Earthquake of 1989 ................................................................................................................ 6-16 Landers and Big Bear Earthquakes of 1992 ......................................................................................... 6-16 Northridge Earthquake of 1994........................................................................................................... 6-16 Hector Mine Earthquake of 1999........................................................................................................ 6-17 Baja California Earthquake of 2010 .................................................................................................... 6-17 Ridgecrest Earthquake Sequence of 2019 ........................................................................................... 6-18 Ojai Earthquake of 2023..................................................................................................................... 6-18 EARTHQUAKE HAZARD ASSESSMENT............................................................................................................... 6-18 Choosing Earthquakes for Planning and Design.................................................................................. 6-18 Seismic Shaking.................................................................................................................................. 6-19 Seismic Shaking Sources ................................................................................................................. 6-22 Surface Fault Rupture ......................................................................................................................... 6-37 Definitions and Statutory Requirements........................................................................................... 6-37 Santa Monica Fault Zone................................................................................................................. 6-39 Liquefaction and Related Ground Failure ........................................................................................... 6-44 Definitions....................................................................................................................................... 6-44 Liquefaction Susceptibility In and Near Santa Monica..................................................................... 6-46 Seismically Induced Slope Failure ...................................................................................................... 6-46 Local Hazard Mitigation Plan
City of Santa Monica, California
2025 Table of Contents – Volume II Page ii
TABLE OF CONTENTS (continued)
SECTION ......................................................................................... PAGE # VULNERABILITY ASSESSMENT ......................................................................................................................... 6-47 Vulnerable Structure Types................................................................................................................. 6-48 Unreinforced Masonry Buildings..................................................................................................... 6-48 Soft-Story Buildings ......................................................................................................................... 6-49 Wood-Frame Structures................................................................................................................... 6-49 Pre-Cast Concrete Structures ........................................................................................................... 6-49 Reinforced Masonry Tilt-up Buildings ............................................................................................. 6-50 Reinforced Concrete-Frame Buildings ............................................................................................. 6-50 Multi-Story Steel-Frame Buildings.................................................................................................... 6-51 Mobile Homes ................................................................................................................................ 6-51 Other Conditions that Can Lead to Structural Failure.......................................................................... 6-52 Building Shape ................................................................................................................................ 6-52 Pounding......................................................................................................................................... 6-52 Critical Facilities and Infrastructure..................................................................................................... 6-52 RISK ANALYSIS............................................................................................................................................. 6-54 HazUS................................................................................................................................................ 6-54 HazUS Earthquake Scenarios for the Santa Monica Area.................................................................... 6-59 Building Damage ............................................................................................................................ 6-61 Total Economic Losses: Sum of Building-Related and Lifeline-Related Losses ................................ 6-69 Casualties........................................................................................................................................ 6-71 Damage to Critical and Essential Facilities ...................................................................................... 6-74 Utility Systems Damage................................................................................................................... 6-75 Fire Following Earthquake ............................................................................................................... 6-77 Debris Generation........................................................................................................................... 6-78 SEISMIC HAZARD MITIGATION ACTIVITIES ....................................................................................................... 6-79 Existing Mitigation Activities............................................................................................................... 6-79 California Earthquake Mitigation Legislation ................................................................................... 6-79 City of Santa Monica Codes ............................................................................................................ 6-80 Businesses/Private Sector................................................................................................................. 6-80 Hospitals......................................................................................................................................... 6-81 Earthquake Education ......................................................................................................................... 6-82 EARTHQUAKE RESOURCE DIRECTORY ............................................................................................................. 6-83 Local and Regional Resources ............................................................................................................ 6-83 State Resources................................................................................................................................... 6-83 Federal and National Resources ......................................................................................................... 6-84 Publications........................................................................................................................................ 6-85 Section 7: Tsunamis
WHY ARE TSUNAMIS A THREAT TO THE CITY OF SANTA MONICA? ....................................................................... 7-1 TSUNAMI BASICS – DEFINITIONS AND CAUSES ................................................................................................... 7-2 What is a Tsunami?............................................................................................................................... 7-2 What Causes a Tsunami?...................................................................................................................... 7-3 HISTORICAL, LOCALLY SIGNIFICANT TSUNAMIS.................................................................................................. 7-5 Santa Barbara Tsunami of 1812 .......................................................................................................... 7-10 Tsunami of May 10, 1877................................................................................................................... 7-10 Tsunami of April 13, 1923.................................................................................................................. 7-11 Earthquake and Tsunami of August 31, 1930...................................................................................... 7-11 Possible Tsunami of August 21, 1934 ................................................................................................. 7-11 Local Hazard Mitigation Plan
City of Santa Monica, California
2025 Table of Contents – Volume II Page iii
TABLE OF CONTENTS (continued)
SECTION ......................................................................................... PAGE # Aleutian Islands Tsunami of March 9, 1957........................................................................................ 7-11 Chilean Tsunami of May 23, 1960...................................................................................................... 7-12 Good Friday (March 28) Earthquake Tsunami of 1964........................................................................ 7-12 Boxing Day (December 26) Earthquake and Tsunami of 2004............................................................ 7-12 Chilean Earthquake and Tsunami of February 27, 2010...................................................................... 7-13 Tohoku, Japan Earthquake and Tsunami of March 11, 2011............................................................... 7-13 Tonga Volcanic Eruption and Tsunami of January 15, 2022 ............................................................... 7-14 TSUNAMI HAZARD ASSESSMENT FOR SANTA MONICA ...................................................................................... 7-14 Hazard Identification.......................................................................................................................... 7-14 Vulnerability and Risk ........................................................................................................................ 7-19 Risk Assessments - Methodology ........................................................................................................ 7-20 Risk Assessments - Results .................................................................................................................. 7-20 TSUNAMI MITIGATION ACTIVITIES .................................................................................................................. 7-24 TSUNAMI RESOURCE DIRECTORY ................................................................................................................... 7-27 City Resources.................................................................................................................................... 7-27 County Resources............................................................................................................................... 7-27 State Resources................................................................................................................................... 7-28 Federal Resources and Programs ........................................................................................................ 7-29 Publications........................................................................................................................................ 7-29 Section 8: Flood Hazards
WHY ARE FLOODS A THREAT TO THE CITY OF SANTA MONICA? .......................................................................... 8-1 HISTORY OF FLOODING IN THE CITY OF SANTA MONICA.................................................................................... 8-4 Historic Flooding in Los Angeles County .............................................................................................. 8-5 WHAT FACTORS CREATE FLOOD RISK? ............................................................................................................. 8-8 Climate................................................................................................................................................. 8-8 Geography and Geology .................................................................................................................... 8-13 Built Environment............................................................................................................................... 8-14 STORM FLOODING CHARACTERISTICS............................................................................................................. 8-14 Coastal Flooding................................................................................................................................. 8-14 Fluvial (Riverine) Flooding.................................................................................................................. 8-15 Pluvial (Urban) Flooding..................................................................................................................... 8-15 HOW FLOOD-PRONE AREAS ARE IDENTIFIED................................................................................................... 8-16 Flood Terminology ............................................................................................................................. 8-16 Floodplain....................................................................................................................................... 8-16 Floodway ........................................................................................................................................ 8-16 Flood Fringe .................................................................................................................................... 8-16 100-Year Flood ............................................................................................................................... 8-17 Development................................................................................................................................... 8-17 Base Flood Elevation (BFE) .............................................................................................................. 8-17 Special Flood Hazard Area (SFHA).................................................................................................. 8-17 Flood Mapping Methods, Products and Programs............................................................................... 8-17 Flood Insurance Rate Maps and Flood Insurance Studies ................................................................ 8-17 Mapped Flood Areas Outside of the 100-Year Flood Zone.............................................................. 8-20 Community Rating System............................................................................................................... 8-20 Risk Mapping, Assessment and Planning (Risk MAP) Program......................................................... 8-20 FLOOD HAZARD ANALYSIS FOR THE SANTA MONICA AREA............................................................................... 8-21 INUNDATION DUE TO CATASTROPHIC FAILURE OF WATER STORAGE STRUCTURES............................................... 8-25 Local Hazard Mitigation Plan
City of Santa Monica, California
2025 Table of Contents – Volume II Page iv
TABLE OF CONTENTS (continued)
SECTION ......................................................................................... PAGE # Flooding Due to Dam Failure ............................................................................................................. 8-25 Flooding Due to Failure of Above-Ground Water Storage Tanks ........................................................ 8-29 HAZARD ASSESSMENT................................................................................................................................... 8-30 Hazard Identification.......................................................................................................................... 8-30 Flood Hazard Mapping and Flooding Potential in the City of Santa Monica ....................................... 8-30 Urban Flooding and Existing Flood Protection Measures in the City of Santa Monica......................... 8-31 Inundation Due to Catastrophic Failure of Water Storage Structures................................................... 8-32 Water-Retaining Structures In and Near Santa Monica .................................................................... 8-32 Water Tank Failure Impacts............................................................................................................. 8-35 Stone Canyon Reservoir .................................................................................................................. 8-36 VULNERABILITY ASSESSMENT – COMMUNITY FLOOD ISSUES .............................................................................. 8-38 What is Susceptible to Damage During a Flood Event?....................................................................... 8-39 100-Year Coastal Flood Event.......................................................................................................... 8-39 Stone Canyon Inundation Event....................................................................................................... 8-39 RISK ANALYSIS............................................................................................................................................. 8-40 General Building Stock Exposure and Potential Building-Related Losses ............................................ 8-41 100-Year Coastal Flood Event.......................................................................................................... 8-41 Stone Canyon Inundation Event....................................................................................................... 8-42 Essential Facility Damage ................................................................................................................... 8-44 Induced Flood Damage: Debris Generation ....................................................................................... 8-45 Social Impact: Shelter Requirements................................................................................................... 8-45 Business and Industry ......................................................................................................................... 8-46 Public Infrastructure ........................................................................................................................... 8-46 FLOOD MITIGATION ACTION ITEMS ............................................................................................................... 8-46 FLOOD RESOURCE DIRECTORY ...................................................................................................................... 8-48 County Resources............................................................................................................................... 8-48 State Resources................................................................................................................................... 8-48 Federal Resources and Programs ........................................................................................................ 8-48 Other National Resources................................................................................................................... 8-49 Publications........................................................................................................................................ 8-52 Section 9: Landslides
WHY ARE LANDSLIDES A THREAT TO THE CITY OF SANTA MONICA? ..................................................................... 9-1 Physiographic Setting ........................................................................................................................... 9-1 Geologic Setting ................................................................................................................................... 9-2 DEFINITIONS ................................................................................................................................................. 9-6 What is a Landslide?............................................................................................................................. 9-6 What is a Debris Flow?......................................................................................................................... 9-6 CONDITIONS CONDUCIVE TO SLOPE FAILURES .................................................................................................. 9-7 Natural Conditions ............................................................................................................................... 9-7 Impacts of Development and Redevelopment ...................................................................................... 9-8 Alterations to Drainage and Groundwater Systems............................................................................... 9-8 Changes in Vegetation.......................................................................................................................... 9-9 HISTORICALLY AND LOCALLY SIGNIFICANT SOUTHERN CALIFORNIA LANDSLIDES ................................................... 9-9 1956 Portuguese Bend Landslide, Palos Verdes.................................................................................. 9-10 1969 Glendora, Los Angeles County .................................................................................................. 9-10 1977-1978 Monterey Park, Baldwin Hills and Repetto Hills, Los Angeles County.............................. 9-10 1978 Bluebird Canyon, Orange County.............................................................................................. 9-10 Local Hazard Mitigation Plan
City of Santa Monica, California
2025 Table of Contents – Volume II Page v
TABLE OF CONTENTS (continued)
SECTION ......................................................................................... PAGE # 1979 Big Rock, Los Angeles County ................................................................................................... 9-10 1980 Southern California Landslides .................................................................................................. 9-10 1978-1979, 1980 San Diego County .................................................................................................. 9-11 1983 San Clemente, Orange County .................................................................................................. 9-11 1983 Big Rock Mesa, Malibu, Los Angeles County............................................................................. 9-11 1994 Northridge Earthquake Landslides ............................................................................................. 9-11 1995 Los Angeles and Ventura Counties Landslides........................................................................... 9-11 2003-Present Del Mar Bluff Stabilization, San Diego County ............................................................. 9-11 2005 La Conchita Landslide, Ventura County..................................................................................... 9-12 2005 Blue Bird Canyon Landslide, Laguna Beach, Orange County .................................................... 9-12 2018 Montecito Landslide, Santa Barbara County .............................................................................. 9-12 2021-2023 San Clemente Landslides, Orange County........................................................................ 9-12 2023 Rolling Hills Estates Landslide, Los Angeles County................................................................... 9-13 2023 Santa Monica Bluff Stabilization Emergency Work, Los Angeles County ................................... 9-13 LANDSLIDE HAZARD ASSESSMENT FOR SANTA MONICA.................................................................................... 9-13 Hazard Identification.......................................................................................................................... 9-13 Vulnerability and Risk ........................................................................................................................ 9-16 SUMMARY OF COMMUNITY LANDSLIDE ISSUES ................................................................................................ 9-18 What is Susceptible to Landslides?...................................................................................................... 9-18 Landslide Mitigation Activities............................................................................................................ 9-19 LANDSLIDE RESOURCE DIRECTORY................................................................................................................. 9-21 City Resources.................................................................................................................................... 9-21 County Resources............................................................................................................................... 9-21 State Resources................................................................................................................................... 9-21 Federal Resources and Programs ........................................................................................................ 9-22 Publications........................................................................................................................................ 9-22 Section 10: Wildfires and Urban Fires
WHY ARE FIRES A THREAT TO THE CITY OF SANTA MONICA?.............................................................................10-1 WILDFIRES ..................................................................................................................................................10-1 What are Wildfires?............................................................................................................................10-1 Wildfire Causes ..................................................................................................................................10-2 History of Wildfires Near Santa Monica and In Southern California ...................................................10-4 Topanga Canyon or Santa Monica Mountains Fire of 1938 .............................................................10-7 Bel Air, Brentwood and Santa Ynez Fires of 1961 ...........................................................................10-8 Los Angeles Fires of 1978................................................................................................................10-8 Tunnel Fire of 1991 (Oakland Firestorm).........................................................................................10-9 Laguna Fire of 1993 ......................................................................................................................10-10 Southern California Fires of 2003, Including Cedar Fire ................................................................10-10 Tubbs Fire of 2017 ........................................................................................................................10-11 Camp Fire of 2018 ........................................................................................................................10-12 Woolsey Fire of 2018 ....................................................................................................................10-13 Franklin (Malibu) Fire of 2024.......................................................................................................10-14 Palisades Fire of 2025 ...................................................................................................................10-14 Eaton Fire of 2025 .........................................................................................................................10-17 Wildfire Characteristics ....................................................................................................................10-17 Wildfire Hazard Identification ..........................................................................................................10-21 Federal Responsibility Areas (FRAs)...............................................................................................10-21 Local Hazard Mitigation Plan
City of Santa Monica, California
2025 Table of Contents – Volume II Page vi
TABLE OF CONTENTS (continued)
SECTION ......................................................................................... PAGE # State Responsibility Areas (SRAs)...................................................................................................10-22 Local Responsibility Areas (LRAs)..................................................................................................10-23 Mapped Wildfire Hazard Zones Near and In Santa Monica..............................................................10-23 STRUCTURE FIRES.......................................................................................................................................10-27 Definition and Recent Statistics ........................................................................................................10-27 Causes of Structure Fires...................................................................................................................10-28 Factors that Impact Structure Fire Risk in a Community ....................................................................10-29 Fires After an Earthquake ..................................................................................................................10-31 VULNERABILITY AND RISK ASSESSMENT .........................................................................................................10-33 What is Susceptible to Wildfires in Santa Monica.............................................................................10-33 Structure Fires...................................................................................................................................10-35 Fires After an Earthquake ..................................................................................................................10-37 Social Vulnerability to Fires..............................................................................................................10-38 FIRE HAZARD MITIGATION ACTIVITIES ..........................................................................................................10-43 Fire Prevention ................................................................................................................................. 10-43 Vegetation Management...................................................................................................................10-44 Construction Requirements...............................................................................................................10-45 Public Awareness .............................................................................................................................10-47 FIRE HAZARDS RESOURCE DIRECTORY..........................................................................................................10-51 Local Resources................................................................................................................................10-51 Santa Monica Fire Department......................................................................................................10-51 Automatic and Mutual Aid Agreements.........................................................................................10-52 Standardized Emergency Management System (SEMS) ..................................................................10-53 Santa Monica Office of Emergency Management ..........................................................................10-55 Santa Monica Police Department ..................................................................................................10-55 Southern California Edison (SCE) ...................................................................................................10-55 County Resources.............................................................................................................................10-56 Los Angeles County Fire Department ............................................................................................10-56 Los Angeles County Chief Executive Office ...................................................................................10-56 State Resources................................................................................................................................. 10-56 California Division of Forestry & Fire Protection and Office of the State Fire Marshal (OSFM) ......10-56 California Governor’s Office of Emergency Services (Cal-OES) .....................................................10-56 California FireSafe Council (CFSC) ................................................................................................10-56 Federal Resources and Programs ......................................................................................................10-56 Federal Emergency Management Agency (FEMA)..........................................................................10-57 U.S. Department of the Interior .....................................................................................................10-57 National Weather Service..............................................................................................................10-57 Other National Resources.................................................................................................................10-57 National Fire Protection Association (NFPA) .................................................................................10-57 National Interagency Fire Center (NIFC)........................................................................................10-57 United States Fire Administration (USFA) ......................................................................................10-58 Additional Resources........................................................................................................................10-58 Firewise USA................................................................................................................................. 10-58 Insurance Services Office (ISO) .....................................................................................................10-58 Publications......................................................................................................................................10-58 Local Hazard Mitigation Plan
City of Santa Monica, California
2025 Table of Contents – Volume II Page vii
TABLE OF CONTENTS (continued)
SECTION ......................................................................................... PAGE # Section 11: Climate Change and Severe Weather
WHY DO CLIMATE CHANGE AND SEVERE WEATHER POSE A THREAT TO THE CITY OF SANTA MONICA? ..................11-1 SNAPSHOT OF SANTA MONICA’S CLIMATE AND WEATHER BASED ON HISTORICAL DATA, AND ELEMENTS IMPACTING FUTURE FORECASTS.....................................................................................................................11-3 Extreme Heat......................................................................................................................................11-5 Social Vulnerability to Extreme Heat ...............................................................................................11-8 Extreme Heat Mitigation Actions and Resources............................................................................11-10 Drought ............................................................................................................................................11-13 Social Vulnerability to Drought .....................................................................................................11-14 Drought Planning and Mitigation Actions......................................................................................11-15 Sea-Level Rise...................................................................................................................................11-17 Sea-level Rise Projections in Santa Monica and Social Impacts.....................................................11-18 Sea-Level Rise Adaptation Strategies in Santa Monica ...................................................................11-22 Severe Windstorms...........................................................................................................................11-23 Social Vulnerability to Windstorms ...............................................................................................11-35 Activities for Windstorm Mitigation and Response ........................................................................11-36 Section 12: Hazardous Materials
WHY ARE HAZARDOUS MATERIALS A THREAT TO THE CITY OF SANTA MONICA? .................................................12-1 DEFINITIONS ...............................................................................................................................................12-2 What are Hazardous Materials?..........................................................................................................12-2 What is a Hazardous Material Release?..............................................................................................12-3 Conditions Conducive to Hazardous Material Releases......................................................................12-3 Releases from Leaking Underground Storage Tanks.........................................................................12-5 Spills Related to Transportation Accidents.......................................................................................12-5 Releases Due to Damaged Pipelines ...............................................................................................12-6 Releases Due to Earthquakes...........................................................................................................12-7 Inappropriate or Accidental Use of Hazardous Materials in Households.........................................12-7 HISTORICALLY AND LOCALLY SIGNIFICANT SOUTHERN CALIFORNIA RELEASES OF HAZARDOUS MATERIALS.............12-8 1987 Releases Related to the Whittier Narrows Earthquake................................................................12-8 1994 Releases Related to the Northridge Earthquake..........................................................................12-8 2019 405-Freeway Closure due to Overturned Truck .........................................................................12-9 2020 405-Freeway Closure due to Hazardous Spill............................................................................12-9 2020 Hazardous Material Spills..........................................................................................................12-9 2021 Hazardous Material Spills..........................................................................................................12-9 2022 Hazardous Material Spills........................................................................................................12-10 2023 Hazardous Material Spills........................................................................................................12-10 July 2024 Closure of I-15 due to Truck Fire ......................................................................................12-11 2024 Hazardous Material Spills........................................................................................................12-11 September 2024 Freeway Closure due to Truck Fire.........................................................................12-11 HAZARDOUS MATERIALS RELEASE ASSESSMENT FOR SANTA MONICA................................................................12-12 Hazard Identification........................................................................................................................12-12 Generators of Hazardous Materials ...............................................................................................12-12 Leaking Underground Storage Tanks (LUSTs) and Cleanup Program Sites.....................................12-17 Transporters of Hazardous Materials .............................................................................................12-19 Pipelines Through the City ............................................................................................................12-20 Vulnerability and Risk ......................................................................................................................12-20 HAZARDOUS MATERIALS MITIGATION ACTIVITIES ..........................................................................................12-23 Local Hazard Mitigation Plan
City of Santa Monica, California
2025 Table of Contents – Volume II Page viii
TABLE OF CONTENTS (continued)
SECTION ......................................................................................... PAGE # HAZARDOUS MATERIALS MANAGEMENT RESOURCE DIRECTORY......................................................................12-25 City Resources..................................................................................................................................12-25 Local Resources................................................................................................................................12-25 County Resources.............................................................................................................................12-26 State Resources................................................................................................................................. 12-26 Federal Resources.............................................................................................................................12-27 Section 13: Airport Hazards
WHAT AIRPORT ACTIVITIES COULD POSE A HAZARD TO THE CITY OF SANTA MONICA? .......................................13-1 BACKGROUND ............................................................................................................................................13-1 Airport Setting.....................................................................................................................................13-1 Santa Monica Airport History .............................................................................................................13-3 Airport Operations..............................................................................................................................13-6 HISTORICALLY AND LOCALLY SIGNIFICANT SOUTHERN CALIFORNIA AVIATION ACCIDENTS ...................................13-8 REGULATORY SETTING................................................................................................................................13-18 Airport Land Use Compatibility Planning Requirements...................................................................13-18 Airport Operations............................................................................................................................13-19 Aviation Noise..................................................................................................................................13-23 VULNERABILITY AND RISK ASSESSMENT .........................................................................................................13-27 Ground and Airspace Safety .............................................................................................................13-27 Aviation Noise..................................................................................................................................13-29 Potential for Hazardous Materials Related to Historical Airport Land Uses.......................................13-33 Social Vulnerability to Airport Hazards.............................................................................................13-35 AIRPORT HAZARDS MITIGATION ACTIVITIES ..................................................................................................13-37 AIRPORT HAZARDS RESOURCE DIRECTORY ...................................................................................................13-38 City Resources..................................................................................................................................13-38 County Resources............................................................................................................................. 13-39 State Resources................................................................................................................................. 13-39 Federal Resources and Programs ......................................................................................................13-40 Publications......................................................................................................................................13-40 VOLUME III – APPENDICES
Appendix A: Master Resource Directory....................................................................... A-1 Appendix B: Public Participation Process and Meeting Materials.................................. B-1 Appendix C: Economic Analysis of Natural Hazard Mitigation Projects........................ C-1 Appendix D: Acronyms............................................................................................... D-1 Appendix E: Glossary .................................................................................................. E-1 Appendix F: California Disasters Since 1950................................................................ F-1 Appendix G: Major Dams in Los Angeles County ........................................................ G-1 Appendix H: Plates ...................................................................................................... H-1 Appendix I: References ................................................................................................ I-1 Appendix J: City Council Adoption Resolution............................................................. J-1 Local Hazard Mitigation Plan
City of Santa Monica, California
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TABLE OF CONTENTS (continued)
SECTION ......................................................................................... PAGE #

TABLES, FIGURES AND PHOTOS
Tables
Table 6-1: Abridged Modified Mercalli Intensity Scale ............................................................................. 6-6 Table 6-2: Historical (1769 – March 2024) Earthquakes in the Southern California Region with Magnitudes > 4.5........................................................................................................... 6-11 Table 6-3: Estimated Horizontal Peak Ground Accelerations and Seismic Intensities in the Santa Monica Area ................................................................................................................ 6-21 Table 6-4: Injury Classification Scale...................................................................................................... 6-58 Table 6-5: HazUS Earthquake Scenarios for the City of Santa Monica.................................................... 6-61 Table 6-6: Building Exposure by Occupancy Type for the Study Area .................................................... 6-62 Table 6-7: Number of Buildings Damaged, by Construction Type.......................................................... 6-62 Table 6-8: Number of Buildings Damaged, by Occupancy Type............................................................ 6-64 Table 6-9: Building-Related Economic Loss Estimates (in Million $) ....................................................... 6-70 Table 6-10: Estimated Transportation and Utility System Economic Losses............................................... 6-71 Table 6-11: Estimated Casualties .............................................................................................................. 6-73 Table 6-12: Estimated Damage to Critical Facilities as a Result of the Earthquake Scenarios .................... 6-74 Table 6-13: Estimated Damage to Pipelines as a Result of Three Earthquake Scenarios............................ 6-76 Table 6-14: Expected Performance of Potable Water and Electricity Services........................................... 6-77 Table 6-15: Fires Following an Earthquake ............................................................................................... 6-78 Table 6-16: Debris Generated by Earthquake Scenarios ........................................................................... 6-79 Table 6-17: Partial List of Over 200 California Laws on Earthquake Safety ............................................... 6-79 Table 7-1: Significant Historical Tsunami Record for Southern California – 1812 to January 2024........... 7-6 Table 7-2: Total Building and Total Residential Building Damage Counts for Los Angeles County Based on a HazUS Tsunami Analysis of the CGS (2021) Tsunami Hazard Maps for California ........ 7-21 Table 7-3: Estimated Tsunami Casualties in Santa Monica by Time of Day, Exposed Population and Community Preparedness Level (for a Local Tsunami Scenario) ............................................ 7-22 Table 7-4: Estimated Economic Losses to Residential and Commercial Buildings in Santa Monica Using Crisis Track® Software ................................................................................................ 7-23 Table 7-5: Tsunami Alerts and Appropriate Responses ........................................................................... 7-24 Table 8-1: Historical Floods in Los Angeles County.................................................................................. 8-5 Table 8-2: Tropical Storms That Affected Southern California Between 1858 and August 2023 ............. 8-10 Table 8-3: Flood Zones Mapped in Santa Monica .................................................................................. 8-21 Table 8-4: Dam Failures in Southern California...................................................................................... 8-26 Table 8-5: Water Storage Reservoirs In and Near Santa Monica Owned by Santa Monica ..................... 8-32 Table 8-6: Building Exposure by Occupancy Type – Stone Canyon Dam Inundation Event ................... 8-40 Table 8-7: Estimated Economic Losses to Residential and Commercial Buildings in Santa Monica Within the 100-Year Flood Zone Using Crisis Track® Software ........................................................ 8-41 Table 8-8: Expected Building Damage by Occupancy as a Result of a Stone Canyon Dam Failure ........ 8-42 Table 8-9: Expected Building Damage by Building Type ........................................................................ 8-42 Table 8-10: Building-Related Economic Loss Estimates Resulting from a Catastrophic Stone Canyon Dam Failure Scenario ............................................................................................................ 8-44 Table 8-11: Estimated Economic Losses to Residential and Commercial Buildings in Santa Monica Within the Stone Canyon Dam Inundation Zone Using Crisis Track® Software..................... 8-44 Table 10-1: Top Twenty Most-Destructive Fires in California (as of April 3, 2025) Based on Total Number of Structures Destroyed............................................................................................10-6 Table 10-2: Zones in Santa Monica Under Evacuation Orders and Evacuation Warnings During the Palisades Fires of January 2025............................................................................................10-16 Local Hazard Mitigation Plan
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2025 Table of Contents – Volume II Page x
TABLE OF CONTENTS (continued)
SECTION ......................................................................................... PAGE # Table 10-3: Causes of Residential Building Fires in the U.S. (2017-2019), Organized from Highest to Lowest Percentages .............................................................................................................10-29 Table 10-4: Fires Following an Earthquake .............................................................................................10-38 Table 10-5: Santa Monica Fire Department Stations, Resources and Minimum Daily Staffing ................10-52 Table 11-1: Summary of Natural Hazard Events in California (1980-Sept. 2024) Costing >$1 Billion.......11-2 Table 11-2: Record Monthly Temperatures (°F) and Years of Record in Santa Monica .............................11-6 Table 11-3: Santa Monica Projected Heat Health Events..........................................................................11-9 Table 11-4: Projected Sea-Level Rise Scenarios for Santa Monica ..........................................................11-19 Table 11-5: Anticipated Costs of Sea Level Rise in Santa Monica (in millions of $) Through the Year 2100........................................................................................................11-22 Table 11-6: Selected Extreme Wind Events Reported in the Santa Monica Region Through June 2024.

Santa Ana Wind events are highlighted in yellow................................................................11-24 Table 11-7: Enhanced Fujita Scale..........................................................................................................11-31 Table 11-8: Tornado Events in the Santa Monica Region........................................................................11-32 Table 12-1: TRI Site, and Large-Quantity Generators (LQGs) of Hazardous Materials in the City of Santa Monica.......................................................................................................................12-14 Table 12-2: Open Leaking Underground Fuel Tank (LUST) and Cleanup Program Sites in the City of Santa Monica.......................................................................................................................12-18 Table 12-3: Routes in the Vicinity of Santa Monica Used to Transport Hazardous Materials and Explosives.....................................................................................................................12-20 Table 13-1: Annual Flight Operations at SMO, 2018-2023 ......................................................................13-7 Table 13-2: Significant Historical Aviation Accidents and Incidents in Southern California......................13-9 Table 13-3: Aircraft Incidents Reported for SMO between 1964 and 2024.............................................13-14 Table 13-4: Noise Complaints Related to SMO Operations, 2018-2023.................................................13-33 Figures
Figure 6-1: Regional Map Showing Select Faults, Physiographic Provinces and Landforms Mentioned in the Text .............................................................................................................................. 6-2 Figure 6-2: Main Types of Faults................................................................................................................ 6-4 Figure 6-3: Earthquake Shaking Levels Anticipated in California ............................................................... 6-8 Figure 6-4: Notable Regional Earthquakes............................................................................................... 6-12 Figure 6-5: ShakeMap for a Magnitude 7.4 Earthquake Scenario on the Malibu Coast-Santa MonicaHollywood Fault System........................................................................................................ 6-24 Figure 6-6: ShakeMap for a Magnitude 7.2 Earthquake Scenario on the Newport-Inglewood Fault......... 6-27 Figure 6-7: ShakeMap for a Magnitude 7.5 Earthquake Scenario on the Compton Thrust Fault ............... 6-29 Figure 6-8: Scenario for a M7.1 Earthquake Scenario on the Puente Hills Fault Showing Estimated Intensity Values in the Region Resulting from this Event ...................................................................... 6-30 Figure 6-9: Estimated Percent Building Loss as a Result of a Mw 7.5 Earthquake on the Puente Hills Thrust ................................................................................................................ 6-31 Figure 6-10: ShakeMap for a Magnitude 7.8 Earthquake Scenario on the San Andreas Fault (repeat of the 1857 Fort Tejon Earthquake) ............................................................................ 6-35 Figure 6-11: ShakeMap for a Magnitude 7.8 Earthquake Scenario (Shakeout) on the San Andreas Fault.... 6-36 Figure 6-12: Faults Mapped and Inferred Through, Near and Below Santa Monica ................................... 6-40 Figure 6-13: Earthquake Fault Zones of Required Investigation In and Near Santa Monica, California ...... 6-43 Figure 6-14: Seismic Hazards Zones Map, Santa Monica, California......................................................... 6-45 Figure 6-15: Census Tracts Used in the HazUS Analyses........................................................................... 6-60 Local Hazard Mitigation Plan
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TABLE OF CONTENTS (continued)
SECTION ......................................................................................... PAGE # Figure 6-16: Total Residential Building Related Losses as a Percentage of Total Residential Building Exposure.................................................................................................................. 6-67 Figure 6-17: Total Commercial Building Related Losses as a Percentage of Total Commercial Building Exposure.................................................................................................................. 6-68 Figure 7-1: Bathymetric Map of Santa Monica Bay Showing Continental Slope and Offshore Canyons that Could be a Source of Tsunami-Generated Landslides....................................................... 7-1 Figure 7-2: Diagram Showing Terms Commonly Used in Tsunami Studies................................................ 7-2 Figure 7-3: World Map Showing Approximate Location of Ring of Fire (pink shaded areas), Subduction Trenches (blue lines), Active Volcanoes (yellow triangles), and Large Earthquakes Between 1900 and 2013 (red, green and blue dots). Cascadia Subduction Zone also shown................. 7-4 Figure 7-4: Tsunami Hazards Map, Santa Monica, California .................................................................. 7-18 Figure 8-1: Annual Rainfall in the City of Los Angeles, California for the Period Between 1877 and 2023................................................................................................................................. 8-2 Figure 8-2: Tile Plot Showing Monthly Discharges in Ballona Creek at Site 11103500 for Years 1928-1978 ..................................................................................................................... 8-4 Figure 8-3: Flood Zones Mapped In and Near Santa Monica................................................................... 8-23 Figure 8-4: Dam Inundation Map ............................................................................................................ 8-37 Figure 8-5: Anticipated Damage in Santa Monica as a Result of a Stone Canyon Dam Failure Scenario . 8-43 Figure 9-1: Physiographic Map of Santa Monica and Surrounding Areas................................................... 9-2 Figure 9-2: Geologic Map of Santa Monica and Vicinity with Unit Descriptions....................................... 9-3 Figure 9-3: Bathymetric Map of Santa Monica Bay Showing Offshore Canyons ........................................ 9-5 Figure 9-4: Slope Distribution Map for Santa Monica and Adjoining Areas ............................................. 9-14 Figure 9-5: Areas in Santa Monica Susceptible to Slope Failure............................................................... 9-16 Figure 10-1: Wildfires Causes for the Period 2010-2020 in California (top) and in Los Angeles County (bottom)................................................................................................................................. 10-3 Figure 10-2: Historical (1930-2025) Wildfires Near Santa Monica ............................................................10-5 Figure 10-3: Federal Responsibility Areas (FRAs) in light green and State Responsibility Areas (SRAs) in yellow to the north and northwest of Santa Monica as of December 31, 2024 ....................10-24 Figure 10-4: Fire Hazard Severity Zones In and Near Santa Monica, California.......................................10-26 Figure 10-5: Various Statistics for Fire Losses in the United States, 1980-2023........................................10-28 Figure 10-6: Wildfire Risk Profile for Santa Monica, California................................................................10-39 Figure 11-1: Impacts of Climate Change on California ..............................................................................11-1 Figure 11-2: Health Impacts of Climate Change ........................................................................................11-2 Figure 11-3: Projections of Maximum Temperatures and Extreme Heat Days in Santa Monica (1950-2100) under RCP 4.5 (teal) and 8.5 (violet)..................................................................11-7 Figure 11-4: 2022 California Drought, Five-Category System ..................................................................11-13 Figure 11-5: Percent of Los Angeles County in U.S. Drought Monitor Categories: 2000-2024.................11-14 Figure 11-6: Projected Sea-Level Rise Scenarios for Santa Monica ..........................................................11-19 Figure 11-7: Flood Extent along the Santa Monica Coastline Through the Year 2100 Using Three Sea-Level Rise Scenarios......................................................................................................11-21 Figure 11-8: Santa Ana Winds.................................................................................................................11-24 Figure 11-9: Funnel Cloud Over the Ocean off Venice Beach, Feb. 22, 2005 .........................................11-31 Figure 12-1: Map of the impacted area where a multi-day power outage was required to complete repairs to the gas main break. Actual damage occurred on Lipton Avenue at Stanford Street ..............12-10 Figure 12-2: Hazardous Materials Sites, Santa Monica, California...........................................................12-16 Figure 13-1: Regional Setting of the Santa Monica Airport.........................................................................13-2 Figure 13-2: Land Use Around Santa Monica Airport ................................................................................13-5 Figure 13-3: Annual Operations by Year at SMO, 2001-2023 ...................................................................13-7 Local Hazard Mitigation Plan
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Figure 13-4: The Four Safety Management System (SMS) Components....................................................13-20 Figure 13-5: Airport Influence Zones.......................................................................................................13-22 Figure 13-6: Typical Noise Levels............................................................................................................13-24 Figure 13-7: Single Event Noise Exposure Level (SENEL) .........................................................................13-26 Figure 13-8: Noise Contours for SMO .....................................................................................................13-32 Figure 13-9: Social Vulnerability of Census Tracts Near the Santa Monica Airport (showing nursing homes, schools, and child care centers)..................................................................13-36 Photos
Photo 8-1: Failure of the Baldwin Hills Dam ....................................................................................... 8-28 Photo 12-1: View to the northwest along Stanford Street, from just south of Lipton Avenue, of the street closures and potholing required to fix the gas main .........................................................12-10 Photo 12-2: View of damaged gas main under repair ..........................................................................12-10 Local Hazard Mitigation Plan Section 6
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SECTION 6: EARTHQUAKES
WHY ARE EARTHQUAKES A THREAT TO THE CITY OF SANTA MONICA? Santa Monica is at risk from many natural and man-made hazards, and an earthquake is an event that poses a great potential for far-reaching loss of life or property, and economic damage. This is true for most of southern California, since damaging earthquakes occur relatively frequently, affect widespread areas, trigger many secondary effects, and can overwhelm the ability of local jurisdictions to respond. Earthquake-triggered geologic effects include ground shaking, surface fault rupture, landslides, liquefaction, subsidence, seiches and if offshore, tsunamis (see Section 7).

Earthquakes can also cause human-made hazards such as dam failures (see Section 8), urban fires (Section 11), and toxic chemical releases (see Section 12).

In California, recent earthquakes in or near urban environments have caused relatively few casualties. This is due more to luck than design. For example, when a portion of the Nimitz Freeway in Oakland collapsed at rush hour during the 1989, MW 7.1 Loma Prieta earthquake, the freeway was uncommonly empty because so many were watching the World Series. The 1994, MW 6.7 Northridge earthquake occurred before dawn, when most people were home safely in bed.

Despite such good luck, California’s urban earthquakes have resulted in significant losses. The moderate-sized Northridge earthquake caused 54 deaths, more than 1,500 injuries and nearly $30 billion in damage. For days afterward, thousands of homes and businesses were without electricity; tens of thousands had no gas; and nearly 50,000 had little or no water. Approximately 15,000 structures were moderately to severely damaged, which left thousands of people temporarily homeless. Santa Monica experienced a relatively large number of damaged buildings, including to its hospitals. Several collapsed bridges and overpasses created commuter and commercial transport havoc on the freeway system. Extensive damage was caused by ground shaking, with shaking-induced liquefaction and dozens of fires after the earthquake causing additional damage. This moderately sized earthquake resulted in record economic losses, and yet the Los Angeles metropolitan area, including the city of Santa Monica, is at risk from earthquakes that could release more than ten times the seismic energy of the Northridge earthquake.

Historical and geological records show that California has a long history of seismic events. The state is probably best known for the San Andreas fault, an approximately 750-mile-long fault zone running from the Mexican border to a point offshore west of San Francisco. Geologic studies show that over the past 1,400 to 1,500 years, large earthquakes have occurred on the southern San Andreas fault at about 130-year intervals. As the last large earthquake on the southern San Andreas occurred in 1857, that section of the fault is considered a likely location for an earthquake within the next few decades. The San Andreas fault, however, is only one of dozens of known faults that extend across southern California. Some of the better-known faults include the Sierra Madre, Newport-Inglewood, Whittier-Elsinore, Santa Monica, Hollywood, Malibu Coast and Palos Verdes faults (see Figure 6-1). The Santa Monica fault extends through the northern portion of the city. The Malibu Coast, Palos Verdes, Hollywood, Newport-Inglewood and Compton faults are located within about 10 miles of Santa Monica, and thus have the capacity to generate strong shaking in the city, stronger than what an earthquake on the San Andreas fault would produce. Each of these faults is discussed in detail later in this section.

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Great advances in earthquake engineering have been made in the last three decades in large part as a result of the lessons learned from the 1994 Northridge, California, 1995 Kobe, Japan, 1999 Izmit, Turkey and 1999 Chi-Chi, Taiwan earthquakes. The 1994 Northridge earthquake in particular engendered several changes first adopted in 1997 to the building code used in California. Nevertheless, many California communities remain unprepared, partly because the Local Hazard Mitigation Plan Section 6
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building code changes are not retroactive and thus many older structures have not been strengthened. In the past decade, however, this has changed in some areas, with several cities, including Santa Monica, requiring seismic retrofits of several construction styles that are known to perform poorly in earthquakes. Thus, with the application of the latest building codes and the enforcement of seismic retrofits for older structures, it is possible to minimize the destructive effects of earthquakes.

Comprehensive hazard mitigation programs that include the identification and mapping of hazards, prudent planning, public education, emergency exercises, enforcement of building codes, and expedient retrofitting and rehabilitation of weak structures can significantly reduce the scope of an earthquake’s effects and avoid disaster. The objective is for local governments, emergency relief organizations, and residents to take action and develop and implement policies and programs designed to reduce the effects of earthquakes. Undoubtedly these improvements will be tested the next time the region experiences a moderate- to large-magnitude earthquake.

EARTHQUAKE BASICS - DEFINITIONS
The outer 10 to 70 kilometers of the Earth consist of enormous blocks of moving rock, called plates. There are about a dozen major plates, which slowly collide, separate, and grind past each other. In the uppermost plates, friction locks the plate edges together, while movement continues at depth. Consequently, the near-surface rocks bend and deform near plate boundaries, storing strain energy. Eventually, the frictional forces are overcome and the locked portions of the plates move. The stored strain energy is released in waves.

By definition, the break or fracture between moving blocks of rock is called a fault, and such differential movement produces a fault rupture. The place where the fault first ruptures is called the focus (or hypocenter). The released energy waves radiate out in all directions from the rupture surface, making the earth vibrate and shake as the waves travel through. This shaking is what we feel in an earthquake.

Although earthquakes can occur in areas with no known faults, most earthquakes occur on or near plate boundaries. Given that California straddles the boundary between the North American and Pacific plates (Figure 6-1), it experiences many earthquakes, and relatively often. The Pacific Plate is moving northwesterly, relative to the North American Plate, at about 52 mm/yr (2 inches/yr).

This is about the rate at which fingernails grow, and seems unimpressive. However, it is enough to accumulate enormous amounts of strain energy over dozens to thousands of years. Despite being locked in place most of the time, in another 15 million years (a short time in the context of the Earth’s history), due to plate movements, Santa Monica will be hundreds of kilometers north of San Francisco.

Although the San Andreas fault marks the actual separation between the Pacific and North American plates, only about 70 percent of the plate motion occurs on the San Andreas fault itself.

The rest is distributed among other faults of the San Andreas system, including the San Jacinto, Whittier-Elsinore, Newport-Inglewood, Palos Verdes, plus several offshore faults; and among faults of the Eastern Mojave Shear Zone, a series of faults east of the San Andreas fault that were responsible for the 1992 Landers, 1999 Hector Mine and 2019 Ridgecrest earthquakes. Thus, the zone of plate-boundary earthquakes and ground deformation covers an area that stretches from the Pacific Ocean to Nevada.

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Because the Pacific and North American plates are sliding past each other, with relative motions to the northwest and southeast, respectively, all of the faults mentioned above are aligned northwestsoutheast, and are strike-slip faults (see Figure 6-1). On average, strike-slip faults are nearly vertical breaks in the rock, and when a strike-slip fault ruptures, the rocks on either side of the fault slide horizontally past each other.

However, about 70 miles northwest of Santa Monica, there is a kink in the San Andreas fault, commonly referred to as the “Big Bend.” Near the Big Bend, the two plates do not slide past each other. Instead, they collide, causing localized compression, resulting in folding and thrust faulting (see Figure 6-2). Thrust faults meet the surface of the Earth at a low angle, dipping 25 to 45 degrees from the horizontal. Thrusts are a type of dip-slip fault, where rocks on opposite sides of the fault move up or down relative to each other. When a thrust fault ruptures, the top block of rock moves up and over the rock on the other side of the fault.

Figure 6-2: Main Types of Faults
Few faults are simple, planar breaks in the Earth. They more often consist of smaller strands, with a similar orientation and sense of movement. Sometimes geologists group strands into sections or segments, which are believed capable of rupturing together during a single earthquake. The more extensive the fault, the bigger the earthquake it can produce. Therefore, multi-strand fault ruptures generally produce larger earthquakes.

Large-magnitude earthquakes that occur near urban centers have the potential to cause the most damage. Thus, fault dimensions and proximity to urban centers are key parameters in any hazard assessment. In addition, it is important to know a fault’s style of movement (i.e., is it dip-slip or strike-slip), the age of its most recent activity, its total displacement, and its slip rate (all discussed below). These values are helpful in estimating how often a fault produces damaging earthquakes, Strike-slip faults are vertical or almost vertical
rifts where the earth’s plates move mostly
horizontally. From the observer’s perspective, if
the opposite block, looking across the fault,
moves to the right, the fault is a right-lateral
fault; if the block moves left, the fault is a leftlateral fault.

Dip-slip faults are slanted structures where the blocks mostly shift vertically. If the rock above an inclined fault moves down, the fault is called a normal fault; when the rock above the fault moves up, the fault is called a reverse fault. Thrust faults are reverse faults with a dip of 45° or less.

Normal fault Reverse fault
Strike-slip fault
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and the size of the earthquake that will be generated the next time that fault ruptures. Most large damaging earthquakes are followed by aftershocks. By definition, these are smaller earthquakes that happen close to the mainshock (the biggest earthquake of the sequence) in time and space.

These smaller earthquakes occur as the Earth adjusts to the regional stress changes created by the mainshock. The bigger the mainshock, the greater the number of aftershocks, the larger the aftershocks will be, and the wider the area in which they might occur. On average, the largest aftershock will be 1.2 magnitude units less than the mainshock. This is an average, and there are many cases where the biggest aftershock is larger than the average predicts. The key point is that any major earthquake will produce aftershocks large enough to cause additional damage, especially to already weakened structures. Consequently, post-disaster response planning must take damaging aftershocks into account.

When comparing the sizes of earthquakes, the most meaningful feature is the amount of energy released. Thus, scientists most often consider seismic moment, a measure of the energy released when a fault ruptures. We are more familiar, however, with scales of magnitude, which measure amplitude of ground motion. Magnitude scales are logarithmic. Each one-point increase in magnitude represents a ten-fold increase in amplitude of the waves as measured at a specific location, and a 32-fold increase in energy. That is, a magnitude 7 earthquake produces 100 times (10 x 10) the ground motion amplitude of a magnitude 5 earthquake. Similarly, a magnitude 7 earthquake releases approximately 1,000 times more energy (32 x 32) than a magnitude 5 earthquake. Scientists now use the moment magnitude (Mw) scale to relate energy release to magnitude; this scale has replaced the Richter scale, which is no longer used by seismologists.

An early measure of earthquake size still used today is the seismic intensity scale, which is a qualitative assessment of an earthquake’s effects at a given location. Although it has limited scientific application, intensity is still widely used because it is intuitively clear and quick to determine. The most commonly used measure of seismic intensity is called the Modified Mercalli Intensity (MMI) scale, which has 12 levels of damage (see Table 6-1).

A given earthquake will have one seismic moment and, in principle, one magnitude, although there are several methods of calculating magnitude, which give slightly different results. However, one earthquake will produce several intensities because intensity effects vary with the location (distance), soil conditions, and perceptions of the observer.

Total displacement is the length, measured in kilometers (km), of the total movement that has occurred along the fault over as long a time as the geologic record reveals. It is usually estimated by measuring the distance between geologic features (such as a distinctive rock formation) that have been split apart and separated (offset) by the cumulative movement of the fault over many earthquakes. Slip rate is a speed, expressed in millimeters per year (mm/yr). Slip rate is estimated by measuring an amount of offset accrued during a known amount of time, obtained by dating the ages of geologic features. Slip rate data also are used to estimate a fault’s earthquake recurrence interval. Sometimes referred to as “repeat time” or “return interval,” the recurrence interval represents the average amount of time that elapses between major earthquakes on a fault.

Geologists generally derive the recurrence interval for a fault by excavating a series of trenches across the fault to obtain paleoseismic evidence of the earthquakes that have occurred during prehistoric time. If the sediments exposed in the trenches are suitable for dating and the earthquake record is well preserved, geologists can date (typically with a certain margin of error) the past earthquakes, and from that data, develop an average earthquake recurrence interval for that fault segment.

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Table 6-1: Abridged Modified Mercalli Intensity Scale
2BIntensity Value and Description
Average Peak
Velocity
(cm/sec)
Average Peak
Acceleration
(g = gravity)
I. Not felt except by a very few under especially favorable circumstances (I Rossi-Forel scale). Damage potential: None. <0.1 <0.0017 II. Felt only by a few persons at rest, especially on upper floors of high-rise buildings. Delicately suspended objects may swing. (I to II Rossi-Forel scale).

Damage potential: None.

III. Felt quite noticeably indoors, especially on upper floors of buildings, but 0.1 – 1.1 0.0017 – 0.014 many people do not recognize it as an earthquake. Standing automobiles may rock slightly. Vibration like passing of truck. Duration estimated. (III Rossi-Forel scale). Damage potential: None.

IV. During the day felt indoors by many, outdoors by few. At night some awakened. Dishes, windows, doors disturbed; walls make creaking sound.

Sensation like a heavy truck striking building. Standing automobiles rocked noticeably. (IV to V Rossi-Forel scale). Damage potential: None. Perceived shaking: Light.

1.1 – 3.4 0.014 - 0.039
V. Felt by nearly everyone; many awakened. Some dishes, windows, and so on broken; cracked plaster in a few places; unstable objects overturned.

Disturbances of trees, poles, and other tall objects sometimes noticed.

Pendulum clocks may stop. (V to VI Rossi-Forel scale). Damage potential:

Very light. Perceived shaking: Moderate.

3.4 – 8.1 0.039-0.092
VI. Felt by all; many frightened and run outdoors. Some heavy furniture moved, few instances of fallen plaster and damaged chimneys. Damage slight. (VI to VII Rossi-Forel scale). Damage potential: Light. Perceived shaking: Strong.

8.1 - 16 0.092 -0.18
VII. Everybody runs outdoors. Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable in poorly built or badly designed structures; some chimneys broken. Noticed by persons driving cars. (VIII Rossi-Forel scale). Damage potential: Moderate.

Perceived shaking: Very strong.
16 - 31 0.18 - 0.34
VIII. Damage slight in specially designed structures; considerable in ordinary substantial buildings with partial collapse; great in poorly built structures.

Panel walls thrown out of frame structures. Fall of chimneys, factory stacks, columns, monuments, and walls. Heavy furniture overturned. Sand and mud ejected in small amounts. Changes in well water. Persons driving cars disturbed. (VIII+ to IX Rossi-Forel scale). Damage potential: Moderate to heavy. Perceived shaking: Severe.
31 - 60 0.34 - 0.65
IX. Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb; great in substantial buildings with partial collapse. Buildings shifted off foundations. Ground cracked conspicuously.

Underground pipes broken. (IX+ Rossi-Forel scale). Damage potential:

Heavy. Perceived shaking: Violent.
60 - 116 0.65 – 1.24
X. Some well-built wooden structures destroyed; most masonry and frame structures destroyed; ground badly cracked. Rails bent. Landslides considerable from river banks and steep slopes. Shifted sand and mud. Water splashed, slopped over banks. (X Rossi-Forel scale). Damage potential: Very heavy. Perceived shaking: Extreme.
> 116 > 1.24
XI. Few, if any, (masonry) structures remain standing. Bridges destroyed. Broad fissures in ground. Underground pipelines completely out of service. Earth slumps and land slips in soft ground. Rails bent greatly.

XII. Damage total. Waves seen on ground surface. Lines of sight and level distorted. Objects thrown into air.

Modified from Bolt (1999); Wald et al. (1999).

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In southern California, ruptures along thrust faults have built the Transverse Ranges geologic province, a region with an east-west trend to its landforms and underlying geologic structures (Figure 6-1). This orientation is anomalous, virtually unique in the western United States, and a direct consequence of the plates colliding at the Big Bend. Many of southern California’s most recent damaging earthquakes have occurred on thrust faults that are uplifting the Transverse Ranges, including the 1971 San Fernando, the 1987 Whittier Narrows, the 1991 Sierra Madre, and the 1994 Northridge earthquakes. In addition to generating stronger ground shaking than a similarmagnitude earthquake on a strike-slip fault, thrust faults are also particularly hazardous because many are blind, that is, they do not extend to the surface of the Earth. These blind thrust faults are extremely difficult to detect before they rupture. Both the 1987 Whittier Narrows and the 1994 Northridge earthquakes occurred on blind thrust faults.

CAUSES OF EARTHQUAKE DAMAGE
Causes of earthquake damage can be categorized into three general areas: strong shaking, various types of ground failure that are a result of shaking, and ground displacement along the rupturing fault.

Ground shaking is the motion felt on the earth's surface caused by seismic waves generated by the earthquake. It is the primary cause of earthquake damage, and is typically reported as the peak horizontal ground acceleration estimated as a percentage of g, the acceleration of gravity. Full characterization of shaking potential, though, requires estimates of peak (maximum) ground displacement and velocity, the duration of strong shaking, and the periods (lengths) of waves that will control each of these factors at a given location. The strength of ground shaking also depends on the source, path, and site effects. Estimates of the ground shaking that different locations in California are likely to experience have been mapped, as shown on Figure 6-3. This figure shows that only the easternmost portion of southern California and a portion of San Diego County may be beyond the reach of a damaging earthquake (green areas). • Source effects include earthquake size, location, and distance, plus directivity of the seismic waves (for example, the 1995, MW 6.9, Kobe, Japan earthquake was not much bigger than the 1994, MW 6.7 Northridge, California earthquake, but Kobe caused much worse damage. During the Kobe earthquake, the fault’s orientation and movement directed seismic waves into the city, whereas during the Northridge earthquake, the fault’s motion directed waves away from populous areas.
• Path effects refer to how the seismic waves change direction as they travel through the Earth’s contrasting layers, just as light bounces (reflects) and bends (refracts) as it moves from air to water. Sometimes seismic energy gets focused into one location and causes damage in unexpected areas (focusing of the seismic waves generated by the 1989 MW 7.1 Loma Prieta earthquake caused damage in San Francisco’s Marina district, some 100 km distant from the rupturing fault).
• Site effects refer to how seismic waves interact with the ground surface. Seismic waves slow down in the loose sediments and weathered rock at the Earth’s surface; as they slow, their energy converts from speed to amplitude, which heightens shaking (amplification).

Therefore, buildings on poorly consolidated and thick soils will typically see more damage than buildings on consolidated soils and bedrock. Amplification can also occur in areas on Local Hazard Mitigation Plan Section 6
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deep, sediment-filled basins and on ridge tops. Seismic waves can also get trapped at the surface and reverberate (resonate). Whether resonance will occur depends on the period (the length) of the incoming waves – long-period seismic waves, which are created by large earthquakes, are most likely to reverberate and cause damage in structures like bridges and high-rises, known as “long-period structures” for their response to these waves.

Shorter-period seismic waves, which tend to die out quickly, will most often cause damage fairly near the fault, and they will cause most damage in “shorter-period structures,” such as one- to three-story buildings. Very short-period waves are most likely to cause nearfault, interior damage, such as to equipment.

Figure 6-3: Earthquake Shaking Levels Anticipated in California Map shows the level of ground shaking with a 2 percent chance of exceedance in 50 years anticipated to occur throughout the state. The pink and red zones can experience higher ground shaking because they are closer to active faults.

The white star shows the approximate location of Santa Monica.

Source: Modified from Branum, D., Chen, R., Petersen, M., and Wills, C., 2016, Earthquake Shaking Potential for California: California Geological Survey Map Sheet 48; available in its entirety from https://www.conservation.ca.gov/cgs/documents/publications/map-sheets/MS_048.pdf.

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Earthquake damage also depends on the characteristics of human-made structures, with the interaction of ground motion with the built environment being complex. Governing factors include a structure’s height, construction, and stiffness, architectural design, condition, and age.

Liquefaction typically occurs within the upper 50 feet of the surface, where saturated, loose, fineto medium-grained soils (sand and silt) are present. Earthquake shaking suddenly increases pressure in the water that fills the pores between soil grains, causing the soil to lose strength and behave as a liquid. This process can be observed at the beach by standing on the wet sand near the surf zone. Standing still, the sand will support your weight. However, when you tap the sand with your feet, water comes to the surface, the sand liquefies, and your feet sink.

Liquefaction-related effects include loss of bearing strength, ground oscillations, lateral spreading and flow failures or slumping. The excess water pressure is relieved by the ejection of material upward through fissures and cracks. When soils liquefy, the structures built on them can sink, tilt, and suffer significant structural damage. Buildings and their occupants are at risk when the ground can no longer support the buildings.

Earthquake-induced landslides and rockfalls are secondary earthquake hazards that occur from ground shaking. Gravity inexorably pulls hillsides down, and earthquake shaking enhances this on-going process. Landslides and rockfalls can destroy roads, buildings, utilities, and other critical facilities necessary to respond and recover from an earthquake. Many communities in southern California with steep slopes have a high likelihood of being impacted by earthquake-induced landslides or rockfalls. Earthquake-induced slope failures are discussed in this section whereas landslide hazards are discussed in Section 9.

Primary ground rupture due to fault movement typically results in a relatively small percentage of the total damage in an earthquake, yet being too close to a rupturing fault can result in extensive damage. It is difficult, although not impossible, to safely reduce the effects of this hazard through building and foundation design. Therefore, the primary mitigation measure is to avoid active faults by setting structures back from the fault zone. Application of this measure is subject to the requirements of the Alquist-Priolo Earthquake Fault Zoning Act and guidelines established by the California Geological Survey. The Santa Monica fault has been mapped through the northern portion of Santa Monica, so this hazard is discussed further in this section.

HISTORY OF EARTHQUAKE EVENTS IN SOUTHERN CALIFORNIA
To better understand earthquake hazards, scientists study past earthquakes by looking at their records, and by studying the effects that past earthquakes had on the ground surface and the built environment. Historical earthquake records are either from the instrumental period (since about 1932, when the first seismographs were deployed), or pre-instrumental. In the absence of instrumentation, the detection and record of earthquakes are based on observations and felt reports, and are dependent upon population density and distribution.

Since California was sparsely populated in the 1800s, our record of pre-instrumental earthquakes is relatively incomplete. However, two very large earthquakes, the Fort Tejon in 1857 (M7.9) and the Owens Valley in 1872 (M7.6), are evidence of the tremendously damaging potential of earthquakes in southern California. More recently, two M7.3 earthquakes struck southern California, in Kern County (1952) and Landers (1992), and two M7.1 earthquakes struck the Local Hazard Mitigation Plan Section 6
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Mojave Desert (Hector Mine, in 1999 and Ridgecrest in 2019). The damage from all of these large earthquakes was limited because they occurred in sparsely populated areas. A similarly sized earthquake closer to southern California’s population centers has the potential to place millions of people at risk.

Since seismologists started recording and measuring earthquakes, there have been millions (if not billions) of recorded earthquakes in southern California. In fact, Ross et al. (2019) described a new earthquake detection technique that they applied to the 2008-2017 waveform archive for southern California and recognized approximately 1.81 million earthquakes during that ten-year time frame, amounting to ten times the number of earthquakes in the Southern California Seismic Network (SCSN) catalog that had been previously recognized for the same time period. That averages to about 495 earthquakes per day in the area, with an interval between earthquakes of 174 seconds.

The vast majority of these earthquakes are very small, of magnitude less than 1.0.

Table 6-2 lists the moderate to large historical earthquake events that have affected southern California, generally of magnitudes greater than about 4.5. Figure 6-4 shows the most significant historical earthquakes in the southern California region. Those earthquakes known or inferred to have been felt strongly in the Santa Monica area, or that led to the passage of important legislation, are discussed further in the sub sections below. For the known or inferred epicentral location of most of these earthquakes, refer to Figure 6-4.

Unnamed Earthquake of 1769
On July 28, 1769, the first recorded earthquake in southern California was noted by Spanish explorers traveling north with Gaspar de Portolá. At the time of the earthquake, the explorers were camped at the location of the present-day community of Olive, in the city of Orange, on the east bank of the Santa Ana River. Father Juan Crespo, who kept a daily account of the expedition, reported a strong mainshock followed by five days of moderate aftershocks. An estimated magnitude of at least 6.0 has been assigned to the main earthquake based on the explorers’ account (Teggart, 1911). The source for this earthquake is unknown, and still being debated by the paleoseismology community. Some researchers have suggested that this earthquake, of possible magnitude 7.3, may have caused coastal uplift in the northern Orange County region, with the causative fault being a blind thrust under the San Joaquin Hills (Grant et al., 2002). The nearby Elsinore and Newport-Inglewood faults have also been considered possible sources for the earthquake.

Wrightwood Earthquake of December 12, 1812
This large earthquake occurred on December 8, 1812 and was felt throughout southern California.

Based on accounts of damage recorded at missions in the earthquake-affected area, an estimated magnitude of 7.5 has been calculated for the event (Toppozada et al., 1981). Subsurface investigations and tree ring studies show that the earthquake likely ruptured the Mojave section of the San Andreas fault near Wrightwood, and may have been accompanied by a significant surface rupture between Cajon Pass and Tejon Pass (Jacoby et al., 1988). The worst damage caused by the earthquake occurred significantly west of the San Andreas fault at San Juan Capistrano Mission, where the roof of the church collapsed, killing 40 people. The earthquake also damaged walls and destroyed statues at San Gabriel Mission and damaged missions in the Santa Barbara area. Strong aftershocks caused earthquake-damaged buildings to collapse for several days after the mainshock.

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Table 6-2: Historical (1769 – March 2024) Earthquakes in the Southern California Region with Magnitudes > 4.5
1769 Orange County - Los Angeles Basin 1970 Lytle Creek, east of Mount Baldy 1800 San Diego Region 1971 San Fernando
1812 Wrightwood 1971 San Fernando aftershocks
1812 Santa Barbara Channel 1973 Point Mugu
1827 Los Angeles Region, offshore Malibu 1978 Offshore Santa Barbara 1852 Fort Tejon area, east of Lebec 1981 Offshore, Channel Islands Region 1855 Los Angeles Region 1986 North Palm Springs
1857 Great Fort Tejon Earthquake 1987 Whittier Narrows
1858 San Bernardino Region 1987 Whittier Narrows aftershock 1862 San Diego Region 1988 Between Lebec and Tehachapi
1880 Moreno Valley – Perris Region 1990 Claremont area
1883 West of Frazier Park 1991 North of Pasadena
1889 Mountains north of Morongo Valley 1992 Landers
1892 San Jacinto or Elsinore Fault 1992 Landers aftershocks 1893 Pico Canyon 1992 Big Bear
1894 Lytle Creek Region 1994 Northridge
1894 San Diego Region 1994 Northridge aftershocks
1899 Lytle Creek region 1997 West of Santa Clarita
1899 San Jacinto and Hemet 1999 Hector Mine
1899 San Jacinto and Hemet aftershocks 1999 Hector Mine aftershocks 1905 San Bernardino Region 2001 Anza
1907 San Bernardino Region 2002 Laguna Salada
1910 Glen Ivy Hot Springs 2002 Yorba Linda
1912 Offshore, west of Malibu 2002 Big Bear
1916 Tejon Pass Region 2004 Offshore, south of Coronado 1918 San Jacinto 2004 Arvin
1923 San Bernardino Region 2005 Wheeler Ridge
1925 Santa Barbara 2005 Southeast of Anza
1925 Santa Barbara aftershocks 2005 Yucaipa
1926 Mountains north of Carpinteria 2005 South shores of Salton Sea (Obsidian Butte) 1930 Offshore Malibu 2008 Chino Hills
1930 Seven Oaks Dam area, two events 2009 Bombay Beach swarm 1933 Long Beach 2009 Inglewood
1933 Long Beach aftershocks 2009 Northern Baja California 1935 Mountains north of Morongo Valley 2010 Baja California, South of Mexicali 1938 Santa Ana Mountains 2010 Baja California aftershocks 1940 Mountains north of Northridge 2010 Northwest of Borrego Springs 1941 Carpinteria and Santa Barbara 2012 North of Brawley 1941 Lebec 2012 North of Brawley
1941 Wilmington 2014 La Habra Heights
1943 Big Bear Lake Region 2019 Ridgecrest, two main shocks 1944 Desert Hot Springs – Cabazon Region 2019 Ridgecrest, several aftershocks 1947 Desert Hot Springs – Yucca Valley Region 2019 Searles Valley, several aftershocks 1951 San Clemente Island 2020 Ridgecrest
1952 Tehachapi, Kern County Region 2020 Lone Pine
1952 Tehachapi aftershocks 2021 Imperial County
1954 West of Wheeler Ridge 2023 Ojai
1968 Near Santa Cruz Island 2024 Thousand Oaks
1969 Offshore San Nicolas Island 2024 Malibu, several minor to moderate shocks Local Hazard Mitigation Plan Section 6
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Unnamed Earthquake of December 21, 1812
The Wrightwood earthquake was followed by a strong earthquake on December 21st that caused widespread damage in the Santa Barbara area. The effects of this second earthquake are sometimes attributed to the December 12th event, giving the impression that a single large earthquake caused significant damage from Santa Barbara to San Diego. This second earthquake had an estimated magnitude 7 and was either located offshore, within the Santa Barbara Channel, or it may have occurred inland, in Santa Barbara or Ventura counties. The earthquake destroyed the church at the Mission in Santa Barbara, the Mission of Purísima Concepción near present-day Lompoc, and the Mission at Santa Inéz. The earthquake also caused a tsunami that may have traveled up to 1/2 mile inland near Santa Barbara.

Unnamed Earthquake of 1855
This earthquake occurred on July 11, 1855 and was felt across southern California from Santa Barbara to San Bernardino. Light to moderate damage was reported in the Los Angeles area, where 26 houses experienced cracked walls and the bell tower of the San Gabriel Mission was knocked down. Because damage was limited primarily to the Los Angeles area, this earthquake is postulated to have occurred on a local fault such as the Hollywood-Raymond, Whittier or Newport-Inglewood faults, or on one of the many blind thrust faults that underlie this area.

Santa Monica Earthquake of 1930
This ML (Richter magnitude) 5.2 earthquake occurred in Santa Monica Bay on August 31, 1930, at 4:40 P.M. PST0F
1 . Using station delays calculated from the 1979 Malibu earthquake (ML 5.0), Hauksson and Saldivar (1986) revised the location of the 1930 earthquake to near the western end of the Santa Monica fault, approximately 1 mile offshore of the eastern Malibu coast. The earthquake produced moderate to strong shaking (Modified Mercalli Intensities of V-VI) at some locations in the Los Angeles basin and San Fernando Valley (Gutenberg et al., 1932). Unusually high intensities were also recorded to the east, in Riverside (MMI V), San Jacinto (MMI IV), and Hemet (MMI IV), located approximately 100 miles away from the epicenter (Gutenberg et al., 1932; Hauksson and Saldivar, 1986).

Long Beach Earthquake of 1933
This Mw 6.4 earthquake occurred on March 10, 1933, at 5:54 P.M. PST, following a strong foreshock the day before. The earthquake’s epicenter was offshore of Newport Beach, approximately 3 miles south of present-day Huntington Beach, on the Newport-Inglewood fault (see Figure 6.4). However, because extensive damage was reported in Long Beach, the earthquake is referred to as the Long Beach earthquake. The earthquake caused 120 deaths, and over $50 million (~$1.20 billion in 2024) in property damage (Wood, 1933). Many strong aftershocks occurred through March 16th.
1 PST stands for Pacific Standard Time, and it is the North American time zone in the western United States, including California, that is eight hours behind Coordinated Universal Time (UTC). Since 2006, PST starts on the first Sunday in November and extends to the second Sunday in March. Pacific Daylight Time (PDT), which is seven hours behind UTC (UTC – 7:00), is used during the remainder of the year. Between 1967 and 2006, PST changed to PDT on the first Sunday in April and reversed back to PST on the last Sunday in October. PDT was first used in the U.S. in 1918, during the first world war, and occasionally, in a hodge-podge manner. California has used PDT since 1954. Earthquake catalogs typically report earthquakes in UTC time to avoid confusion caused by local time zones.

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Although primary ground rupture of the Newport-Inglewood fault was not observed, secondary cracking, minor slumping, and lateral movement of unconsolidated sediments occurred throughout the region. Road surfaces along the shore between Long Beach and Newport Beach were damaged by settlement of road fills that had been placed on marshy land. In urban areas, unreinforced masonry buildings were most severely damaged, especially in areas of artificial fill or water-saturated alluvium. In one part of Compton, most buildings built on unconsolidated sediments and artificial fill were destroyed. In Long Beach, many buildings collapsed, were pushed off their foundations, or had walls or chimneys knocked down. In Newport Beach, 800 chimneys were knocked down at the roofline and hundreds of houses were destroyed. As a result, building codes were improved. Damage to school buildings was especially severe; fortunately, children were not present in the classrooms at that time, otherwise, the death toll would have been much higher. This earthquake led to the passage of the Field and Riley Acts by the State legislature. The Field Act regulates school construction, and gives the Division of the State Architect authority and responsibility for approving the design and supervising the construction of public schools. The Riley Act regulates the construction of buildings larger than two-family dwellings.

Torrance-Gardena Earthquakes of 1941
In 1941, two small earthquakes struck the southern Los Angeles basin, affecting surrounding communities. Although these earthquakes were relatively minor, they occurred close to the surface and caused significant, although localized damage. The first Torrance earthquake (with a magnitude of 4.7) occurred on October 21st at 10:57 P.M. PST, and was located east of Carson, near the present-day interchange of the 405 and 710 freeways. Shaking up to intensity level VII was reported in the communities of Wilmington, Gardena, Lynwood, Hynes and Signal Hill where walls were cracked and chimneys damaged. In some cases, houses that had not been adequately repaired after the 1933 Long Beach earthquake were damaged again. No injuries were reported and damage estimates totaled $100,000 (~$2.1 million in 2024).

A second earthquake (of magnitude 5.1) occurred less than a month later, on November 14 at 12:42 A.M. PST, near Wilmington. Shaking during the second earthquake was reportedly stronger than the first, locally reaching intensity level VIII (Table 6-1) and felt as far away as Cabazon, Carpinteria, and San Diego. Gas and water mains burst near the epicenter and storefronts in the business districts of Torrance and Gardena collapsed, crushing parked cars. Damage to local oilfields was significant –- well casings and equipment were damaged and a 55,000-gallon oil tank ruptured, flooding nearby streets with oil. Production of several wells was lowered or stopped. No injuries were reported, although damage attributed to the second event was estimated at $1 million (~$21.1 million in 2024).

San Fernando (Sylmar) Earthquake of 1971
This Mw 6.6 earthquake occurred on the San Fernando fault zone, the western-most segment of the Sierra Madre fault, on February 9, 1971, at 6:00 A.M. PST. The surface rupture caused by this earthquake was nearly 12 miles long, and occurred in the Sylmar-San Fernando area, approximately 24 miles (38 km) northwest of Santa Monica. The maximum slip measured at the surface was nearly 6 feet.

The earthquake caused over $38.3 million in property damage (~$500 million in 2024) and 65 deaths. Most of the deaths occurred when the Veteran's Administration Hospital in Sylmar collapsed. Several other hospitals, including the Olive View Community Hospital in Sylmar suffered severe damage. Newly constructed freeway overpasses also collapsed, in damage scenes Local Hazard Mitigation Plan Section 6
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similar to those that occurred 23 years later during the 1994 Northridge earthquake. Loss of life could have been much greater had the earthquake struck at a busier time of day. As with the Long Beach earthquake, legislation was passed in response to the damage caused by the 1971 earthquake. In this case, the building codes were strengthened and the Alquist Priolo Special Studies (now known as the Earthquake Fault Zone) Act was passed in 1972.

Point Mugu Earthquake of 1973
This magnitude 5.3 earthquake occurred on the Malibu Coast fault, the westernmost segment of the Malibu Coast-Santa Monica-Hollywood fault zone, on February 1, 1973, at 6:46 A.M. PST. The Point Mugu earthquake was responsible for at least five injuries and more than $1 million in damage ($7 million in 2024 dollars) in the Point Mugu-Oxnard area, although damage was mainly confined to the epicentral area (https://scedc.caltech.edu/earthquake/pointmugu1973.html). Large boulders fell down onto Highway 1 at Point Mugu, partially blocking the road. Over 7,000 customers were without electricity for hours. Most damage reported was to windows, ceilings, plaster, chimneys and shelved goods, but structural damage and broken pipes were also reported.

Malibu Earthquake of 1979
This magnitude 5.2 earthquake occurred on January 1, 1979, at 3:15 P.M. PST, along the AnacapaDume fault. The Anacapa-Dume fault is located approximately 3.2 km (2 mi) offshore the Malibu coast, and it is separated from the Santa Monica fault by a 5- to 6-mi (8- to 9.7 km) north-south sidestep (Hauksson and Saldivar, 1986). The 1979 Malibu earthquake caused only minor damage in the areas closest to the epicenter, although it was felt as far away as Kings, Kern, and San Diego counties (https://scedc.caltech.edu/earthquake/malibu1979.html). Stover et al. (1980) reported intensities of VI (along the Malibu coast and at several sites in the San Fernando Valley. Moderate to strong shaking (MMIs of V-VI) was felt in the Santa Monica area.

Whittier Narrows Earthquake of 1987
The Whittier Narrows earthquake occurred on October 1, 1987, at 7:42 P.M. PST, (Hauksson and Jones, 1989), with its epicenter located approximately 20.5 miles (33 km) to the east of Santa Monica. This magnitude 5.9 earthquake occurred on a previously unknown, north-dipping concealed thrust fault (blind thrust) now called the Puente Hills fault (Shaw and Shearer, 1999).

Modified Mercalli intensities in the VII to VIII range were reported from Monrovia and Pasadena in the north, to beyond Whittier in the southeast. Modified Mercalli intensities in the VI range were reported farther out, in an approximately 580-square-mile area.

The earthquake caused eight fatalities, over 900 injured, and $358 million (~$978 million in 2024) in property damage. Severe damage was confined mainly to communities east of Los Angeles and near the epicenter. Areas with high concentrations of unreinforced masonry (URM) buildings, such as the “uptown” district of Whittier, the old downtown section of Alhambra, and the “Old Town” section of Pasadena, were severely impacted. Several tilt-up buildings partially collapsed, including tilt-up buildings built after 1971, that were built to meet improved building standards, but were of irregular configuration, revealing seismic vulnerabilities not previously recognized.

Residences that sustained damage usually were constructed of masonry, were not fully anchored to their foundations, or were houses built over garages with large openings. Many chimneys collapsed, and in some cases, fell through roofs. Wood-frame residences, in contrast, sustained relatively little damage, and no severe structural damage was reported to high-rise structures in downtown Los Angeles.

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Malibu Earthquake of 1989
This magnitude 5.0 earthquake occurred on January 18, 1989, at 10:53 P.M. PST. This earthquake is spatially associated with the Palos Verdes-Shelf Projection anticlinorium, a fold broadly located offshore in the Santa Monica Bay (Sorlien et al., 2004). As a result of the shaking caused by this earthquake, several people were injured, shelved items fell in local stores, and some windows broke. Hardest hit was the coastal region encompassing Malibu, Santa Monica, and Redondo Beach, although damage was low even in that area
(https://scedc.caltech.edu/earthquake/chronological.html). Slight damage was also reported in Los Angeles, Hollywood, Monterey Park, and as far away as Lancaster.

Landers and Big Bear Earthquakes of 1992
On the morning of June 28, 1992, most people in southern California were awakened at 4:57 A.M. by the largest earthquake to strike California in 40 years. Named “Landers” after a small desert community near its epicenter, the earthquake had a magnitude of 7.3. Centered in the Mojave Desert, approximately 120 miles from Los Angeles, the earthquake caused relatively little damage for its size (Brewer, 1992). It released about four times as much energy as the very destructive Loma Prieta earthquake of 1989, but fortunately, it did not claim as many lives (one child died when a chimney collapsed). The power of the earthquake was illustrated by the length of the ground rupture it left behind. More than 50 miles of surface rupture occurred as a result of this earthquake. The earthquake ruptured five separate faults: Johnson Valley, Landers, Homestead Valley, Emerson, and Camp Rock faults (Sieh et al., 1993). Other nearby faults also experienced triggered slip and minor surface rupture. The average right-lateral strike-slip displacement was about 10 to 15 feet, with a maximum of up to 18 feet observed. Modified Mercalli Intensities of between V and VI were retroactively reported to the U.S. Geological Survey’s “Did You Feel It?” application by Santa Monica residents as a result of this earthquake.

The magnitude 6.4 Big Bear earthquake struck little more than 3 hours after the Landers earthquake on June 28, 1992 at 8:05:30 A.M. PST. This earthquake is technically considered an aftershock of the Landers earthquake (indeed, the largest aftershock), although the Big Bear earthquake occurred over 20 miles west of the Landers rupture, on a fault with a different orientation and sense of slip than those involved in the main shock. From its aftershock, the causative fault was determined to be a northeast-trending left-lateral fault. The Big Bear earthquake did not break the ground surface, and, in fact, no surface trace of a fault with the proper orientation has been found in the area. The Big Bear earthquake caused a substantial amount of damage in the Big Bear area, but fortunately, it claimed no lives. However, landslides triggered by the quake blocked roads in the mountainous areas, aggravating the clean-up and rebuilding process.

Northridge Earthquake of 1994
The Northridge Earthquake of January 17, 1994 woke up most of southern California at 4:30 in the morning. The earthquake’s epicenter was located 20 miles to the west-northwest of downtown Los Angeles, on a previously unknown blind thrust fault now called the Northridge Thrust. Although moderate in size, this earthquake produced the still-strongest ground motions (1.2g horizontal and

1.0g vertical) instrumentally recorded in an urban setting in the United States (https://scedc.caltech.edu/earthquake/northridge1994.html).

The Mw 6.7 earthquake is one of the most expensive natural disasters to have impacted the United States, with $20 billion ($41.9 billion in 2024 dollars) in property damage and more than $40 Local Hazard Mitigation Plan Section 6
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billion ($83.8 billion in 2024) in economic losses. Damage was widespread, sections of major freeways, parking structures and office buildings collapsed, and numerous apartment buildings suffered irreparable damage. Damage to wood-frame apartment houses was very widespread in the San Fernando Valley and Santa Monica areas, especially to structures with "soft" first floor or lower-level parking garages. The high accelerations, both vertical and horizontal, lifted structures off of their foundations and/or shifted walls laterally. Up to 72 people died, and more than 1,500 people were seriously injured.

The Santa Monica area was hit particularly hard, in part because the seismic waves were focused along and bounced back from shallow sections of bedrock along the Santa Monica fault (Bausch and Gonzalez, 1994; Graves et al., 1998; Davis et al., 2000). The earthquake affected almost every building on the Santa Monica College campus, with extensive damage also reported at the Santa Monica Hospital Medical Center, Saint John’s Hospital, and St. Monica’s Catholic Church.

Both hospitals experienced significant damage, with sections of the buildings closed for months for repairs. The north wing of Saint John’s had to be demolished. More than 1,600 housing units were damaged, and a section of the I-10 freeway across Centinela Boulevard collapsed, making travel into and out of the city difficult. Following the earthquake, Santa Monica issued more than 3,400 repair permits for earthquake-damaged properties, and fifty-three (53) buildings were demolished because they were determined to be damaged too extensively. Despite the losses, gains made through earthquake hazard mitigation efforts implemented during the previous two decades, including retrofits of masonry buildings, helped reduce the loss of life. Strengthening of hospitals following the 1971 San Fernando earthquake also helped.

Hector Mine Earthquake of 1999
Widely felt magnitude 7.1 event that occurred on October 18, 1999 in a remote region of the Mojave Desert, 47 miles east-southeast of Barstow. The Hector Mine earthquake occurred on a series of north-northwest trending strike-slip faults within the Eastern Mojave Shear Zone.

Geologists documented a 25-mile (40-km) long surface rupture and a maximum right-lateral strikeslip offset of about 16 feet on the Lavic Lake fault. Surface rupture was confined entirely within the Twentynine Palms Marine Corps Base, and did not extend across any paved roads or buildings. As a result, the earthquake caused very little damage to structures or infrastructure; the most significant damage reported was the derailment of an Amtrak train.

Baja California Earthquake of 2010
A magnitude 7.2 earthquake that occurred just south of the U.S. / Mexico border on Easter Sunday, April 4, 2010, at 3:40:42 PM PST, was felt throughout Mexico, southern California, Arizona, and Nevada. Based on analysis of the waveforms, seismologists identified two sub-events: one rupturing an 18-km section of the Pescadores fault, followed, six to 12 seconds later by a second, larger event on the Borrego fault. Both of these faults are part of the Laguna Salada fault system, which is the southern extension of the Elsinore fault. Surface rupture continued northward to just past the border into California. The main earthquake caused triggered slip of up to a few centimeters on several faults in the Salton Sea area, and as far north as in the Mecca Hills.

Secondary effects, including liquefaction, rockfalls and shattering were reported along a wide area in the El Centro and Brawley region, and westward toward San Diego. More than 4,000 aftershocks were recorded within the first ten days after the main shock, and more than 9,500 were recorded in the two months after the main event (Kroll et al., 2013). A peak instrumental ground acceleration of 1.1g was recorded at the Salton Sea. Similar or stronger shaking may have occurred closer to the epicenter, but given the lack of instrumentation in that area, went Local Hazard Mitigation Plan Section 6
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unrecorded. Many of the aftershocks occurred along the Elsinore, San Jacinto, and the southern extension of the San Andreas fault through the Brawley area. Based on observations reported by many residents, shaking in the Santa Monica area as a result of this earthquake was light, in the Modified Mercalli intensity III range
(https://earthquake.usgs.gov/earthquakes/eventpage/ci14607652/map).

Ridgecrest Earthquake Sequence of 2019
The Ridgecrest sequence in eastern California included two moderate (Mw>6) earthquakes and thousands of relatively small (Mw<6) earthquakes on a previously unnoticed section of the Little Lake Fault Zone southwest of Searles Valley. The first moderate earthquake (Mw6.4) occurred on July 4th, 2019, at 10:33 A.M. PDT, whereas the second moderate earthquake (Mw7.1) occurred on July 5th, 2019, at 8:19 P.M. PDT. The damage was most severe close to the epicenter, near the town of Ridgecrest, where buildings, roads, and public utility lines were damaged. Shaking was widely felt throughout California, including light to moderate ground shaking in Los Angeles, weak shaking in the San Francisco Bay Area, and some minor but noticeable shaking as far east as Phoenix, Arizona and as far north as Sacramento. In the Santa Monica area, shaking felt on July 5th as a result of the Mw 7.1 Ridgecrest earthquake resulted in MMI levels between III and IV (https://earthquake.usgs.gov/earthquakes/eventpage/ci38457511/dyfi/intensity).

Ojai Earthquake of 2023
This M 5.1 earthquake occurred on August 20, 2023 at about 2:45 PM local time, the same day coastal southern California was being battered by the remains of Tropical Storm Hilary. The epicenter was approximately 4 miles southeast of Ojai, in Ventura County, and was felt throughout the region. Shaking near the epicenter was reported in the MMI levels of between V and VI. In Santa Monica and greater Los Angeles region, shaking was reported in the MMI range of III to IV (https://earthquake.usgs.gov/earthquakes/eventpage/ci39645386/dyfi/intensity). The main quake was preceded by several foreshocks, and aftershocks were felt several days after.

EARTHQUAKE HAZARD ASSESSMENT
Choosing Earthquakes for Planning and Design
It is often useful to create a design earthquake scenario to study the effects of a particular earthquake on a building or a community. Typically, such scenarios have considered the largest earthquake believed possible to occur on a fault or fault segment, referred to as the maximum magnitude earthquake (Mmax). Building codes usually consider other scenarios for the design of structures, using the ground motion with a statistical probability of being exceeded in a given length of time, with different earthquake scenarios considered depending on the application, such as the planned use, lifetime, or importance of a facility. Traditionally, the more critical the structure, the longer the time period used between earthquakes and the larger the design earthquake that has been used. Seismic design parameters in the most recent version of the California Building Code (2023 edition) are based on the risk-targeted maximum considered earthquake, with a ground motion that has a 2 percent probability of being exceeded in 50 years and a recurrence interval of about 2,500 years, with an adjustment for risk. Risk is defined as the probability that damage will occur to the proposed structure. Seismic design parameters define what kinds of earthquake effects a structure must be able to withstand. These include peak ground acceleration, duration of strong shaking, the periods of incoming strong motion waves, and the orientation of maximum response of the earthquake’s motion.

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Geologists, seismologists, engineers, emergency response personnel and urban planners typically have used maximum magnitude and maximum considered earthquakes to evaluate the seismic hazard of a site or area. The assumption is that if we plan for the worst-case scenario, we establish safety margins. As a result, smaller earthquakes, which are more likely to occur, can be dealt with effectively.

As is true for most of the Los Angeles Basin, many potential earthquake sources pose a threat to Santa Monica (see Table 6-1). Thus, it is also important to consider the overall likelihood of damage from a plausible suite of earthquakes. This approach is called probabilistic seismic hazard analysis (PSHA), and typically considers the likelihood of exceeding a certain level of damaging ground motion that could be produced by any or all faults within a 100-km (62-mile) radius of the project site, or in this case, the city of Santa Monica.

In California, many agencies are focused on seismic safety issues, including the California Geological Survey (CGS), the State’s Seismic Safety Commission (SSC), the Governor’s Office of Emergency Services (OES), the Applied Technology Council (ATC), and the California Institute of Technology (Cal Tech). A number of other universities and private foundations also invest time and resources in seismic studies and seismic hazard mitigation. Many of these organizations, often in partnership with other State and Federal agencies such as the U.S. Geological Survey and FEMA, have undertaken a rigorous program in California to identify seismic hazards and risks, including active fault identification, ground shaking, ground motion amplification, liquefaction, earthquake induced landslides, and for coastal areas, tsunami inundation zones. Seismic hazard maps that identify areas thought to be susceptible to earthquake-induced fault rupture, liquefaction, slope failures, and tsunamis, have been published and are available for many communities in California through the California Geological Survey. This report refers to several of these maps to discuss the most significant earthquake-induced hazards with the potential to impact the city of Santa Monica. These hazards are described below and in other pertinent sections of this document.

Seismic Shaking
Seismic shaking is the seismic hazard that has the greatest potential to severely impact Santa Monica given the city’s proximity to several active seismic sources (faults). To give the City a better understanding of the hazard posed by these faults, we compiled, using a series of ShakeMaps prepared by the U.S. Geological Survey (USGS) Earthquake Hazards Program, the peak horizontal ground accelerations (PHGA) and Modified Mercalli intensities (MMIs) anticipated in the city as a result of an earthquake on several of the known Holocene-active faults within about 62 miles (100 km) from Santa Monica. The ShakeMap scenarios use realistic assumptions about the size and location of a future earthquake to estimate its impact in the epicentral area; this information is in turn used for loss estimations, preparedness exercises and disaster planning. The distances between the faults and the city were measured using the Community Fault Model (CFM, version

6.0) by the Southern California Earthquake Center (SCEC; Marshall et al., 2022).

Peak ground acceleration (ground shaking) depends on the size of the earthquake (which is in turn dependent in the dimensions of the ruptured fault plane), the proximity of the rupturing fault, and local soil conditions. Effects of soil conditions are estimated using attenuation relationships developed from empirical data. Specifically, to develop these relationships, scientists analyze recordings of earthquake shaking on similar soils during earthquakes of various sizes and distances. The peak ground acceleration estimates obtained from these analyses can be then used Local Hazard Mitigation Plan Section 6
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to provide a general indication of relative earthquake risk at a given site. For individual projects however, site-specific analyses that consider the precise distance from a given site to the various faults in the region, as well as the local near-surface soil types, should be conducted.

The PGAs and intensities listed on Table 6-3 below should be considered as approximate values, as different areas of Santa Monica are expected to feel and respond to each earthquake differently in response to site-specific conditions. These conditions may include the geometry of the basin and the possible reflection, and amplification, of the seismic waves off the harder rock forming the Santa Monica Mountains to the north. The models may not consider some of these local conditions, and as a result, localized higher accelerations than those listed here are possible.

Moreover, please note that the PGA values listed on Table 6-3 provide a general indication of relative earthquake risk throughout the Santa Monica area, but these values are not to be used for building design purposes.

Table 6-3 shows:
• The closest approximate distance, in miles and kilometers, between each of the faults listed and various points in the Santa Monica region, given as a range. Since these measurements are based on specific, but randomly selected points in the study area; other points in the city could be closer or farther away from the faults than the distances provided herein; • the maximum magnitude earthquake (Mmax) each fault is estimated capable of generating; • the range in peak ground accelerations (PGAs) expressed as a fraction of the acceleration of gravity (g), that could be experienced in the Santa Monica area if the Mmax occurs on one of these faults (values given range from the median to median plus 1 sigma standard deviation); and
• the range in Modified Mercalli seismic Intensity (MMI) values estimated to be felt in the city as a result of the Mmax on each one of these faults.

Those faults that can cause peak horizontal ground accelerations of about 0.1g or greater (Modified Mercalli Intensities greater than VII) in the Santa Monica area are listed in Table 6-3. For a map showing most of these faults, refer to Figures 6-1 and 6-4. Those faults included in Table 6-3 that would have the greatest impact on the Santa Monica area, or that are thought to have a higher probability of causing an earthquake, are described in more detail in the following pages.

The PGA and MMI values extracted from USGS ShakeMaps indicate that many nearby faults, such as the Santa Monica, Malibu Coast, Compton blind thrust, Hollywood, Palos Verdes, NewportInglewood, Anacapa-Dume, and Puente Hills blind thrust, have the potential to generate severe to extreme ground shaking in the city of Santa Monica. The worst-case scenario event would likely result from rupture of the Santa Monica fault together with the Malibu Coast fault to the west and the Hollywood fault to the east in an estimated Mw 7.4 earthquake. This potential scenario would create extreme perceived shaking in the Santa Monica area, with potential PGA values ranging from about 0.3g to more than 0.75g (compare Tables 6-1 and 6-3). Shaking at these levels can cause significant damage to older structures, and moderate damage to even newer buildings constructed in accordance with the latest building code provisions.

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Table 6-3: Estimated Horizontal Peak Ground Accelerations and Seismic Intensities in the Santa Monica Area
0BFault Name or Fault Zone
Distance to
Santa Monica
β (km)
Distance to
Santa Monica
β (mi)
Magnitude
of Mmax*
PGA (g)
from Mmax*
MMI
from Mmax*
Santa Monica rupturing together with
the Malibu Coast and Hollywood faults 0 – 12 0 – 7.5 7.4 0.75+ – 0.3 VIII+
Santa Monica 0 – 4 0 – 2.5 6.8 0.65 – 0.2 VIII – VII
Malibu Coast 2 – 10 1 – 6 7.0 0.65 – 0.3 VIII – VII+
Anacapa-Dume 3.5 – 10.5 2 – 6.5 7.2 0.75+
– 0.2 VIII+ – VII
Newport-Inglewood (Onshore) 5 – 16 3 - 10 7.2 0.75+
– 0.2 VIII+ – VII
Hollywood 7 – 12 4 – 7.5 6.7 0.35 – 0.2 VII – VI
Compton Blind Thrust 11 - 18 6.8 - 11 7.5 0.63+ VIII+ – VIII Palos Verdes 11 – 23 7 – 14.5 7.7 0.75+
– 0.2 VIII+ – VII
Upper Elysian Park Thrust 18 – 24 11 – 15 6.7 0.35+
– 0.1 VII+ – VI
Puente Hills (Los Angeles segment) 11 – 19 7 – 12 7.0 0.35+ – 0.1 VII+ – VI
Puente Hills (Santa Fe Springs segment) 31 – 39 19.5 – 24 6.7 0.2+ – 0.1 VI+
Puente Hills (Coyote Hills segment) 40 – 47 25 – 29 6.9 0.3 – 0.1 VII – VI Raymond 21 – 27 13 – 17 6.8 0.2+
– 0.1 VI+
Verdugo 23 – 29 14 – 18 6.9 0.2+
– 0.1 VI+
Sierra Madre 37 – 43 23 – 26 7.2 0.2+
– 0.1 VI+
San Andreas;
Shakeout2 Full Scenario 67 – 104 41 – 65 7.8 0.2 – 0.05 VI – V San Jacinto fault rupture consisting of
San Bernardino + San Jacinto Valley
+ Anza + Clark Segments
101 – 108 63 – 67 7.7 0.2 – 0.05+ VI – V+
Notes and Abbreviations used in Table 6-3:
β Distances measured from the Community Fault Model (CFM, version 6.0) by the Southern California Earthquake Center (SCEC; Marshall et al., 2022). For the blind faults, the distances indicated are to the surface projection of the fault ramp and/or, in the case of the Compton fault, to the fault at depth under Santa Monica. * Data from USGS ShakeOut Scenarios (https://earthquake.usgs.gov/scenarios/catalog/sclegacy/). The peak ground accelerations and MMI values are listed from high to low in an effort to show that locations closer to the fault will experience the higher peak ground accelerations and stronger shaking. mi – miles; km – kilometer; Mmax – magnitude of the largest earthquake the fault is thought capable of generating, based on its length; PGA – peak ground acceleration as a percentage of g, the acceleration of gravity; MMI – Modified Mercalli Intensity.

The ground motions presented in Table 6-3 are based on the largest earthquake that each fault, or fault segment, is believed capable of generating, referred to as the maximum magnitude earthquake (Mmax). This deterministic approach is useful to study the effects of a particular earthquake on a building or community. However, because many potential earthquake sources pose a hazard to the region, it is also important to consider the overall likelihood of damage from a plausible suite of earthquakes, including earthquakes of different sizes on the same fault. This approach is called probabilistic seismic hazard analysis (PSHA), and typically considers the Local Hazard Mitigation Plan Section 6
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likelihood of exceeding a certain level of damaging ground motion that could be produced by any or all faults within a given radius of the study area, which in this case is the city of Santa Monica.

Most seismic hazard analyses consider a distance of 100 km (62 miles), but this is arbitrary.

PSHA is used by the USGS to produce national and state seismic hazard maps for risk assessment, such as those used by the Uniform Building Code and the California Building Code. Specifically, the Unified Hazard Tool is a PSHA platform developed and maintained by the National Seismic Hazard Mapping Project (NSHMP) within the USGS earthquake hazards program that computes the statistical likelihood of an earthquake occurrence in terms of PGA (https://earthquake.usgs.gov/hazards/interactive/). This analysis differs from ShakeMaps because the computed PGA is based on the combined effects of multiple seismic sources.

The Unified Hazard Tool using the “Dynamic: Conterminous U.S. 2014 (update) (v.4.2.0)” edition was utilized to estimate the ground motions in the city of Santa Monica that have a 10% probability of exceedance in 50 years (equivalent to a return period of ∼475 years), and a 2% probability of exceedance in 50 years (equivalent to a return period of ∼2,475 years1F 2 ). Site class
“C/D boundary” for soils with a shear wave velocity in the upper 100 feet (30 m) of approximately 1,200 feet per second (equivalent to 365 meters per second [m/sec]) was selected for the analysis based on data obtained from the CGS2F
3 . The Unified Hazard Tool estimated levels of ground
motion for Santa Monica with a 10% and a 2% probability of being exceeded in 50 years of approximately 0.51g and 0.93g, respectively. The principal sources responsible for these levels of shaking are the Palos Verdes fault, the various faults of the Malibu Coast-Santa Monica-Hollywood fault system, the Newport-Inglewood fault zone, and the Compton blind thrust. These levels of shaking correspond to Modified Mercalli Intensity values of VIII to IX, and can be expected to cause significant damage, particularly to older and poorly constructed buildings. The maximum considered earthquake geometric mean PGA adjusted for site effects using the ASCE Tool 7-223F 4 ,
and a stiff soil (soil class C/D; for soils with shear wave velocities of 365 m/sec), returned a value for Santa Monica of 0.91g, nearly the same as the 2% in 50-years PGA value calculated using the Unified Hazard Tool.

Seismic Shaking Sources
Santa Monica Fault
The Santa Monica segment is a north-dipping, high angle fault that extends for a distance of approximately 25 miles (40 km) through the communities of Beverly Hills, Westwood, Santa Monica, and Pacific Palisades, before going offshore parallel to the Malibu coastline. The Santa Monica fault is one segment of a much longer system of faults, referred to as the Southern Frontal Fault System, that forms the southern boundary of the Transverse Ranges geomorphic province.

From west to east these faults include the Malibu, Anacapa–Dume, Santa Monica, Hollywood and Raymond (Figure 6-1). These individual fault segments are discussed in the following subsections.

Specifics regarding the Santa Monica fault as it pertains to its location through the city of Santa Monica and its hazard of surface fault rupture are also discussed later in this section.

2 Seismic design parameters in the 2022 California Building Code are based on the maximum considered earthquake, which is an earthquake that is expected to occur once in approximately 2,475 years. 3 Based on California Geological Survey (CGS) Map Sheet 48: Shear-wave Velocity in Upper 30m of Surficial Geology (Vs30) available from https://gis.data.ca.gov/datasets/cadoc::cgs-map-sheet-48-shear-wave-velocity-inupper-30m-of-surficial-geology-vs30/explore?location=34.027555%2C-118.483092%2C13.66, accessed on October 26, 2023. 4
https://asce7hazardtool.online/; accessed on October 26, 2023 and March 18, 2024.

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A limited paleoseismic study of the Santa Monica fault at the West Los Angeles Veteran’s Health Administration (VA) Medical Center just one mile east of Santa Monica suggests that the fault has experienced two to three surface-rupturing events in the last 16,000 to 17,000 years (the most recent event probably occurred between 1,000 and 3,000 years ago), and at least six rupture events in the past about 50,000 years (Dolan and others, 2000c). Based on these data, the rate of slip on the Santa Monica fault is poorly constrained, but is estimated at between 0.5 and 1.5 mm/yr (Dolan and others, 1995; Dolan and others, 2000c).

Given its length, rupture on the Santa Monica fault on its own is thought capable of generating a magnitude 6.8 earthquake. Such an earthquake would result in PGAs of between about 0.2g and

0.65g in Santa Monica (Table 6-3). If the Santa Monica fault ruptures together with the Malibu Coast fault to the west and the Hollywood fault to the east, an earthquake as large as Mw7.4 is possible. Such an earthquake could produce peak ground accelerations in excess of 0.75g and cause moderate to severe damage to many structures in the city of Santa Monica. The ShakeMap for a magnitude 7.4 earthquake scenario on the combined Malibu Coast–Santa Monica– Hollywood fault system is shown on Figure 6-5. Note that Modified Mercalli intensities in the city as a result of this scenario earthquake are expected to exceed VIII.

Malibu Coast Fault
The Malibu Coast fault is a complex zone of reverse and left-stepping, en echelon4F 5 , left-lateral
strike-slip faults that parallel the west-trending coastline in the Malibu area of southern California.

The onshore traces of the fault zone are moderately well expressed by an alignment of benches, saddles and linear drainages, but youthful landsliding and erosion make it difficult to determine the fault’s recency of activity. The offshore section of the fault zone is not well defined.

The Malibu Coast fault is known to have moved in the late Quaternary, with the timing of the most recent surface-rupturing earthquake on this fault being poorly constrained. Treiman (1994) reported no evidence for Holocene displacement along most of the onshore fault traces. Probable Holocene displacement was reported on one, and possibly two, secondary faults (Drumm, 1992; Rzonca and others, 1991), but later studies indicated that some of these secondary faults have not moved in the past 11,000 years (Leighton and Associates, 1994a; Earth Consultants International, 2000; 2002). These data suggest that many of the Malibu Coast fault traces onshore either are no longer active, or they move very infrequently. Treiman (1994, 2000) favored the first alternative, suggesting that the slip rate of the fault has diminished from about 1 to 2 mm/yr in the Quaternary, to 0.5 mm/yr in the late Quaternary, to nearly zero in the Holocene. Alternatively, the active strands of the fault may have moved southward (and beachward), analogous to several other range-fronting faults such as the Sierra Madre and Cucamonga faults. Further studies are necessary to resolve these questions.

Nevertheless, the Malibu Coast fault is included in both the Uniform California Earthquake Rupture Forecast, Version 3 (UCERF3; Field and others, 2013) and the California Fault Model, Version 6.1 (Marshall et al., 2022) databases, albeit with a preferred relatively low slip rate of about 0.3 mm/yr. As a result, the fault is also considered in this analysis. Given its length, the Malibu Coast fault is thought capable of generating a magnitude 7.0 earthquake. An earthquake of that size on this fault is estimated to cause ground shaking in the Santa Monica area with PGAs between 0.3g and 0.65g, with MMI values in the VII to VIII range.

5 En echelon refers to closely spaced parallel or subparallel, overlapping or step-like features Local Hazard Mitigation Plan Section 6
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Figure 6-5: ShakeMap for a Magnitude 7.4 Earthquake Scenario on the Malibu Coast–Santa Monica–Hollywood Fault System
Source:
https://earthquake.usgs.gov/scenarios/eventpage/sclegacysantamonicaconnectedalt2m7p4_se/shakemap/intensity Anacapa-Dume Fault
The Anacapa-Dume fault is an offshore fault that is poorly studied. Some researchers suggest that the Anacapa and Dume faults are separate structures, with the Anacapa fault being a segment of the Malibu Coast fault, and the Dume fault being a possible extension of the Santa Monica fault.

As traditionally mapped, the Anacapa-Dume fault is thought to be a left-lateral oblique fault approximately 47 miles (75 km) long that is receiving slip from both the Palos Verdes and Santa Monica faults. UCERF3 (Field et al., 2013) assign it a preferred slip rate of 0.39 mm/yr, with minimum and maximum slip rate estimates of 0.2 and 1.0 mm/yr, respectively. Given its length, the fault is thought capable of generating a magnitude 7.2 earthquake. Such an earthquake would generate PGAs in the Santa Monica area of about 0.2 to 0.75+ g, with MMIs in the VII to VIII+
range. Future studies of this fault system are likely to modify significantly our current understanding of this fault, which may require a revision to this section of the report when it is updated.

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Hollywood Fault
The Hollywood fault is the eastern 9-mile (14 km) long segment of the Malibu–Santa Monica– Hollywood fault system that forms the southern margin of the Santa Monica Mountains (locally known as the Hollywood Hills). It has also been considered the westward extension of the Raymond fault. Movement on the Hollywood fault over geologic time is thought responsible for the growth of the Hollywood Hills, which is why earlier researchers characterized this fault as a northward-dipping reverse fault. However, studies by Dolan et al. (1997, 2000a) and Tsutsumi et al. (2001) showed that the Hollywood fault is primarily a left-lateral strike-slip fault, consistent with its linear trace and steep, 80- to 90-degree dips (reverse faults typically have irregular, arcuate traces and shallow dips).

The Santa Monica–Hollywood fault system has not produced any damaging historical earthquakes and it has had only relatively minor microseismic activity. Subsurface studies by Dolan et al. (2000a) suggest that the Hollywood fault moves infrequently. The most recent surface-rupturing earthquake on this fault appears to have occurred 7,000 to 9,500 years ago, and another earthquake appears to have occurred between about 10,000 and 22,000 years ago (Dolan et al., 2000a). These data suggest that the fault either has a slow rate of slip (of between 0.33 and 0.75 mm/yr), or that it breaks in infrequent but large-magnitude earthquakes. UCERF3 (Field and others, 2013), assigns this fault a preferred slip rate of 0.9 mm/yr, with minimum and maximum rates of

0.2 and 1.5 mm/yr, respectively.

Based on its length, the Hollywood fault is thought capable of generating a Mw ~6.4 to 6.7 earthquake. A magnitude 6.7 earthquake on the Hollywood fault is thought capable of generating PGAs of about 0.2g to 0.35g in Santa Monica, with MMIs of VI to VII.

Palos Verdes Fault
The Palos Verdes fault zone is located primarily offshore, extending in a southeasterly direction from Santa Monica Bay to its possible connection with the Coronado Bank fault zone at about the latitude of La Jolla in San Diego County, a distance of more than 68 miles (110 km). The fault extends onshore a short distance, between Redondo Beach and San Pedro, and follows the northeastern flank of the Palos Verdes Hills, along the southern limits of the city of Torrance. The fault accommodates both right-lateral strike-slip and reverse dip-slip motion, with some segments experiencing primarily strike-slip movement, whereas other sections, like the one under the Palos Verdes peninsula, exhibiting primarily vertical displacements in the form of uplift.

Detailed recent three-dimensional modeling of the Palos Verdes fault zone (Wolfe et al., 2022) indicates that the fault has multiple segments and bends, but three main geographic and structural segments can be defined. These include the Santa Monica Bay, San Pedro Bay, and Lasuen Knoll segments. The Santa Monica Bay segment has been difficult to study because multiple, recurrent episodes of sea level drop during glacial periods in the Quaternary have exposed the sea floor, allowing for erosion to erase evidence of faulting in the sediments. However, the broad zone of diffuse seismicity in the bay indicates that the area is underlain by seismically active faults. Wolfe et al. (2022) studied a dense grid of marine seismic reflection data collected in the bay and determined, based on well-imaged offsets in the bedrock, that the Palos Verdes fault zone is comprised of multiple splays, with the main Palos Verdes fault strand consisting of a moderately southwest-dipping structure that includes several high-angle splays on its hanging wall. The fault zone bends northwesterly as it approaches the offshore Anacapa-Dume-Santa Monica fault system and appears to be truncated by it. Displacement on the Palos Verdes fault appears to decrease Local Hazard Mitigation Plan Section 6
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northward, but slip also seems to be transferred onto the Redondo Canyon fault zone. Several of these faults appear to have moved in the Holocene, and are therefore considered Holocene-active.

Slip rate on the Palos Verdes fault is poorly constrained at between 1.1 and 5.9 mm/yr. These are moderate to high rates of slip compared to other faults in the southern California region, and suggest that earthquakes on this fault occur about every 600 to 1,000 years (Wolfe et al., 2022). It is unknown when the Palos Verdes fault last ruptured in an earthquake. Based on the length of individual fault segments and the complexities of the fault zone, Wolfe et al. (2022) calculated that if a single segment of the Palos Verdes fault ruptures, the resulting earthquake would range between magnitude (Mw) 7.0 and 7.4, with the Santa Monica Bay segment likely causing a Mw 7.0 to 7.1 earthquake. However, if several segments rupture together, similar to the 1992 Landers earthquake, the resulting earthquake would range between Mw 7.4 and 7.8. Earthquakes this large would pose a significant hazard to the Los Angeles region, causing considerable damage and loss of life. Moreover, given that the Palos Verdes fault extends across both the ports of Los Angeles and Long Beach, and is located near the Los Angeles airport (LAX), rupture of this fault would have a significant impact on the economy of southern California. Rupture of the offshore sections could also result in local tsunamis that could impact these ports and other nearby coastal areas. The USGS has modeled a magnitude 7.7 earthquake on the Palos Verdes fault; the ShakeMap for that earthquake scenario indicates that PGAs in Santa Monica would range between about 0.2g and

0.75+g, with MMIs locally exceeding level VIII. Rupture of the offshore section of the fault could result in a significant tsunami in the Santa Monica area (see Section 7).

Newport-Inglewood Fault Zone
The onshore northwest-trending Newport-Inglewood fault zone (NIFZ) is discontinuous at the surface, consisting of a series of primarily left-stepping en echelon fault strands, each up to 4 miles (6.5 km) long, that extend from near Beverly Hills south to Newport Beach, a distance of approximately 41 miles (65 km). At Newport Beach, the fault continues offshore where it lines up with the deeply incised Newport Submarine Canyon (Fischer and Mills, 1991), and is comprised of five strands and three stepovers (Sahakian et al., 2017). Farther south and back onshore, the fault continues as the Rose Canyon fault, extending in a southeasterly direction through San Diego and the international border to Baja California, where it continues as the Agua Blanca fault. Overall, from Beverly Hills to Baja California, the fault zone is more than 185 miles (300 km) long (Grant and Rockwell, 2002).

The NIFZ is for the most part a right-lateral strike-slip fault. Vertical movement on this fault can occur and is inferred locally, the result of either a change in strike, or steps between the en echelons sections, with areas between left steps experiencing uplift, and areas between right steps experiencing extension and subsidence. Thus, the onshore section of the fault zone that extends through western Los Angeles and Orange counties is marked by a series of uplifts (anticlines) and low-lying areas typically occupied by salt marshes or estuaries. The uplifts include, from northwest to southeast, the Baldwin Hills, Rosecrans Hills, Dominguez Hills, Signal Hill and Reservoir Hill, Alamitos Heights and Landing Hill, Bolsa Chica Mesa, Huntington Mesa, and Newport Mesa (Barrows, 1974). These uplifts are petroleum traps that have been drilled and pumped successfully since the beginning of the 20th century. Several studies have identified multiple strands of the onshore segment of the NIFZ that have displaced Holocene-age terraces and sediments, showing that this fault is Holocene-active (Barrows, 1974; Shlemon et al., 1995; Grant et al., 1997).

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Figure 6-6: ShakeMap for a Magnitude 7.2 Earthquake Scenario on the Newport-Inglewood Fault Source: https://earthquake.usgs.gov/scenarios/eventpage/sclegacynewportinglewoodalt1m7p2_se/executive The slip rate for the Rose Canyon-NIFZ is poorly constrained at between 0.3 to 3.5 mm/yr (Woodward-Clyde Consultants, 1979; Fischer and Mills, 1991; Freeman, 1992; Lindvall and Rockwell, 1995; Grant et al., 1997). Fischer and Mills (1991) estimated an earthquake recurrence interval of between 200 and 800 years for the offshore segment of the NIFZ between San Mateo Point and Newport Beach. Paleoseismic studies by Shlemon et al. (1995) and Grant et al. (1997) suggest that the onshore segment of the NIFZ has had three to five ground rupturing earthquakes in the last 11,700±700 years. More recently, a study in the Seal Beach area (Leeper et al., 2017) uncovered evidence for several late Holocene subsidence events that the authors suggest are associated with earthquakes on the NIFZ. These researchers found evidence for three subsidence events in the past about 2,000 years, with the most recent event (MRE) occurring about 350 to 650 years ago. The recurrence interval between these burial events is, on average, approximately 700 years. A similar earthquake history and recurrence interval between earthquake events has been recently documented on the Rose Canyon fault from a trenching site in Old Town, San Diego (Singleton et al., 2019). The similarity between earthquake records has led these researchers to suggest that these faults rupture in sequence, with earthquakes on the Rose Canyon fault triggering ruptures on the NIFZ.

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The last significant earthquake on the NIFZ was the magnitude 6.3 Long Beach earthquake that occurred in 1933. This earthquake did not break the ground surface, but it caused significant damage to the region and caused an estimated 120 deaths. An earthquake of magnitude 7.2 on the onshore segment of the NIFZ has the potential to generate ground motions with PGAs of between

0.2g and 0.75+
g in the Santa Monica area, with the higher accelerations likely if the fault ruptures from south to north. MMIs in the city as a result of this earthquake scenario are estimated at between VII and VIII+
. The ShakeMap for this earthquake scenario is presented above, on Figure 6-6.

Compton Thrust Fault
The Compton Thrust fault is an inferred blind thrust fault in the southwestern portion of the Los Angeles basin. Because the fault does not extend upward to the ground surface, its location, extent, and even geometry are inferred. At least two different interpretations of the Compton Thrust fault have been proposed.

The first is based on high-resolution seismic reflection profiles of the Los Angeles basin that Shaw and Suppe (1996) analyzed to model the Compton fault as a ~25-mile- (~40-km-) long, northeastdipping low-angle blind thrust ramp that bends at two locations deep in the subsurface: 1) where the ramp meets the Central Basin decollement, a nearly flat fault surface that underlies the Los Angeles basin at a depth of about 6 miles (10 km), and 2) where the thrust ramp flattens at a depth of about 3 miles (5 km) just east of the Palos Verdes Hills and extends offshore into the Santa Monica Bay. In this model, when the thrust ramp moves during an earthquake, the sediments above these two locations are deformed. The northeastern zone of deformation is referred to as the Compton-Los Alamitos trend, whereas the southwestern zone is referred to as the Compton buried thrust or the Torrance-Wilmington-Belmont trend. Sorlien et al. (2004) have suggested that the San Pedro Escarpment and the Shelf Projection Anticlinorium, the structure thought responsible for the 1979 and 1989 earthquakes offshore Malibu, in the Santa Monica Bay, may be the updip projections of the Compton thrust ramp. Shaw and Suppe (1996) estimated a slip rate on this fault of 1.4 ± 0.4 mm/yr. This is the model used by the USGS in the ShakeMap magnitude 7.5 earthquake scenario presented in Figure 6-7. This earthquake scenario has the potential to cause severe to violent shaking in the Los Angeles region, including in the city of Santa Monica.

The second interpretation is that the Compton fault is a relatively high-angle, west-dipping, reverse oblique structure that bounds the eastern edge of the anticlines associated with the NewportInglewood fault. This interpretation is based on micro-seismicity (Hauksson, 1987), steep dips in the sedimentary units penetrated by oil exploratory wells (Wright, 1991; Yeats and Verdugo, 2010), and uplift to the east of the mapped traces of the NIFZ during the 1933 Long Beach earthquake (Barrows, 1974). Yeats and Verdugo (2010) used deep oil well records to look for evidence of repeated stratigraphic layers that would indicate faulting rather than folding. Their analysis found that strata at depth are faulted, with vertical separation across the fault measuring “hundreds of meters.” Combined, the data strongly suggest that the NIFZ and Compton-Los Alamitos faults are related, with strain partitioning occurring such that the NIFZ accommodates primarily strike-slip displacement while the Compton-Los Alamitos accommodates reverse movement. Yeats and Verdugo (2010) suggest, however, that the Compton-Los Alamitos is capable of producing earthquakes on its own. An earthquake scenario based on this fault interpretation is not available, but ground shaking in the region as a result of this earthquake is likely to be about the same or less than that presented in Table 6-3 and Figure 6-7.

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Figure 6-7: ShakeMap for a Magnitude 7.5 Earthquake Scenario on the Compton Thrust Fault The black rectangle with black lines shows the approximate location and extent of the Compton Thrust fault underneath the western edge of the Los Angeles basin. The fault is defined by a very low angle plane that is located several kilometers below the ground surface. Because of its shape and regional extent, this fault would cause strong ground shaking over a broad area if it causes an earthquake.

Source: https://earthquake.usgs.gov/scenarios/eventpage/bssc2014comptonellbgeol_m7p45_se/shakemap/intensity Puente Hills Thrust Fault
The ML 5.9 Whittier Narrows earthquake of October 1, 1987 first alerted geologists to the presence of seismically active blind thrust faults underneath the Los Angeles basin. Researchers like Davis et al. (1989), Shaw and Suppe (1996), and Shaw and Shearer (1999) began using oil field data to construct cross-sections through the area and interpret the sub-surface geology. Shaw and Shearer (1999) and Shaw et al. (2002) relocated the main shock and aftershocks of the 1987 earthquake, and showed that the earthquake sequence occurred on a section of a west- to northwest-trending Local Hazard Mitigation Plan Section 6
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buried thrust they called the Puente Hills blind thrust fault. Although the fault is blind, it is expressed at the surface by a series of low hills, including the Puente Hills on its eastern end.

Based on further work, these researchers divided the thrust fault into three segments referred to as the Los Angeles, Santa Fe Springs and Coyote Hills segments. The Santa Fe Springs segment is considered responsible for the 1987 earthquake. Dolan et al. (2003) and Leon et al. (2007) conducted subsurface studies using boreholes of the sediments folded above the Santa Fe Springs segment, and suggested that this fault has experienced at least four large (Mw 7.2 to 7.5) earthquakes in the past 11,000 years. Leon et al. (2007) also calculated a minimum Holocene rate of slip on this fault of between 0.9 and 1.6 mm/yr. Based on these data, UCERF3 (Field et al., 2013) assigned the Puente Hills thrust fault a preferred slip rate estimate of 0.9 mm/yr, and upper and lower slip rate bounds of 1.5 and 0 .2 mm/yr, respectively.

Figure 6-8: Scenario for a M7.1 Earthquake on the Puente Hills Fault Showing Estimated Intensity Values in the Region Resulting from this Event The black polygon shows the approximate location and extent of this buried thrust fault in the subsurface, underneath the north-central portion of the Los Angeles basin.

Source: http://earthquake.usgs.gov/earthquakes/shakemap/sc/shake/Puente_Hills_se/ Local Hazard Mitigation Plan Section 6
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The Los Angeles segment of the Puente Hills thrust fault, the segment closest to Santa Monica, is estimated to be about 11 miles (18 km) long, with a dip of about 26 degrees to the northeast. If this segment ruptures by itself, it could generate a magnitude 6.7 to 7.0 earthquake. The M 7.1 ShakeMap earthquake scenario for the Puente Hills fault (see Figure 6-8 above), which is based on rupture of the Los Angeles segment, shows estimated PGAs in the Santa Monica area of between about 0.1g and 0.35+
g, with MMIs of between about VI and VII+. Stronger ground shaking would occur in the city, and in the region, if the Los Angeles segment of the Puente Hills thrust fault ruptures together with the Santa Fe Springs segment, or if the entire Puente Hills thrust fault ruptures together in one Mw 7.2 to 7.5 earthquake.

Field et al. (2005) conducted a loss-estimation analysis for different sized earthquakes on the Puente Hills thrust fault and found that an earthquake on this fault could be a worst-case scenario for the Los Angeles region, and possibly the costliest natural disaster in U.S. history. The scenarios suggest economic losses ranging between $82 and $252 billion (2005 dollars; approximately equivalent to $130.3 and $400.4 billion, respectively, in 2024), and between 3,000 and 18,000 fatalities. According to this study, some parts of Santa Monica could lose 10-25% of its building inventory (Figure 6-9).

Figure 6-9: Estimated Percent Building Loss as a Result of a Mw 7.5 Earthquake on the Puente Hills Thrust Fault
Source: Field and others, 2005, Loss Estimates for a Puente Hills Blind-Thrust Earthquake in Los Angeles, California: Earthquake Spectra, DOI: 10.1193/1.1898332.

Upper Elysian Park Thrust
The Elysian Park Fold and Thrust Belt (EPFT) was initially described by Davis et al. (1989), the first researchers to postulate that the Los Angeles area is underlain by a deep master detachment fault5F 6 and that most of the uplift in the region is caused by folding and blind thrust faulting at bends and kinks in the detachment fault. Subsequent work (cf., Bullard and Lettis, 1993; Shaw and Suppe, 1996; Shaw and Shearer, 1999; Oskin et al., 2000; Shaw et al., 2002) has highly modified the

6 A basal detachment fault, also referred to as a décollement (from French décoller 'to detach from'), is a horizontal to sub-horizontal gliding plane at the contact between two distinct rock masses that results in different styles of deformation in each block. In a compressional setting like the Los Angeles basin, the block above the detachment fault typically develops more intense deformation in the form of folding and faulting compared to the layer or block below.

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original model. The detachment fault at depth is now referred to as the Lower Elysian Park blind thrust, and a shallower reverse fault is called the Upper Elysian Park blind thrust. Both of these faults, like the Compton and Puente Hills thrusts, are “blind” because they are buried thousands of feet under the basin and are only expressed at the ground surface by the low hills that overlie them (such as the Elysian Park, Repetto, Montebello and Puente hills). The Lower Elysian Park blind thrust fault is the deepest, with its top about 6 miles (9.8 km) below the ground surface; the Puente Hills thrust is at a minimum about 3.1 miles (5 km) below the ground surface; and the Upper Elysian Park thrust is the shallowest, with the tip of the fault occurring about 1.9 miles (3 km) under the ground surface. The Lower Elysian Park thrust fault is interpreted to have a low seismic hazard, with a preferred slip rate of 0.01 mm/yr, and upper and lower bounds of 0.2 and 0 mm/yr respectively (UCERF3; Field et al., 2013). Given these values, this deep fault is typically no longer considered in seismic hazard assessments of the Los Angeles region. The Upper Elysian Park thrust fault, on the other hand, is considered a significant seismic source.

The length, location and slip rate of the Upper Elysian Park blind thrust were better defined by Oskin et al. (2000), who analyzed a series of small-scale folds on the south side of the Elysian Park hills (referred to as the Elysian Park anticline). These researchers modeled the fault as northeastdipping structure, and calculated a contraction rate in this part of the Los Angeles region that resolves into a slip rate of about 0.8 to 2.2 mm/yr on the Upper Elysian Park thrust fault. UCERF3 (Field et al., 2013) used the Oskin et al. (2000) values and assigned a best-estimate slip rate to this thrust fault of 1.9 mm/yr, with minimum and maximum rates of 0.8 and 2.2 mm/yr, respectively.

The approximately 12-mile- (20-km-) long fault is thought capable of producing Mw 6.2 to 6.7 earthquakes with a recurrence between earthquakes of 500 to 1,300 years. The USGS modeled a maximum credible earthquake of Mw 6.7 on the Upper Elysian Park thrust fault. The ShakeMap for that earthquake scenario, with the epicenter located north of downtown Los Angeles, estimates PGAs in the Santa Monica area of between about 0.1g and 0.35g, with MMIs of VI to VII+ .

Raymond Fault
The Raymond fault zone is about 15.5 miles (25 km) long and extends southwesterly from Monrovia, where it intersects the Sierra Madre fault at the foot of the San Gabriel Mountains, across the eastern and southern margins of Pasadena, to the northern reaches of Arcadia, San Marino and South Pasadena. A sharp gravity gradient and aerial photo lineaments suggest that the western end of the fault may connect to the eastern end of the Hollywood fault (Chapman and Chase, 1979; Crook et al., 1987; Dolan et al., 1995).

The fault is gently arcuate and convex to the south, and locally produces a very obvious southfacing scarp; the scarp is especially well defined near the middle of the fault trace (Crook et al., 1987; Weaver and Dolan, 2000). This led many geologists to originally favor reverse slip as the predominant sense of motion for the fault (Buwalda, 1940, as reported in Crook et al., 1987).

However, deflected channels, shutter ridges, sag ponds and pressure ridges along the fault’s main traces indicate that the Raymond fault is primarily a strike-slip fault (Jones et al., 1990). This sense of motion was substantiated by the seismological record, especially by the mainshock and aftershock sequence to the 1988 Pasadena earthquake of local magnitude (ML) 5.0 that occurred on this fault (Jones et al., 1990; Hauksson, 1994).

Several paleoseismic studies of the Raymond fault have been conducted, mostly across the North Branch of the fault. Crook et al. (1987) excavated a trench near Eaton Wash that exposed a 36- foot-wide fault zone that reportedly disturbed all sedimentary units to within about 1 foot of the surface, and they also investigated the fault at other locations. Dolan et al. (2000c) excavated more Local Hazard Mitigation Plan Section 6
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than 25 trenches in almost the same location as Crook et al.’s (1987) trench, while Weaver and Dolan (2000) conducted paleoseismic trenching of the Raymond fault in San Marino and at the Los Angeles Arboretum in Arcadia. Together these studies show that the most recent surface rupture on the western and central portions of the Raymond fault occurred between 1,000 and 2,000 years ago (Weaver and Dolan, 2000), and that between five and eight earthquakes occurred on this fault between 2,000 and 41,500 years ago. This yields a maximum average recurrence interval of between 5,700 and 10,000 years for earthquakes on the Raymond fault (Crook et al., 1987; Weaver and Dolan, 2000). Weaver and Dolan (2000) calculated a shorter recurrence interval of ~ 3,300 years for events between 41,500 and 31,500 years ago, based on evidence for three to five events during this time period. All of these earthquakes may be the result of clustering of activity on the Raymond fault during that 10,000-year period (not an unusual occurrence based on studies of other faults, such as the San Andreas), or it could be that the fault has caused several other earthquakes in the more recent past that were not well expressed in the trenches studied, and are therefore missing in the paleoseismic record. Dolan et al.’s study yielded a minimum late Holocene slip rate for the Raymond fault of ~ 4 +1/-0.5 mm/yr (Marin et al., 2000).

A conservative magnitude 6.9 earthquake on the Raymond fault would generate peak ground accelerations in the Santa Monica area of about 0.23g to 0.47g. The shaking would be perceived as very strong, and damage would be in the moderate to heavy ranges. However, the paleoseismic data suggest that this fault is capable of generating larger earthquakes, in the 7.0 magnitude range (Dolan et al., 2000b). If this is the case, stronger ground shaking as a result of an earthquake on this fault could be experienced in the Los Angeles region, including Santa Monica.

Verdugo Fault
The Verdugo fault is an approximately 13-mile- (21-km-) long, northwest-striking fault that extends along the northeastern edge of the San Fernando Valley, at or near the southern flank of the Verdugo Mountains. The fault dips to the northeast at between 55 and 80 degrees (Marshall et al., 2022). The Verdugo fault was first identified from scarps, offset alluvial deposits and differences in groundwater levels (Weber et al., 1980). Subsequent researchers relied on oil-well records and geophysical data to infer the fault’s subsurface characteristics (Tsutsumi and Yeats, 1999; Langenheim et al., 2000; Pujol et al., 2001). Vertical separation on the Verdugo fault is at least 0.6 mi (1 km) based, in part, on the structural relief between the valley floor and the crest of the Verdugo Mountains (Tsutsumi and Yeats, 1999), but the fault is also thought to have a left-lateral component of motion (Walls et al., 1998). Slip rate on the Verdugo fault is poorly constrained.

UCERF3 (Field et al., 2013) assigned it a preferred, geologically derived slip rate of 0.39 mm/yr, with a range between 0.12 and 0.99 mm/yr. The fault’s recurrence interval is unknown; however, the fault’s southern segment is thought to have ruptured during the Holocene, and the fault is therefore considered Holocene-active (Jennings, 1994). Based on its length, the Verdugo fault is thought capable of generating magnitude 6.0 to 6.9 earthquakes. A maximum magnitude 6.9 earthquake on this fault would generate peak ground accelerations in the Santa Monica area of about 0.1g to 0.2g, with MMIs in the VI to VII range.

Sierra Madre Fault
The Sierra Madre fault zone is a north-dipping reverse fault zone approximately 47 miles (75 km) long that extends along the southern flank of the San Gabriel Mountains from San Fernando to San Antonio Canyon, where it continues southeastward as the Cucamonga fault. The Sierra Madre fault has been divided into five segments, each with a different rate of activity.

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The northwestern-most segment of the Sierra Madre fault (the San Fernando segment) ruptured in 1971, causing the Mw 6.7 San Fernando (or Sylmar) earthquake. As a result of this earthquake, the Sierra Madre fault has been known to be active. In the 1980s, Crook et al. (1987) studied the Transverse Ranges using general geologic and geomorphic mapping, coupled with a few trenching locations. Based on this work, they suggested that segments of the Sierra Madre fault east of the San Fernando segment have not generated major earthquakes in several thousands of years, and possibly as long as 11,000 years. By California’s definitions of active faulting, most of the Sierra Madre fault would therefore be classified as not Holocene-active. Then, in the late-1990s, Rubin et al. (1998) trenched a section of the Sierra Madre fault in Altadena and determined that this segment had ruptured at least twice in the last 15,000 years, causing magnitude 7.2 to 7.6 earthquakes. This suggests that the Los Angeles area is susceptible to infrequent, but large nearfield earthquakes on the Sierra Madre fault. Rubin et al.’s (1998) trenching data show that during the last earthquake, the ground at the surface was displaced along the fault as much as 13 feet (4 meters), and that total displacement in the last two events adds up to more than 34 feet (10.5 meters).

Although the fault apparently slips at a slow rate of between 0.5 and 1 mm/yr (Walls et al., 1998), over time, it can accumulate a significant amount of strain. This relatively slow slip rate was also measured by Burgette et al. (2020) along the central Sierra Madre fault, where they infer an average slip rate of 1.1 +1.1/-0.4 mm/yr based on offset terrace and fan surfaces. The paleoseismic data obtained at the Altadena site (Rubin et al., 1998) were insufficient to estimate the recurrence interval and the age of the last surface-rupturing event on this segment of the fault. However, Tucker and Dolan (2001) trenched the east Sierra Madre fault at Horsethief Canyon and obtained data consistent with Rubin et al.’s (1998) findings. At Horsethief Canyon, the Sierra Madre fault last ruptured about 8,000 to 9,000 years ago. A recurrence interval of about 8,000 years was calculated using a slip rate of 0.6 mm/yr and a slip per event of 15 feet (5 meters). Therefore, if the last event occurred more than 8,000 years ago, it is possible that these segments of the Sierra Madre fault are near the end of their cycle, and are likely to generate an earthquake in the not-toodistant future.

The deterministic analysis for the Santa Monica area estimates peak ground accelerations of about

0.1g to 0.2g, based on a magnitude 7.2 earthquake on the central segment of the Sierra Madre fault. A larger earthquake on this fault, up to magnitude 7.6, would generate significantly stronger peak ground accelerations in the region, including in the Santa Monica area.

San Andreas Fault Zone
The San Andreas fault is the principal boundary between the Pacific and North American plates, and as such, it is considered the “Master Fault” because it has frequent (geologically speaking), large earthquakes, and it controls the seismic hazard in southern California. The fault extends nearly 800 miles (1,300 kilometers), from near Cape Mendocino in northern California to the Salton Sea region in southern California. Many refer to an earthquake on the San Andreas fault as “The Big One,” and for many parts of southern California, this is indeed true. Although, as shown in Table 6-3, several other faults closer to Santa Monica have the potential to cause stronger ground shaking, and therefore more local damage than the San Andreas fault, an earthquake on the San Andreas fault would cause extensive economic damage to the region, with, for example, potential losses to utilities (water, gas, and sewer lines, and the aqueducts that bring water to the Los Angeles area), and damage to the interstate freeways that are used to transport goods in and out of the State). Thus, the San Andreas fault should be considered in all seismic hazard assessment studies in southern California given its high probability of causing an earthquake in the Local Hazard Mitigation Plan Section 6
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near future. In 2007-2008, a group of scientists referred as the 2007 Working Group on California Earthquake Probabilities (WGCEP, 2008) calculated that the southern San Andreas fault had a 59% probability of causing an earthquake of at least magnitude 6.7 in the next 30 years. That probability increases with each passing year without an earthquake.

Large faults, such as the San Andreas fault, are generally divided into segments in order to evaluate their future earthquake potential. The segments are generally defined at discontinuities along the fault that may affect the rupture length. Each segment is assumed to have a characteristic slip rate (rate of movement averaged over time), recurrence interval (time between moderate to large earthquakes), and displacement (amount of offset during an earthquake). While this methodology has some value in predicting earthquakes, historical records and studies of prehistoric earthquakes show that it is possible for more than one segment to rupture during a large quake or for ruptures to overlap into adjacent segments. In central and southern California, the San Andreas fault zone is divided into five segments named, from north to south, the Cholame, Carrizo, Mojave, San Bernardino Mountains, and Coachella Valley segments. At its closest approach, the Mojave Section of the San Andreas fault is approximately 41 miles (66 km) northeast of Santa Monica.

The last major earthquake on the southern portion of the San Andreas fault was the 1857 Fort Tejon (M 7.7-8) event, which ruptured the Cholame, Carrizo, Big Bend, and Mojave North and Mojave South sections of the fault. This is the largest earthquake reported in California. The 1857 surface rupture broke a 186-mile- (300-km-) long section of the fault, resulting in displacements of as much as 27 feet (9 meters) along the rupture zone. Rupture of these fault segments as a group, during a single earthquake, is thought to occur with a recurrence interval of between 104 and 296 years. Figure 6-11 shows the PGAs and seismic intensities that would be expected in the southern California area if a repeat of the 1857 earthquake occurred.

Figure 6-10: ShakeMap for a Magnitude 7.8 Earthquake Scenario on the San Andreas Fault (repeat of the 1857 Fort Tejon Earthquake) Source: http://earthquake.usgs.gov/eqcenter/shakemap/sc/shake/1857_se/ Local Hazard Mitigation Plan Section 6
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In 2008, the USGS modeled a magnitude 7.8-8 earthquake on the southern section of the San Andreas fault to identify the physical, social and economic consequences of a major earthquake in southern California. Referred to as the “ShakeOut scenario,” the earthquake modeled starts in the Salton Sea area, with the fault rupturing northward for 200 miles, toward Lake Hughes. This section of the San Andreas fault is thought to be long overdue for an earthquake because the average interval between earthquakes is about 115 to 220 years, and it has been more than 330 years since it last ruptured. Figure 6-11 is the ShakeMap for this hypothetical magnitude 7.8 “Shakeout” earthquake scenario. Comparison with the ShakeMap for the repeat of the Fort Tejon earthquake shown on Figure 6-10 shows that the epicenter location and fault rupture directivity have a significant impact on the ground shaking experienced in the region. The ShakeMap on Figure 6-10 shows that the area in and around Santa Monica would experience strong to very strong shaking (Modified Mercalli Intensity VI-VII), whereas the ShakeOut scenario is anticipated to cause very strong to severe shaking (Modified Mercalli intensity of VII-VIII) of long duration, possibly lasting several minutes. The long-period seismic waves caused by the ShakeOut earthquake in the Los Angeles region are expected to damage high-rise structures preferentially, but moderate damage to shorter, older structures is also likely. Economic losses in the entire southern California region as a result of the ShakeOut scenario were estimated at $200 billion (in 2008 dollars), with about 1,800 fatalities (Jones et al., 2008). Rose et al. (2011) revisited the losses as a result of the ShakeOut scenario and calculated $113 billion in property damage and more than $100 billion in business interruption (in 2008 dollars). These values are approximately equivalent to $162.9 and $144.1 billion, respectively, in 2024 dollars.

Figure 6-11: ShakeMap for a Magnitude 7.8 Earthquake Scenario (Shakeout) on the Southern San Andreas Fault
Source: https://earthquake.usgs.gov/scenarios/eventpage/sclegacyshakeout2full_se/executive Local Hazard Mitigation Plan Section 6
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San Jacinto Fault Zone
The San Jacinto fault zone consists of a series of closely spaced faults that form the western margin of the San Jacinto Mountains. The zone extends from its junction with the San Andreas fault in San Bernardino, just north of Rialto, southeasterly toward the Brawley area, where it continues south of the international border as the Imperial fault. This fault zone has a high level of historical seismic activity, having generated at least ten moderate (M6-7) earthquakes between 1890 and 1986.

Movement on the San Jacinto fault is predominantly right-lateral strike-slip, similar to the San Andreas fault, although Brown (1990) suggested that vertical motion contributes up to 10% of the net slip.

The San Jacinto fault zone has been divided into seven segments. Each segment, in turn, consists of a series of subparallel faults. The segment of the San Jacinto fault closest to Santa Monica is the San Bernardino segment, which is approximately 63 miles (101 km) west-northwest of the city at its closest. Slip rates on the various segments of the San Jacinto fault are less well constrained than for the San Andreas fault, but the data available suggest right-lateral slip rates of 12±6 mm/yr for the northern segments of the fault (including the San Bernardino segment) and slip rates of 4±2 mm/yr for the southern segments (WGCEP, 1995; Field et al., 2013).

Petersen and Wesnousky (1994) compiled data from various studies to suggest a recurrence interval of between 150 and 300 years for large ground-rupturing earthquakes on the San Jacinto fault. Since then, several new studies have helped refine these estimates. The most relevant of these studies were conducted at Mystic Lake, a site at the southern end of the San Bernardino segment, on the Claremont fault. Here, researchers have interpreted 12 surface-rupturing earthquakes in the past about 2,000 years (Marliyani et al., 2013; Onderdonk et al., 2015), which resolves into an average recurrence rate for the Claremont fault of 164 -8/+31 years (Onderdonk et al., 2014). The data also indicate that the average recurrence rate for the last four earthquakes is shorter, between about 100 and 140 years (Onderdonk et al., 2015), with the most recent groundrupturing earthquake thought to have occurred between A.D. 1800 and 1850 (Onderdonk et al., 2013). Thus, based on these data, the northern section of the San Jacinto fault is thought to be near the end of its earthquake cycle, and likely to rupture again in the not-too-distant future.

A maximum credible earthquake of magnitude 6.7 on the San Bernardino segment of the San Jacinto fault has the potential to generate light to moderate ground motions in the Santa Monica area. If the San Bernardino segment of the San Jacinto fault ruptures together with the San Jacinto Valley, Anza, and Clark segments to the south in an estimated Mw 7.7 earthquake, PGAs in Santa Monica are expected to range between about 0.05g and 0.2g, with MMIs in the V to VI range.

Shaking as a result of such an earthquake could potentially last at least a minute, if not longer, posing a risk to high rises and infrastructure such as bridges and freeway overpasses. Similar large rupture scenarios are also likely if the San Jacinto fault functions as an “earthquake gate” at Cajon Pass and ruptures together with the Mojave section of the San Andreas fault to the north (Onderdonk et al., 2018).

Surface Fault Rupture
Definitions and Statutory Requirements
Primary fault rupture refers to fissuring and offset of the ground surface along a rupturing fault during an earthquake. Primary fault rupture is rarely confined to a simple line along the fault trace.

As the rupture reaches the ground surface, it commonly spreads out into complex fault patterns of Local Hazard Mitigation Plan Section 6
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secondary faulting and ground deformation, especially in areas underlain by unconsolidated to poorly consolidated sediments, such as those underlying most of Santa Monica. Primary ground rupture typically results in a relatively small percentage of the total damage in an earthquake, but straddling a rupturing fault can cause severe damage to structures.

Secondary fault rupture refers to ground surface displacements along faults other than the main traces of active regional faults. Most of these subsidiary faults are not deeply rooted in the Earth’s crust and are not capable of producing damaging earthquakes on their own; movement along these faults generally occurs in response to movement on a nearby regional fault. Secondary ground deformation includes fracturing, shattering, warping, tilting, uplift and/or subsidence.

Surface displacement and distortion associated with secondary faulting and deformation can be relatively minor or can be large enough to cause significant damage to structures. Triggered slip as a result of a regionally large earthquake can also occur in faults many miles away from the causative fault. For example, as a result of the April 4, 2010 Sierra El Mayor earthquake in Baja California, triggered slip was reported on fault zones 40 to 60 miles away.

Faults have formed over millions of years, usually in response to regional stresses. Shifts in these stress regimes do occur over millennia. As a result, some faults change in character. For example, millions of years ago, the now right-lateral strike-slip NIFZ used to be a normal fault. Some faults may be abandoned altogether, and previously not active faults may be reactivated. Consequently, the State of California, under the guidelines of the Alquist-Priolo Earthquake Fault Zoning Act (APEFZA) of 1972, and in accordance with the most recent language in Special Publication 42 (CGS, 2018a) classifies faults according to the following criteria:
■ Holocene-Active: faults showing proven displacement of the ground surface within the past 11,700 years (within the Holocene Epoch);
■ Pre-Holocene: faults that have not moved in the past 11,700 years; and ■ Age-undetermined faults: faults whose recency of movement has not been determined.

Within the framework of the APEFZA, these faults are considered Holocene-active until proved otherwise.

In the past, faults were generally described as active, potentially active and not active, but these terms are not defined in the APEFZA, and often caused confusion from a regulatory perspective. In California, these definitions apply to structures intended for human occupancy, including some residential structures, and commercial and industrial buildings. Different definitions of activity are used by other agencies or organizations depending on the type of facility being planned or developed. For example, longer periods of inactivity are typically required for dams or nuclear power plants. An important subset of active faults is those with historical earthquakes.

The underlying assumption in this classification system is that if a fault has not ruptured the ground surface in the past 11,700 years, it is not likely to be the source of a damaging earthquake in the future. In reality, however, many faults have been insufficiently studied to determine their hazard level. Case in point, some of the faults that ruptured in the 1992 Landers, 1999 Hector Mine, and 2019 Ridgecrest earthquakes were previously thought to be not active, as they appeared to have not moved in at least 11,700 years, or were not even recognized as faults. Moreover, although simple in theory, the evidence necessary to determine whether a fault has or has not moved during the past 11,700 years can be difficult to obtain.

Development constraints within active fault zones were implemented in 1972 with passage of the Local Hazard Mitigation Plan Section 6
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APEFZA. This law prohibits the construction of new habitable structures astride a Holocene-active fault and requires special geologic studies to locate, and evaluate whether a fault has ruptured the ground surface in the last 11,700 years (CGS, 2018a). Because it is difficult and costly to safely reduce the effects of surface fault rupture through building and foundation design, the preferred and State-mandated mitigation measure is to avoid Holocene-active faults by avoiding placement of structures meant for human habitation across the trace of a Holocene-active fault. This is typically accomplished by establishing a structural setback from the surface projection of the fault as exposed in a trench or inferred from a subsurface study using borings, cone penetration tests (CPTs), and/or geophysical methods.

Santa Monica Fault Zone
The only fault currently mapped as extending through the city of Santa Monica that poses a surface-fault rupture hazard is the Santa Monica fault zone. As previously mentioned, this fault zone is part of the series of east- to east-northeast-trending faults that form the northern boundary of the Los Angeles basin. Defining the near-surface location and character of the Santa Monica fault has been difficult, as discussed further in the following paragraphs, with various interpretations provided by different researchers. Thus, depending on the source referenced, the Santa Monica fault zone occurs at or near the mountain-front of the Santa Monica Mountains, out in or underneath the coastal plain, or connecting both areas (see Figure 6-12).

The presence of a fault along the southern margin of the Santa Monica Mountains was suggested as early as 1914, when McLaughlin and Waring described the Santa Monica fault as one of the structural features of the northern Los Angeles region (based on the summary in Olson, 2018).

Hoots and Kew (1931), in the first comprehensive geologic map of the northern portion of the Los Angeles basin and the southern face of the Santa Monica Mountains, identified two east-trending faults in the Pacific Palisades area north of Santa Monica and several short, discontinuous bedrock faults in the mountains. Hoots and Kew (1931), however, did not identify any faults through the city of Santa Monica proper. The southernmost of the two short faults that these geologists mapped in the Pacific Palisades area is the Potrero fault. This fault used to be exposed in the cliff faces where Potrero Canyon meets the Pacific Ocean. The exposures were photographed and mapped by Johnson (1932) as part of a geologic feasibility study for the Quelinda Estate, a now fully developed residential area in the western half of Pacific Palisades. Johnson and his colleagues extended the Potrero fault eastward at least 1 mile from the mouth of Potrero Canyon (and thus about 0.5 miles farther east than Hoots and Kew) based on their analysis of changes in stream gradient and breaks in slope of the terrace surfaces. McGill (1989) extended the Potrero Canyon fault farther eastward based on breaks in slope and surface mapping of terrace deposits.

The next detailed geologic map of the area was prepared by Castle in 1960, who did not map any faults through the city. Then in 1965, Yerkes and others published a seminal study on the geology of the Los Angeles basin where they describe the Santa Monica fault as a major reverse fault at the boundary between geomorphic and geologic provinces. In their maps they show the fault as approximately located and consisting of two parallel to subparallel strands in the Santa Monica region, with these strands converging into one strand to the east, in the Hollywood area. These authors relied on subsurface data collected from oil wells to infer the location of the fault and evaluate its impact on the geologic units at depth. Specifically, Yerkes and others (1965) indicated that the basement rocks are vertically offset about 7,500 feet by the fault, with decreasing separation of the geologic units as the fault approaches the ground surface. In the Beverly Hills area, they reported that the Santa Monica fault dips about 50 degrees to the north at depth, and Local Hazard Mitigation Plan Section 6
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that matching the core of the Elysian Hills anticline across the fault suggests left-lateral offset in addition to vertical movement.

Figure 6-12: Faults Mapped and Inferred Through, Near and Below Santa Monica Local Hazard Mitigation Plan Section 6
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More detailed studies of the faults along the southern base of the Santa Monica Mountains began in the late 1970s and have continued to this day. Hill et al. (1978, 1979) and Hill (1979) mapped discontinuous scarps in the Pacific Palisades area and in the alluvial fans at the base of the Santa Monica Mountains. They used these features, combined with leveling studies across the extensively developed areas of Los Angeles, to postulate the presence of at least two faults in the subsurface east of the 405 Freeway. Other than the Potrero Canyon fault in Pacific Palisades, however, these researchers did not map any faults across Santa Monica. Wright (1991) relied on oil well data to conclude that the faults of the Santa Monica fault system are for the most part buried “under extensive landslides along the north shore of Santa Monica Bay and by a thick alluvial apron farther east.” A well drilled for petroleum in 1960 about four miles west of Beverly Hills confirmed the existence of a major fault in the subsurface, but this information was not disclosed by Wright until 1991. In the Santa Monica area, Wright showed the Santa Monica fault consisting of potentially two fault strands, but his maps are too small to reliably transfer that information to a map like the one on Figure 6-12. Wright (1991) referred to the southern of these faults as the Santa Monica fault proper, and the second, more northerly trace as the Potrero Canyon fault. Other geologists that have used oil and/or groundwater data to infer the location of the Santa Monica fault include Lang and Dreessen (1975), Dolan and Sieh (1992), and Yeats (2001).

Other methods that have been used to identify or infer these fault traces include mapping of surface or near-surface exposures (Johnson, 1932, and McGill, 1989, as discussed above; Hill et al., 1979; Crook et al., 1983; Dolan et al., 2000b), differential leveling analyses that have documented regional uplift or subsidence along discrete zones (Hill et al., 1979; Chapman and Chase, 1979), geomorphic mapping of folds that are thought to be the near-surface expression of the fault (Castle, 1960; Dolan and Sieh, 1992; Dolan et al., 2000a,b), and geophysical imaging (Dolan and Pratt, 1997; Pratt et al., 1998; Catchings et al., 2001; 2008). Work conducted by Dolan et al. (1992, 1997, 2000a) that built on the earlier work by Crook et al. (1983, 1992) concluded that the Santa Monica fault zone is comprised of north-dipping, high-angle, left-lateral reverse faults expressed at the surface by stepping broad folds that stretch across the western Los Angeles and Santa Monica region. These folds and scarps are located approximately 1.25 to 2.5 miles south of the base of the Santa Monica Mountains, in the plain that is covered in Quaternaryaged alluvial fan deposits (see the mapped scarps and folds in yellow on Figure 6-12). The 1992 work by Dolan and Sieh was referred to extensively by Leighton and Associates (1995) in defining the Fault Hazard Management Zones that were presented in the City of Santa Monica 1995 Safety Element of the General Plan.

Geophysical studies by Pratt et al. (1998) and Catchings et al. (2001, 2008), in addition to the work by Tsutsumi et al. (2001), confirmed that the Santa Monica fault zone consists of two main north-dipping branches that merge at a depth of approximately 1.25 miles (2 km). The northern branch has been investigated at the Veteran’s Administration property (Crook et al., 1983, Dolan et al., 2000a), and at University High School (Crook et al., 1983; MACTEC, 2004a, 2004b, 2007), where a series of steeply dipping to sub-vertical faults that offset late Quaternary deposits were exposed in trenches or interpreted from closely spaced borings and CPTs. The dip and normal sense of movement on the faults exposed by Dolan et al. (2000a) and MACTEC (2004a) suggest that these are upper-plate features rather than the main fault zone. However, these studies were the first to suggest that the Santa Monica fault has ruptured in the Holocene and should therefore be considered Holocene-active. Furthermore, Dolan et al.’ (2000a) study in the grounds of the Veteran’s Administration property suggests that this fault has moved five, and possibly six, times in the past 50,000 years, with an average recurrence interval of between 7,000 and 8,000 years.

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These studies were followed in 2011 by a fault investigation report issued by the Los Angeles County Metropolitan Transportation Authority (Metro) for the Century City area east of the 405 Freeway (Parsons Brinckerhoff, 2011) that summarized their seismic study of the west side of Beverly Hills, in the area of the proposed metro line and subway station on Constellation Boulevard. The report and accompanying maps inferred a complex zone of northeast-trending faulting along Santa Monica Boulevard, and north- to northwest-trending faulting associated with the northward extension of the Newport-Inglewood fault. Both fault systems were inferred to be Holocene-active although no age dating of the faulted sediments was conducted.

Based on a review of vintage topographic maps and photographs, new LiDAR technology, and the work conducted by Dolan, Crook and others, as discussed above, the CGS began to review the Hollywood and Santa Monica faults to determine whether they should be zoned following the criteria of the APEFZA. The Hollywood fault was zoned in 2014, and the State issued new APEFZ maps for the Santa Monica and Beverly Hills region in early 2018 (CGS, 2018a, 2018b). The zones of required investigation mapped by the CGS through the city of Santa Monica are presented in Figure 6-13. These zones supersede the Fault Hazard Management Zones presented in the 1995 City of Santa Monica Safety Element.

The APEFZ map (Figure 6-13) shows six generally east-west striking fault traces extending across the northern approximately one-third of the city (the official APEFZ maps for both the Topanga and Beverly Hills quadrangles were issued on January 11, 2018). The mapped or inferred fault traces are shown by the dashed red lines. The yellow shading surrounding the red fault lines are the zones of required investigation. If a property located inside or partially inside one of these yellowshaded areas is to be developed or re-developed, the applicant first needs to retain a Californiaregistered geologist to conduct a fault investigation before the building permit is granted. These studies need to be conducted in accordance with the guidelines provided in Special Publication 42 (CGS, 2018a). The only exception to this requirement is if the proposed structure is a singlefamily residence two stories or less in height. The study needs to conclusively show that the footprint of the proposed structure(s), and a certain distance beyond the structure(s), is not underlain by Holocene-active faults.

To date, approximately two dozen fault studies have been conducted in Santa Monica to comply with this requirement (Michael Phipps, geological reviewer for the City of Santa Monica, personal communication, 2023), and several more have been conducted to the east, in the city of Los Angeles. A few of these investigations have inferred faults at depth, typically at least 35 feet below the ground surface, but these inferred faults are capped by unbroken sediments and soils that, based on their characteristics and geomorphic position, are interpreted to be Pleistocene in age.

Thus, to date, no study has yet to expose a Holocene-active fault in the city of Santa Monica.

Similar findings have been reported from studies along Santa Monica Boulevard in the city of Los Angeles. The only two fault traces that, based on the data available to date, appear to extend upward to, or near the ground surface, include the fault mapped across the campus of University High School, and the Potrero Canyon fault in the Pacific Palisades area and northeastern corner of the city. Holocene activity on these faults has not been conclusively demonstrated, and as such, until proved otherwise, are considered Holocene-active.

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Liquefaction and Related Ground Failure
Definitions
Liquefaction is a geologic process that causes various types of ground failure. Liquefaction typically occurs in loose, saturated sediments primarily of sandy composition, in the presence of ground accelerations over 0.2g (Borchardt and Kennedy, 1979; Tinsley and Fumal, 1985).

Moderate magnitude (M>5.5) earthquakes can trigger liquefaction (Sana, 2021). Earthquake shaking suddenly increases pressure in the water that fills the pores between soil grains, causing the soil to have a total or substantial loss of shear strength, and behave like a liquid or semiviscous substance. Liquefaction can cause structural distress or failure due to ground settlement, a loss of bearing capacity in the foundation soils, and the buoyant rise of buried structures such as underground storage tanks. The excess hydrostatic pressure generated by ground shaking can result in the formation of sand boils or mud spouts, and/or seepage of water through ground cracks.

The types of ground failure typically associated with liquefaction are explained below.

Lateral Spreading – Lateral displacement of surficial blocks of soil as the result of liquefaction in a subsurface layer is called lateral spreading. Even a very thin liquefied layer can act as a hazardous slip plane if it is continuous over a large enough area. Once liquefaction transforms the subsurface layer into a fluid-like mass, gravity plus inertial forces caused by the earthquake may move the mass downslope towards a cut slope or free face (such as a river channel or a canal). Lateral spreading most commonly occurs on gentle slopes that range between 0.3° and 3°, and can displace the ground surface by several meters to tens of meters. Such movement damages pipelines, utilities, bridges, roads, and other structures. During the 1906 San Francisco earthquake, lateral spreads with displacements of only a few feet damaged every major pipeline. Thus, liquefaction compromised San Francisco’s ability to fight the fires that caused about 85 percent of the damage (Tinsley et al., 1985).

Flow Failure – The most catastrophic mode of ground failure caused by liquefaction is flow failure.

Flow failure usually occurs on slopes greater than 3 degrees. Flows are principally liquefied soil or blocks of intact material riding on a liquefied subsurface. Displacements are often in the tens of meters, but in favorable circumstances, soils can be displaced for tens of miles, at velocities of tens of miles per hour. For example, the extensive damage to Seward and Valdez, Alaska, during the 1964 Great Alaskan earthquake was caused by submarine flow failures (Tinsley et al., 1985).

Ground Oscillation – When liquefaction occurs at depth but the slope is too gentle to permit lateral displacement, the soil blocks that are not liquefied may separate from one another and oscillate on the liquefied zone. The resulting ground oscillation may be accompanied by the opening and closing of fissures (cracks) and sand boils, potentially damaging structures and underground utilities (Tinsley et al., 1985).

Loss of Bearing Strength – When a soil liquefies, loss of bearing strength may occur beneath a structure, possibly causing the building to settle and tip. If the structure is buoyant, it may float upward. During the 1964 Niigata, Japan earthquake, buried septic tanks rose as much as 3 feet, and structures in the Kwangishicho apartment complex tilted as much as 60 degrees (Tinsley et al., 1985).

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Ground Lurching – Soft, saturated soils have been observed to move in a wave-like manner in response to intense seismic ground shaking, forming ridges or cracks on the ground surface. At present, the potential for ground lurching to occur in a given area can be predicted only generally.

Areas underlain by thick accumulation of colluvium and alluvium appear to be the most susceptible to ground lurching. Under strong ground motion conditions, lurching can be expected in loose, cohesionless soils, or in clay-rich soils with high moisture content. In some cases, the deformation remains after the shaking stops (Barrows et al., 1994).

Liquefaction Susceptibility In and Near Santa Monica
As indicated above, there are three general conditions that need to be met for liquefaction to occur. The first of these–strong ground shaking of relatively long duration–can be expected to occur in the Santa Monica area as a result of an earthquake on any of several active faults in the region. The second and third conditions–loose, unconsolidated, water-saturated sediments consisting primarily of silty sand and sand within about 50 feet of the ground surface occur in some areas in and near Santa Monica. These areas include the coastline, where young beach and eolian sand deposits are mapped, and the portions of the channels of Santa Monica and Rustic canyons north of Santa Monica that are underlain by young alluvial deposits. Other areas where groundwater within 30 feet of the ground surface have been reported historically include 1) along the southwestern limits of the city, extending northward from Marina del Rey to the city’s southern boundary across Marine Park and eastward to near the southwestern corner of the Santa Monica airport; 2) near the northeastern corner of the city, between Brentwood Country Club to the north and just south of Santa Monica Boulevard on the south; and 3) an area approximately between Olympic Boulevard to the north, Pearl Street to the south, Gateway Boulevard on the east, and the southeast projection of Franklin Street on the west. This third area extends into the east-central edge of the city. Areas that the CGS has mapped as potentially susceptible to liquefaction are shown in light blue on Figure 6-14. These maps are typically regional in extent, and may not capture all areas susceptible to this hazard, so sites are typically evaluated on a case-by-case basis when a proposed new development or redevelopment project triggers a geotechnical investigation.

It is unlikely that new residential or commercial development will occur in some of the potentially liquefiable areas such as along the coastline and in the bottom of stream channels. However, structures such as bridges, roadways, major utility lines, and park improvements that could occupy these areas would be vulnerable to damage from liquefaction if mitigation measures are not included in their design. Any construction planned in these areas should include liquefaction mitigation measures, weighing the factors of public safety, the impact to the environment, and the risk of economic loss. For instance, a parking lot at the beach may not warrant ground modification measures, especially if the mitigation measures would be destructive to the environment, but a bridge abutment for a busy roadway would.

Seismically Induced Slope Failure
Strong ground shaking can worsen existing unstable slopes, particularly if coupled with saturated ground conditions. Seismically induced landslides can overrun structures, people or property, sever utility lines, and block roads, thereby hindering rescue operations after an earthquake. Over 11,000 landslides were mapped shortly after the Northridge earthquake, all within a 45-mile radius of the epicenter, including along the coastal bluffs in Santa Monica (Harp and Jibson, 1995, 1996). Although numerous types of earthquake-induced landslides have been identified, the most widespread type generally consists of shallow failures involving surficial soils and the uppermost Local Hazard Mitigation Plan Section 6
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weathered bedrock in moderate to steep hillside terrain (these are also called disrupted soil slides).

Rock falls and rockslides on very steep slopes are also common. The 1989 Loma Prieta and Northridge earthquakes showed that reactivation of existing deep-seated landslides also occurs (Spittler et al., 1990; Barrows et al., 1995).

A combination of geologic conditions leads to landslide vulnerability. These include high seismic potential; rapid uplift and erosion resulting in steep slopes and deeply incised canyons; highly fractured and folded rock; and rock with inherently weak components, such as silt or clay layers.

The orientation of the slope with respect to the direction of the seismic waves (which can affect the shaking intensity) can also control the occurrence of landslides.

Groundwater conditions at the time of the earthquake play an important role in the development of seismically induced slope failures. For instance, the 1906 San Francisco earthquake occurred in April, after a winter of exceptionally heavy rainfall, and produced many large landslides and mudflows, some of which were responsible for several deaths. The 1987 Loma Prieta earthquake however, occurred in October during the third year of a drought, and slope failures were limited primarily to rock falls and reactivation of older landslides that was manifested as ground cracking in the scarp areas but with very little movement (Griggs et al., 1991).

Areas that the State Geologist identifies as susceptible to earthquake-induced slope failure are shown in red on Figure 6-14. Within or immediately adjacent to the city of Santa Monica these areas include the coastal bluffs on the northern half of the city, the northwest-facing slopes along Santa Monica Canyon, some of the steeper slopes along the Brentwood Knoll, slopes along the I10 freeway, the slope on the southwestern side of the Santa Monica airport, and slopes in the neighborhood north of Dewey Street. For additional information regarding slope instability refer to Section 9.

VULNERABILITY ASSESSMENT
The effects of earthquakes span a large area, and large earthquakes occurring in the southern California area can and will be felt throughout the region. Although it is not possible to prevent earthquakes from occurring, their destructive effects can be minimized by strengthening buildings and structures. This is because most loss of life and injuries associated with earthquakes are related to the collapse of hazardous human-made structures. At risk from earthquake damage are large stocks of older buildings, bridges, facilities that house hazardous materials, dams, critical facilities, and private properties and businesses. Earthquakes can also cause extensive damage to the sewer, water, and natural gas systems; petroleum pipelines; and communication towers. Failure of these systems can fuel fires following an earthquake, or limit the water resources available to fight fires, and can severely limit the availability of utilities for days, weeks, or even months following an earthquake. This in turn can result in substantial losses in business interruption and secondary but significant health effects.

Other earthquake hazards, such as surface fault rupture, liquefaction and earthquake-induced landslides, can be as devastating as ground shaking. These hazards, and the areas of the City that could be impacted as a result of surface fault rupture, liquefaction and earthquake-induced slope failure, were previously described in this section. The potential impact of a tsunami is described in Section 7.

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Vulnerable Structure Types
Damage to the building stock in the southern California region is expected to vary due to differences in construction and construction quality. Some common building types and their earthquake performance are discussed in the sub-sections below. However, it is important to know that buildings are often a combination of steel, concrete, reinforced masonry and wood, with different structural systems on different floors or different sections of the building. Because different materials and construction types often react differently to seismic shaking, the overall combination can result in structural damage. Combination types that are potentially hazardous include:
concrete frame buildings without special reinforcing, precast concrete and precast-composite buildings, steel frame or concrete frame buildings with unreinforced masonry walls, reinforced concrete wall buildings with no special detailing or reinforcement, large capacity buildings with long-span roof structures (such as theaters and auditoriums), large un-engineered wood-frame buildings, buildings with inadequately anchored exterior cladding and glazing, and buildings with poorly anchored parapets and appendages (FEMA, 1985).

The seismic performance of buildings is tested when exposed to strong earthquakes. Building codes have often been modified in response to lessons learned in previous earthquakes. Thus, the 1997 version of the building code and all later building codes, including the 2022 edition of the California Building Code now being used, was the result of observations of intense ground shaking at levels higher than expected at Northridge in 1994, and again one year later, in Kobe, Japan.

These earthquakes and the 1992 Landers earthquake also showed that the direction of fault rupture plays a significant role in the distribution of ground shaking. As a result of these observations, current building codes incorporate near-fault site factors, soil-site effects and directivity effects.

As a result of the damage observed following the Northridge earthquake, many communities have implemented seismic retrofit ordinances for certain types of construction that are now known to not perform well when shaken by strong earthquakes. The City of Santa Monica is a leader in this effort, with seismic retrofit ordinances that target unreinforced masonry buildings (URMs), softstories, tilt-up buildings, non-ductile concrete buildings that were built before 1977, and steelmoment frame buildings that were built before 1990. This is discussed further in the following subsections. Additional information on the City’s Seismic Retrofit Program can be obtained directly from the City’s Building and Safety Division.

Unreinforced Masonry Buildings
Unreinforced masonry buildings (URMs) are prone to earthquake failure due to inadequate anchorage of the masonry walls to the roof and floor diaphragms, lack of steel reinforcing, the limited strength and ductility of the building materials, and sometimes, poor construction workmanship. Furthermore, as these buildings age, the bricks and mortar tend to deteriorate, making the buildings even weaker. Recognition of this issue led the State Legislature to pass Senate Bill 547, addressing the identification and seismic upgrade of URMs.

As of June 2023, Santa Monica staff had identified 89 URMs in the city in need of a seismic retrofit pursuant to the City’s Seismic Retrofit Ordinance. Of the 89 URMs, 56 cases were closed as a result of Requests for Reconsiderations and 16 URMs had successfully completed the required retrofit work and were in compliance. Of the remaining non-compliant URMs, 17 had been forwarded to Code Enforcement. The due date for the retrofit of URM buildings in Santa Monica was August 2019.

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Soft-Story Buildings
Soft-story buildings have a story, generally the first floor, lacking adequate strength or toughness due to too few shear walls. Apartments above glass-fronted stores, and buildings perched atop parking garages are common examples of soft-story buildings. Collapse of a soft story and “pancaking” of the remaining stories killed 16 people at the Northridge Meadows apartments during the 1994 Northridge earthquake (EERI, 1995). There are many other cases of soft-story collapses in past earthquakes. In response, the State encourages the identification and mitigation of seismic hazards associated with these types of potentially hazardous buildings, and others such as pre-1971 concrete tilt-ups, mobile homes, and pre-1940 homes.

As part of the City’s Seismic Retrofit Program, 1,686 soft-story buildings in the city of Santa Monica were identified as in need of retrofit. Of these soft-story buildings, 495 cases were closed as a result of Requests for Reconsiderations and 560 have had permits issued for retrofit. The remaining (43%) have until September 2025-July 2026 to submit plans to the City and complete the required retrofit, with the range in due date dependent on the number of stories and number of units in the structure. This due date for soft-story building types reflects a two-year extension granted pursuant to supplements to the City’s local COVID emergency order and codified on January 25, 2022, by Ordinance No. 2689 (CCS).

Wood-Frame Structures
Most wood-frame structures in Santa Monica can be anticipated to experience slight to extensive damage as a result of ground shaking expected in the region, the level of damage generally associated with the age of the structure, its condition, and its location relative to the causative fault, among other conditions. Structural damage to wood-frame structures often results from an inadequate connection between the superstructure and the foundation. These buildings may slide off their foundations, with consequent damage to plumbing and electrical connections.

Unreinforced masonry chimneys may also collapse. These types of damage are generally not life threatening, although they may be costly to repair. Wood frame buildings with stud walls generally perform well during an earthquake, unless they have no foundation or have a weak foundation constructed of unreinforced masonry or poorly reinforced concrete. In these cases, damage is generally limited to cracking of the stucco, which dissipates much of the earthquake's induced energy. The collapse of wood frame structures, if it happens, generally does not generate heavy debris, but rather, the wood and plaster debris can be cut or broken into smaller pieces by handheld equipment and removed by hand in order to reach victims (FEMA, 1985). These types of structures are currently not part of the City’s seismic retrofit program.

Pre-Cast Concrete Structures
Partial or total collapse of buildings where the floors, walls and roofs fail as large intact units, such as large pre-cast concrete panels, cause the greatest loss of life and difficulty in victim rescue and extrication (FEMA, 1985). In southern California, many of the parking structures that failed during the Northridge earthquake, such as the Cal-State Northridge and City of Glendale Civic Center parking structures, consisted of pre-cast concrete components (EERI, 1995).

Collapse of this type of structure generates heavy debris, and removal of this debris requires the use of heavy mechanical equipment. Consequently, the location and extrication of victims trapped under the rubble is generally a slow and dangerous process. Extrication of trapped victims within the first 24 hours after the earthquake becomes critical for survival. In most instances, however, post-earthquake planning fails to quickly procure equipment needed to move heavy debris. The Local Hazard Mitigation Plan Section 6
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establishment of Heavy Urban Search and Rescue teams, as recommended by FEMA (1985), has improved victim extrication and survivability. Buildings that are more likely to fail and generate heavy debris need to be identified, so that appropriate mitigation and planning procedures are defined prior to an earthquake.

Reinforced Masonry Tilt-up Buildings
Tilt-up buildings have concrete wall panels, often cast on the ground, or fabricated off-site and trucked in, which are then tilted upward into their final position. Connections and anchors have pulled out of walls during earthquakes, causing the floors or roofs to collapse. A high rate of failure was observed for this type of construction in the 1971 San Fernando and 1987 Whittier Narrows earthquakes. Tilt-up buildings can also generate heavy debris. This type of construction is often seen in commercial and industrial “box” type buildings like strip malls because it is fairly quick and economical way to build a structure.

The City’s Seismic Retrofit Program identified 42 tilt-up buildings in Santa Monica in need of a seismic retrofit pursuant to the City’s Seismic Retrofit Ordinance. As of June 2023, 5 cases were closed as a result of Requests for Reconsiderations, 17 had been retrofitted in compliance with the requirements of the Seismic Retrofit Ordinance, and 22 had been forwarded to Code Enforcement where compliance cases had been opened. The remaining properties were seeking compliance either through an active building permit or demolition application, or are City-owned and under consideration for pursuing a path to completing retrofit. Specifically, one project was in plan review, four projects permits were in ‘Issued’ status, and seven tilt-up buildings were City-owned.

The City-owned properties were pending review of a structural analysis with a preliminary determination that further analysis was required for three buildings located at Memorial Park, two buildings at Reed Park, and a commercial building (Bank of America) at 1301 4th Street. No further structural analysis was recommended for one City-owned building at Virginia Park. The due date for the retrofit of tilt-up buildings was August 2020.

Reinforced Concrete-Frame Buildings
Reinforced concrete frame buildings, with or without reinforced infill walls, display low ductility and are thus also referred to as non-ductile concrete (NDC) buildings. Earthquakes may cause shear failure (due to large tie spacings in columns, or have insufficient shear strength), column failure (due to inadequate rebar splices, inadequate reinforcing of beam-column joints, or insufficient tie anchorage), hinge deformation (due to lack of continuous beam reinforcement), and non-structural damage (due to the relatively low stiffness of the frame). A common type of failure observed following the Northridge earthquake was confined column collapse (EERI, 1995), where infilling between columns confined the length of the columns that could move laterally in the earthquake. These types of buildings are common not only in southern California, but abroad.

Casualties as a result of the “pancake-style” collapse of these structures in past earthquakes, including Mexico (1985), Armenia (1988), Nicaragua (1972), El Salvador (1986 and 2001), the Philippines (1990), Turkey (1999), China (2008) and most recently, in Turkey and Syria (2023), add to hundreds of thousands, with millions of people displaced from their homes in these types of structures.

Santa Monica’s Seismic Retrofit Program Update, dated March 8, 2022, indicated that 66 NDC buildings constructed prior to 1977 were identified in the city as requiring a seismic retrofit in compliance with the City’s Seismic Retrofit Ordinance. Of these, 32 have been closed as a result of Requests for Reconsiderations and 3 have been retrofitted in compliance with the requirements Local Hazard Mitigation Plan Section 6
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of the Seismic Retrofit Ordinance. Plans for retrofit have been submitted to the City for three NDC buildings, and 17 have had their cases sent to Code Enforcement. The City’s expected retrofit completion due date for NDC buildings is October 2027.

Multi-Story Steel-Frame Buildings
Multi-story steel frame buildings generally have concrete floor slabs. However, these buildings are less likely to collapse than concrete structures. Common damage to these types of buildings is generally non-structural, including collapsed exterior curtain wall (cladding), and damage to interior partitions and equipment. Overall, modern steel frame buildings have been expected to perform well in earthquakes, but the 1994 Northridge earthquake broke many welds in these buildings, a previously unanticipated problem.

Older, pre-1945 steel frame structures may have unreinforced masonry such as bricks, clay tiles and terra cotta tiles as cladding or infilling. Cladding in newer buildings may be glass, infill panels or pre-cast panels that may fail and generate a band of debris around the building exterior (with considerable threat to pedestrians in the streets below). Structural damage may occur if the structural members are subject to plastic deformation, which can cause permanent displacements.

If some walls fail while others remain intact, torsion or soft-story problems may result.

Another category of steel frame structures is referred to as welded steel moment frame buildings (SMF). These are structures where the connections between the steel framing members were welded together (as opposed to bolted together). The welded connections lack ductility and when subjected to seismic shaking, are likely to be stressed and deformed, resulting in structural damage. Welding instead of bolting was accepted by building codes and standards in the 1950s and into the 1990s. Many steel moment-resisting frame structures experienced unexpected brittle damage to their beam-to-column connections as a result of the 1994 Northridge earthquake, prompting a change to the building code standards.

The City of Santa Monica requires the seismic retrofit of all SMF structures built prior to January 1996. The City has identified 80 such structures in Santa Monica as requiring a seismic retrofit in compliance with the City’s Seismic Retrofit Ordinance. As of June 2023, of the 80 SMFs, 11 were closed, and 2 retrofits had been completed in compliance with the requirements of the City’s Seismic Retrofit Ordinance. The plan submittal due date for SMFs is October 2029, with a retrofit completion due date of October 2037.

Mobile Homes
Mobile homes are prefabricated housing units that are placed on isolated piers, jackstands, or masonry block foundations (usually without any positive anchorage). Floors and roofs of mobile homes are usually of plywood, and outside surfaces are covered with sheet metal. Mobile homes typically do not perform well in earthquakes. Severe damage occurs when they fall off their supports, severing utility lines and piercing the floor with their jackstands. Loss estimation analyses typically show that a large percentage of the mobile homes in the southern California area are likely to experience more than moderate damage as a result of an earthquake on a local fault.

Often, when manufactured homes fall off their supports, gas line connections are often severed.

The resultant gas leaks can ignite, which can lead to the complete loss of the structure due to fire.

This happened more recently in several older mobile homes in Ridgecrest following the July 4 and July 5, 2019 earthquakes in the area (Maison and Martinez, 2020).

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There is only one mobile home park in the city of Santa Monica (Mountain View Mobile Home Park at 1930 Stewart Street) with approximately 70 residences, and this mobile home park is currently not in the City’s seismic retrofit program.

Other Conditions that Can Lead to Structural Failure
In addition to building types, there are other factors associated with the design and construction of the buildings that also have an impact on the structures’ vulnerability to strong ground shaking.

Two of these conditions are discussed below:

Building Shape
A building’s vertical and/or horizontal shape can be important. Simple, symmetric buildings generally perform better than non-symmetric buildings. During an earthquake, non-symmetric buildings tend to twist as well as shake. Wings on a building tend to act independently during an earthquake, resulting in differential movements and cracking. The geometry of the lateral loadresisting systems also matters. For example, buildings with one or two walls made mostly of glass, while the remaining walls are made of concrete or brick, are at risk. Asymmetry in the placement of bracing systems that provide a building with earthquake resistance, can result in twisting or differential motions.

Pounding
Site-related seismic hazards may include the potential for neighboring buildings to "pound," or for one building to collapse onto a neighbor. Pounding occurs when there is little clearance between adjacent buildings, and the buildings "pound" against each other as they deflect during an earthquake. The effects of pounding can be especially damaging if the floors of the buildings are at different elevations, so that, for example, the floor of one building hits a supporting column of the other. Damage to a supporting column can result in partial or total building collapse.

Critical Facilities and Infrastructure
Earthquake damage to the region’s critical facilities and infrastructure needs to be considered and planned for. Critical (or essential) facilities are those parts of a community's infrastructure that must remain operational after an earthquake. Critical facilities include hospitals, fire and police stations, emergency operation centers, communication centers, and schools. These structures can be subclassified into the following groups based on their every-day function, and the risk that they pose to the community should they fail.
• High-risk facilities (also referred to as high potential loss facilities) are those that if severely damaged, may result in a disaster far beyond the facilities themselves. Examples include power plants, dams and flood control structures, freeway interchanges, bridges, and industrial plants that use or store explosives, toxic materials or petroleum products. • High-occupancy facilities have the potential to result in a large number of casualties or crowdcontrol problems. This category includes high-rise buildings, large assembly facilities, and large multi-family residential complexes.
• Dependent-care facilities, such as preschools and schools, rehabilitation centers, prisons, group care homes, nursing homes, animal shelters, and other facilities that house populations with special evacuation considerations.

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• Economic facilities are those that should remain operational to avoid severe economic impacts, such as banks, archiving and vital record-keeping facilities, airports, and large industrial or commercial centers.
• Lifelines (also referred to as utility and transportation systems) are those services that are critical to the health, safety and functioning of the community. They are particularly essential for emergency response and recovery after an earthquake. Lifeline systems include water, sewage, electrical power, communication, transportation (highways, bridges, railroads, and airports), natural gas, and liquid fuel systems. Critical facilities that remain functional during and immediately following an earthquake may not be able to provide the functions that they are designed for if the lifelines that they depend on are disrupted.

It is crucial that critical facilities have no structural weaknesses that can lead to collapse. For example, seismic performance goals for health care facilities typically include the following:
• The damage to the facilities should be limited to what might be reasonably expected after a destructive earthquake and should be repairable and not be life-threatening. • Patients, visitors, and medical, nursing, technical and support staff within and immediately outside the facility should be protected during an earthquake. • Emergency utility systems in the facility should remain operational after an earthquake. • Occupants should be able to evacuate the facility safely after an earthquake. • Rescue and emergency workers should be able to enter the facility immediately after an earthquake and should encounter only minimum interference related to out-of-place nonstructural components. • The facility should be available for its planned disaster response role after an earthquake.

The improved performance of critical structures and lifelines in the 1994 Northridge earthquake relative to the 1971 San Fernando earthquake, shows that the seismic codes upgraded and implemented after 1971 were effective. Nevertheless, the impact of the Northridge quake on lifeline systems was widespread and illustrates the continued need to study earthquake impacts, upgrade substandard elements in the systems, provide redundancy in systems, improve emergency response plans, and provide adequate planning, budgeting and financing for seismic safety.

Some of the observations and lessons learned from the Northridge earthquake are summarized below (from Savage, 1995; Lund, 1996). These observations have led to improvements and redundancies that will hopefully reduce the impact of future earthquakes. Nevertheless, all lifeline providers should prioritize the evaluation, upgrade and maintenance of their systems, with an emphasis on seismic strengthening.
• Several electrical transmission towers were damaged or totally collapsed. Collapse was generally due to foundation distress in towers that were located near ridge tops where amplification of ground motion may have occurred. One collapse was the result of a seismically induced slope failure at the base of the tower. • Damage to above ground water tanks typically occurred where piping and joints were rigidly connected to the tank, due to differential movement between the tank and the piping. Older steel tanks not seismically designed under current standards buckled at the Local Hazard Mitigation Plan Section 6
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bottom (called “elephant’s foot”), in the shell, and on the roof. Modern steel and concrete tanks generally performed well.
• The most vulnerable components of pipeline distribution systems were older threaded joints, cast iron valves, cast iron pipes with rigid joints, and older steel pipes weakened by corrosion. In the case of broken water lines, the loss of fire suppression water forced fire departments to utilize water from swimming pools and tanker trucks. • Significant damage occurred in water treatment plants due to sloshing in large water basins.
• A number of facilities did not have an emergency power supply or did not have enough power supply capacity to provide their essential services. • Lifelines within critical structures, such as hospitals and fire stations, were found to be vulnerable. For instance, rooftop mechanical and electrical equipment is not generally designed for seismic forces. During the Northridge quake, rooftop equipment failed causing malfunctions in other systems.
• A 70-year-old crude oil pipeline leaked from a cracked weld, spreading oil for 12 miles down the Santa Clara River.
• A freight train carrying sulfuric acid was derailed causing an 8,000-gallon acid spill and a 2,000-gallon diesel spill from the locomotive.

RISK ANALYSIS
Risk analysis is the third phase of a hazard assessment. Risk analysis involves estimating the damage and costs likely to be experienced in a geographic area over a period of time. Factors that can be considered in assessing earthquake risk include population and property distribution in the hazard area; the frequency of earthquake events; surface fault rupture, liquefaction and landslide susceptibility; type, age and number of buildings and infrastructure elements; and disaster preparedness of the region. This type of analysis can generate estimates of the damages to the region due to an earthquake event in a specific location. FEMA's software program HazUS uses mathematical formulas and information about building stock, local geology and the location and size of potential earthquakes, economic data, and other information to estimate losses from a potential earthquake. HazUS loss estimations were conducted for the City of Santa Monica as part of this study in September and October 2023 using Version 6.0 of the software. This methodology, including its benefits and limitations, is discussed further in the following sub-section.

HazUS
HazUS (which stands for Hazard US) is a standardized methodology for earthquake loss estimation based on a geographic information system (GIS). The methodology is a project of the National Institute of Building Sciences and is funded by the Federal Emergency Management Agency (FEMA). The HazUS project developed guidelines and procedures to make standardized earthquake loss estimates at a regional scale. With standardization, estimates can be compared from region to region. HazUS is designed for use by state, regional and local governments in planning for earthquake loss mitigation, emergency preparedness, response and recovery. HazUS addresses nearly all aspects of the built environment, and many different types of losses. The methodology has been tested by comparing scenario results with actual losses generated by several past earthquakes.

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The loss estimates include physical damage to buildings of different construction and occupancy types, damage to essential facilities and lifelines, number of after-earthquake fires and damage due to fire, and the amount of debris that is expected. The model also estimates the direct economic and social losses, including casualties and fatalities for three different times of the day, the number of people left homeless and number of people that will require shelter, number of hospital beds anticipated to be available, and the economic losses due to damage to the places of businesses, loss of inventory, and business interruption.

Although the loss estimation products are extremely helpful in assessing and comparing losses across regions, or as a result of different hazards, which in turn helps to prioritize mitigation actions, the results must be used with discretion. Loss estimation analyzes casualties, damage and economic loss in great detail based on a series of empirically derived functions applied to datasets that have some inherent limitations.

Some of the basics, assumptions and limitations in the methodology include:
• The basic level of analysis relies on databases built into the HazUS software that are based on generalized building square footage and value, population characteristics, costs of building repair, and basic economic data. Although these databases are based on the best available scientific, engineering, census and economic knowledge, there are inherent uncertainties. For example, there is incomplete scientific knowledge concerning earthquakes and their effects on buildings and facilities, and the software relies on approximations and simplifications to generate the analyses. As a result, the loss estimates may have a significant level of uncertainty. More accurate results may be possible if the basic databases are replaced with user-provided datasets, but this requires an extensive, time- and resource-consuming effort.
• HazUS is driven by statistics, and thus is most accurate when applied to a region, or a class of buildings or facilities. It is least accurate when considering a particular site, building or facility. Census tracts are the smallest regional units considered, with results averaged across individual census tracts. Ground motion demand for the general building stock is averaged over each census tract.
• The results of the building damage estimates are used to calculate the estimated number of casualties, including fatalities; monetary losses, including both cost to repair the structures and business interruption losses; and societal impacts, such as shelter needs. • Casualty estimates are computed from the structural damage output and do not include casualties that might occur during or after the earthquake that are not related to building damage (such as heart attacks, for example).
• The total system performance of the transportation system is not computed. • The baseline inventory database in HazUS includes an estimate of the potable water distribution pipeline length aggregated at the Census tract level, with 80% of the pipelines assumed to be brittle, and 20% ductile. For the wastewater distribution system, 60% of the pipelines are assumed to be brittle, and the remaining 40% are assumed to be ductile. For the gas distribution system, 10% of the pipes are assumed to be brittle; the rest are assumed to be ductile. For all of these systems, the methodology assumes that damage due to seismic waves will result in 80% leaks and 20% breaks, and that damage due to ground failure will result in 20% leaks and 80% breaks.
• The methodology does not consider the impact that failure of one lifeline system may have on another (cascading effects).

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• The methodology does not conduct a detailed system performance of the utility system. • Functionality estimates are based solely on damage to the buildings/structures and do not take emergency response or contingency plans into consideration. Thus, a hospital may be able to continue functioning by re-purposing their parking lots into treatment areas, but the software does not take that into consideration. Functionality estimates also do not consider damage to utilities or cascading effects.
• Shelter requirements are based on income of the displaced households, with 62% of the displaced households making less than $10,000 a year assumed to need shelter, and up to 13% of the displaced households making more than $40,000 per year assumed to need shelter. Given California’s high cost of living, use of these national income values for the HazUS scenarios likely underestimate the number of households that would seek shelter following an earthquake on a local fault.
• Loss of power and water are not considered in the estimates of short-term shelter needs.

The model also does not account for shelter needs in response to after-earthquake fire, dam failure inundation, or releases of hazardous materials. • The model does not consider the long-term impacts of housing damage on shelter needs.

The HazUS methodology produces seemingly precise numbers that can be easily misinterpreted.

Loss estimation's results, for example, may cite 454 left homeless by a scenario earthquake. This is best interpreted by its magnitude. That is, an event that leaves 400 people homeless is clearly more manageable than an event causing 4,000 or 40,000 homeless people; and an event that leaves 400,000 homeless would overwhelm the state’s resources. However, another loss estimation that predicts 600 people homeless should probably be considered equivalent to the 454 result. Because HazUS results make use of a great number of parameters and data of varying accuracy and completeness, it is not possible to assign quantitative error bars. Although the numbers should not be taken at face value, they are not rounded or edited because detailed evaluation of individual components of the disaster can help mitigation agencies ensure that they have considered all important options.

HazUS breaks critical facilities into two groups: essential facilities and high potential loss (HPL) facilities. Essential facilities provide services to the community and should be functional after an earthquake. Essential facilities include hospitals, schools, fire stations, police stations and emergency operations facilities. The essential facility module in HazUS determines the expected loss of functionality for these facilities. The damage probabilities for essential facilities are determined on a site-specific basis (i.e., at each facility). Economic losses associated with these facilities are computed as part of the analysis of the general building stock. Data required for the analysis include occupancy classes (current building use) and building structural type, or a combination of essential facilities building type, design level and construction quality factor. High potential loss facilities include dams, levees, military installations, nuclear power plants and significant hazardous material sites.

HazUS divides the lifeline inventory into two systems: 1) transportation and 2) utility lifelines. The transportation system includes seven components: highways, railways, light rail, bus, ports, ferry and airports. The utility lifelines include potable water, wastewater, natural gas, crude and refined oil, electric power and communications. If site-specific lifeline utility data are not provided for these analyses, HazUS performs a statistical calculation based on the population served.

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General Building Stock Type and Classification: HazUS provides damage data for buildings based on these structural types:
• Concrete
• Mobile (manufactured) home
• Precast concrete
• Reinforced-masonry bearing walls
• Steel
• Unreinforced-masonry bearing walls, and
• Wood frame
and based on these occupancy (usage) classifications:
• Residential (single-family and other residential)
• Commercial
• Industrial
• Agriculture
• Religion
• Government, and
• Education
As mentioned above, loss estimation for the general building stock is averaged for each census tract. Building damage classifications range from slight to complete. As an example, the building damage classification for light wood frame buildings is provided below. Wood-frame structures comprise the most numerous building type in Santa Monica.

Wood, Light Frame:
• Slight Structural Damage: Small plaster or gypsum-board cracks at corners of door and window openings and wall-ceiling intersections; small cracks in masonry chimneys and masonry veneer.
• Moderate Structural Damage: Large plaster or gypsum-board cracks at corners of door and window openings; small diagonal cracks across shear wall panels exhibited by small cracks in stucco and gypsum wall panels; large cracks in brick chimneys; toppling of tall masonry chimneys.
• Extensive Structural Damage: Large diagonal cracks across shear wall panels or large cracks at plywood joints; permanent lateral movement of floors and roof; toppling of most brick chimneys; cracks in foundations; splitting of wood sill plates and/or slippage of structure over foundations; partial collapse of "room-over-garage" or other "softstory" configurations; small foundations cracks. • Complete Structural Damage: Structure may have large permanent lateral displacement, may collapse, or be in imminent danger of collapse due to cripple wall failure or failure of the lateral load resisting system; some structures may slip and fall off the foundations; large foundation cracks.

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HazUS estimates two types of debris. The first is debris that falls in large pieces, such as steel members or reinforced concrete elements. These require special treatment to break into smaller pieces before they are hauled away. The second type of debris is smaller and more easily moved with bulldozers and other machinery and tools. This type includes brick, wood, glass, building contents and other materials.

Casualties are estimated based on the observation that there is a strong correlation between building damage (both structural and non-structural) and the number and severity of casualties. In smaller earthquakes, non-structural damage, (such as toppled bookshelves and broken windows) is typically responsible for most of the casualties. In severe earthquakes where there is a large number of collapses and partial collapses, there is a proportionately larger number of fatalities.

Data regarding earthquake-related injuries are, however, not of the best quality, nor are they available for all building types. Available data often have insufficient information about the type of structure in which the casualties occurred and the casualty-generating mechanism. HazUS casualty estimates are based on the injury classification scale described in Table 6-4.

Table 6-4: Injury Classification Scale
Injury Severity Level Injury Description
Severity 1 Injuries requiring basic medical aid without requiring hospitalization.

Severity 2
Injuries requiring a greater degree of medical care and hospitalization, but not expected to progress to a life-threatening status.

Severity 3
Injuries which pose an immediate life-threatening condition if not treated adequately and expeditiously. The majority of these injuries are the result of structural collapse and subsequent entrapment or impairment of the occupants.

Severity 4 Instantaneously killed or mortally injured.

The Severity 1 and Severity 2 casualties are most likely related to people running outside and, in the process, bumping into overturned furniture, walking barefoot on broken glass, and otherwise being hurt by non-structural elements, and by structural damage to residential structures and manufactured housing. Severity 3 and Severity 4 casualties are anticipated as a result of damage to residential structures other than single-family housing, and damage to commercial and industrial buildings.

In addition, HazUS produces casualty estimates for three times of day:
• Earthquake striking at 2:00 a.m. (most population at home) • Earthquake striking at 2:00 p.m. (most population at work/school) • Earthquake striking at 5:00 p.m. (commuting time)
Displaced Households/Shelter Requirements – Earthquakes can cause loss of function or habitability of buildings that contain housing. Displaced households may need alternative shortterm shelter provided by family, friends, temporary rentals, or public shelters established by the Local Hazard Mitigation Plan Section 6
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City, County or by relief organizations such as the American Red Cross. Long-term alternative housing may require import of mobile homes, occupancy of vacant units, net emigration from the impacted area, or, eventually, the repair or reconstruction of new public and private housing. The number of people seeking short-term public shelter is of most concern to emergency response organizations. The longer-term impacts on the housing stock are of great concern to local governments, such as cities and counties. As mentioned above, however, the software does not calculate the long-term shelter needs.

Economic Losses – HazUS estimates structural and nonstructural repair costs caused by building damage and the associated loss of building contents and business inventory. Building damage can cause additional losses by restricting the building's ability to function properly. Thus, business interruption and rental income losses are estimated. HazUS divides building losses into two categories: (1) direct building losses and (2) business interruption losses. Direct building losses are the estimated costs to repair or replace the damage caused to the building and its contents.

Business interruption losses are associated with inability to operate a business because of the damage sustained during the earthquake. Business interruption losses also include the temporary living expenses for those people displaced from their homes because of the earthquake. HazUS does not calculate business interruption losses due to failure of the lifeline systems (such as electric power outages).

Earthquakes may produce indirect economic losses in sectors that do not sustain direct damage.

All businesses are forward-linked (if they rely on regional customers to purchase their output) or backward-linked (if they rely on regional suppliers to provide their inputs) and are thus potentially vulnerable to interruptions in their operation. Note that indirect losses are not confined to immediate customers or suppliers of damaged enterprises. All of the successive rounds of customers of customers and suppliers of suppliers are affected. In this way, even limited physical earthquake damage causes a chain reaction, or ripple effect, that is transmitted throughout the regional economy.

HazUS Earthquake Scenarios for the Santa Monica Area
HazUS relies on census data, which are reported by geographical areas or tracts. The City of Santa Monica is defined by nineteen census tracts that combined match the city boundaries (see Figure 6-15). We used the population counts provided by the HazUS (version 6.0) 2020 database, which lists a population of 93,043 people in the 8.41 square mile area covered by the city. This number is slightly smaller (0.04% less) than the 2020 US Census figure of 93,076, and slightly larger (by

3.4%) than the American Communities 2022 population estimate for Santa Monica of 89,9476F7.

7 https://www.census.gov/quickfacts/fact/table/santamonicacitycalifornia/PST045222; accessed September 2023- March 2024.

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Figure 6-15: Census Tracts Used in the HazUS Analyses
(for a scaled, larger version of this map, refer to Plate H-6 in Appendix H) Four earthquake scenarios were modeled:
1) an earthquake on the Santa Monica fault that incorporates the Malibu fault to the west, and the Hollywood fault to the east, with the epicenter occurring in the Santa Monica Mountains, in the area of Universal City (see ShakeMap for this scenario on Figure 6-5) 2) an earthquake along the full-length of the Palos Verdes fault, from offshore Santa Monica southward to near the international border with Mexico. This scenario was used for a tsunami analysis (see Section 7), but the earthquake-related loss estimations are provided in this section, as a comparison with the other three earthquakes. 3) an earthquake on the onshore section of the Newport Inglewood fault, with the epicenter occurring near the southern end of the onshore section, in Orange County, and the fault unzipping northward (see ShakeMap for this scenario on Figure 6-6) 4) an earthquake on the southern San Andreas fault rupturing the southernmost section of the fault (the ShakeOut scenario prepared by the U.S. Geological Survey in the Fall of 2008 (Jones et al., 2008) (see the ShakeMap for this scenario on Figure 6-10) Specifics about each of these earthquake-producing faults were provided in the seismic sources section above, and in Table 6-5.

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Table 6-5: HazUS Earthquake Scenarios for the City of Santa Monica Fault Source Magnitude Description
Santa Monica 7.4
Low probability but high-risk earthquake event on a westtrending left-lateral oblique fault zone mapped through the northern portion of the city and extending eastward as the Hollywood fault, and westward as the Malibu fault. The fault can generate strong ground shaking and ground deformation in the form of surface fault rupture and folding in Santa Monica.

Palos Verdes 7.7
Medium probability, high-risk event on the fault that extends from the Santa Monica Bay in a southeastward direction
through the eastern edge of the Palos Verdes peninsula and then offshore to near the border with Mexico. Given the length of the fault, the earthquake would be large. Both strong ground shaking and a potential tsunami would impact the city of Santa Monica. This event would be devastating to the regional economy as it crosses both the Los Angeles and Long Beach ports.

NewportInglewood 7.2
Low to medium probability, medium to high-risk event on a northwest-trending right-lateral strike-slip event on the onshore Newport-Inglewood fault located near to, but outside City limits. This fault is considered to pose a significant hazard the Los Angeles basin.

Southern San
Andreas 7.8
High probability large earthquake that ruptures the entire southern San Andreas fault using the USGS ShakeOut scenario, with the epicenter near the Salton Sea, and the fault rupturing northward. Intense shaking of long duration is anticipated to rock the Los Angeles basin, with potentially devastating impacts to tall buildings and infrastructure, such as bridges. The impact to the region is expected to be substantial7F
8 .

Of the four earthquake scenarios modeled for the city, the results indicate that both a Mw 7.4 earthquake on the Santa Monica fault or a Mw 7.7 earthquake on the Palos Verdes fault have the potential to cause more damage in Santa Monica than a larger, but more distant earthquake on the San Andreas fault, or a not so-distant earthquake on the Newport-Inglewood fault. Specifics regarding the anticipated damage as a result of these four earthquake sources are summarized in the sub-sections below.

Building Damage
The HazUS database estimates that there are about 27,500 buildings in the city, with a total building replacement value (excluding contents) of nearly $19,682 million ($19,682,000,000).

Approximately 84% of the buildings considered in the analysis (and 69% of the building value, see Table 6-6) are associated with residential housing. In terms of building construction types found in the region, wood-frame construction makes up approximately 82% of the building inventory; the remaining 18% is distributed between the other general building types. Table 6-7 shows the estimated number of buildings by construction type anticipated to be damaged by the modeled

8 https://pubs.usgs.gov/of/2008/1150/
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earthquake scenarios.

Table 6-6: Building Exposure by Occupancy Type for the Study Area Occupancy Exposure
(in thousands of $) Percent of Total
Residential 13,512,940 68.7
Commercial 4,488,462 22.8
Industrial 672,168 3.4
Agricultural 7,255 0.0
Religion 200,401 1.0
Government 79,341 0.4
Education 721,343 3.7
19 681 910 Totals 19,681,910 100.0
Table 6-7: Number of Buildings* Damaged, by Construction Type * Based on an estimated total of about 27,500 buildings in the region.

Scenario Structure Type None Slight Moderate Extensive Complete Santa Monica Wood 7,808 9,527 4,470 463 142
Steel 102 145 347 313 144
Concrete 255 309 322 211 124
Precast 129 162 278 169 73
Reinforced Masonry 450 325 421 242 94
Unreinforced Masonry 47 71 116 82 77
Manufactured Housing 1 4 20 31 21
Totals 8,792 10,543 5,974 1,511 675
Palos Verdes
Wood 16,753 2,355 696 77 26
Steel 846 69 123 95 70
Concrete 923 98 105 59 20
Precast 658 48 86 87 47
Reinforced Masonry 2,689 289 436 357 105
Unreinforced Masonry 247 18 19 8 3
Manufactured Housing 78 1 2 1 0
Totals 22,194 2,878 1,467 684 271
Newport-Inglewood
Wood 14,401 6,464 1,457 71 17
Steel 499 233 237 70 12
Concrete 646 294 196 72 13
Precast 409 181 167 49 6
Reinforced Masonry 984 265 214 64 4
Unreinforced Masonry 185 101 77 24 6
Manufactured Housing 14 19 28 14 2
Totals 17,138 7,557 2,376 364 60
San Andreas
Wood 22,022 371 16 0 0
Steel 1,007 32 11 1 0
Concrete 1,186 30 6 0 0
Precast 778 24 9 1 0
Reinforced Masonry 1,496 25 9 1 0
Unreinforced Masonry 368 18 6 1 0
Manufactured Housing 68 6 3 0 0
Totals 26,925 506 60 4 0
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The HazUS models estimate that between about 570 and 18,703 buildings in Santa Monica will be at least slightly damaged in response to the earthquake scenarios modelled, with the lower number representative of damage as a result of an earthquake on the San Andreas fault, and the higher number representing damage as a result of an earthquake on the Santa Monica fault. These figures represent about 2 and 68 percent, respectively, of the total number of buildings considered in the analysis.

Similarly, the models estimate that between about 64 and 8,160 structures in Santa Monica will be at least moderately damaged by these earthquake scenarios. These figures represent about 0.2 and 30 percent, respectively, of the total number of buildings considered, with the lowest number representative of the damage as a result of the San Andreas fault earthquake scenario, and the higher number representing damage as a result of the Santa Monica fault earthquake scenario. The models also estimate that approximately 675 buildings in Santa Monica will be damaged beyond repair by an earthquake on the Santa Monica fault. That amounts to about 2 percent of the entire building stock in the city. The modeled Palos Verdes and Newport-Inglewood fault earthquakes are estimated to damage beyond repair about 236 and 60 buildings, respectively, in Santa Monica.

Given that wood-frame buildings comprise the most common construction type in Santa Monica, the largest number of buildings in the city damaged by the earthquake models are wood-frame structures. Specifically, the models indicate that approximately 14,602 wood-frame buildings in the city will be at least slightly damaged by a Santa Monica fault earthquake event. This amounts to 65 percent of the total number of wood-frame buildings in Santa Monica. The Palos Verdes and Newport-Inglewood earthquake scenarios return estimates of 2,743 and 8,009 wood-frame buildings, respectively, that are at least slightly damaged. These numbers amount to about 16 and 36 percent, respectively, of the total number of wood-frame buildings in the city. The San Andreas ShakeOut earthquake scenario is estimated to result in at least slight damage to about 386 woodframe structures, which amounts to about 2 percent of the more than 22,000 wood-frame structures present in the study area.

However, as a percentage of total number of buildings of each construction type, the models suggest that an earthquake on the Santa Monica fault will cause at least slight damage to nearly 99 percent of the manufactured houses in the city, and at least slight damage to 90 percent of the steel-frame buildings. Similarly, about 88, 84 and 79 percent of the unreinforced masonry, pre-cast and concrete structures, respectively, will be at least slightly damaged by that earthquake scenario.

Even the distant San Andreas ShakeOut scenario is anticipated to at least slightly damage about 12 percent of the manufactured homes stock, 6 percent of the unreinforced masonry buildings, and 4 percent of the steel and pre-cast concrete structures in Santa Monica. Based on these data, woodframe buildings are actually the safest building type in the city, consistent with observations made in many previous earthquakes that have found that wood-frame structures perform well during earthquakes8F
9 . Reasons for this include that wood-frame buildings are lightweight, ductile but strong, and structurally redundant (meaning that that the hundreds of structural elements and thousands of nail connections can and do compensate if adjacent members and joints fail). Woodframe structures in particular perform quite well in an earthquake caused by a distant fault because wood has the flexibility necessary to withstand the long-period waves associated with such an earthquake, unlike other less-ductile construction types. This in part also explains why the earthquake scenarios other than the San Andreas fault result in a lower percentage of damaged wood-frame structures.

9 https://cwc.ca/wp-content/uploads/publications-BP5_WoodFramesAndEarthquakes.pdf, accessed April 2, 2024.

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Table 6-8 below summarizes the modeled damage to buildings in Santa Monica by occupancy type based on the four earthquake scenarios considered.

Table 6-8: Number of Buildings* Damaged, by Occupancy Type Scenario Occupancy Type Slight Moderate Extensive Complete Total Santa Monica
Commercial 806 1,051 636 320 2,813
Education 21 19 9 4 53
Government 8 10 7 3 28
Industrial 105 161 105 58 429
Single-Family Residential 6,944 3,218 298 76 10,536
Other Residential 2,611 1,465 425 198 4,699
Agriculture 7 7 4 2 20
Religion 41 43 27 14 125
Total 10,543 5,974 1,511 675 18,703
Palos Verdes
Commercial 306 367 308 116 1,097
Education 7 6 4 1 18
Government 3 4 4 2 13
Industrial 45 63 63 28 199
Single-Family Residential 1,780 625 83 44 2,532
Other Residential 724 378 200 71 1,373
Agriculture 2 2 2 1 7
Religion 14 15 13 6 48
Total 2,881 1,460 677 269 5,287
Newport-Inglewood
Commercial 817 607 167 26 1,617
Education 17 10 3 0 30
Government 8 6 2 1 16
Industrial 119 105 34 6 264
Single-Family Residential 4,649 1,008 44 10 5,711
Other Residential 1,903 612 106 16 2,637
Agriculture 6 4 1 0 11
Religion 38 24 7 1 70
Total 7,557 2,376 364 60 10,356
San Andreas
Commercial 87 23 2 0 112
Education 1 0 0 0 1
Government 1 0 0 0 1
Industrial 16 6 1 0 23
Single-Family Residential 264 12 0 0 276
Other Residential 132 18 1 0 151
Agriculture 1 0 0 0 1
Religion 4 1 0 0 5
Total 506 60 4 0 570
* Based on a total of about 27,500 buildings in the region.

As a percentage of building damage by occupancy type, the models estimate that about 66 percent of the residential structures, including single-family homes and others (i.e., multi-family residential buildings, including duplexes, condominiums and apartments, and manufactured homes) will suffer at least slight damage as a result of a magnitude 7.4 earthquake on the Santa Monica fault; more than 24 percent will have at least moderate damage. Nearly 84 percent of the industrial structures and about 80 percent of the commercial structures in the city will be at least slightly Local Hazard Mitigation Plan Section 6
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damaged by an earthquake on the Santa Monica fault; 63 and 57 percent, respectively will suffer at least moderate damage. These figures indicate that an earthquake on the Santa Monica fault will pose a significant economic impact to the city and its residents.

The Palos Verdes fault earthquake scenario is estimated to result in at least slight damage to about 17 percent of the residential structures in the city, with about six percent experiencing moderate to complete damage. About 31 percent of the commercial structures and about 38 percent of the industrial structures are also expected to be at least slightly damaged by this earthquake event.

Twenty-three percent of the commercial structures and nearly 30 percent of the industrial structures in the city are estimated to experience moderate to complete damage as a result of large-magnitude earthquake on the Palos Verdes fault.

The Newport-Inglewood earthquake scenario is estimated to result in at least slight damage to about 36 percent of the residential structures in the city, with about 8 percent experiencing moderate to complete damage. About 46 percent of the commercial structures and 52 percent of the industrial structures are also expected to be at least slightly damaged by this earthquake event.

The models also suggest that 23 percent of the commercial structures and 28 percent of the industrial structures in the city will experience moderate to complete damage as a result of the Newport-Inglewood fault scenario.

The San Andreas ShakeOut scenario, on the other hand, is modeled to result in at least slight damage to less than 2 percent of the residential structures in the city, with about 3 percent of the commercial and nearly 4.5 percent of the industrial buildings modeled to be at least slightly damaged by a ShakeOut-type earthquake. These numbers drop to about 0.7 and 1.4 percent, respectively, for commercial and industrial structures experiencing moderate to extensive damage.

None of the structures in Santa Monica are expected to be completely damaged by this earthquake scenario.

The citywide distribution, by census tract, of the damage expected to residential structures in Santa Monica as a result of the four earthquake scenarios modeled using HazUS is shown on Figure 6-16. The percentages shown for each tract are calculated using the estimated total residential building loss (in dollars) divided by the total residential building exposure (in dollars). The maps show that the Santa Monica fault earthquake scenario would do more damage to the central and northern portions of the city closer to the Santa Monica fault, where both surface fault rupture and strong ground shaking would be expected. An earthquake on the offshore Palos Verdes fault is expected to cause most damage in the western portion of the city closest to that fault. An earthquake on the Newport-Inglewood fault, which is located to the southeast of Santa Monica, is expected to cause a higher percentage of damage to structures on the eastern and central portions of the city. The San Andreas fault is far enough from Santa Monica that damage as a result of that earthquake is approximately equal and relatively low throughout the city.

Figure 6-17 show the regional distribution of the damage to commercial structures as a percentage of total commercial building exposure by census tract. The regional impact to commercial structures by each earthquake scenario generally matches the regional distribution of damaged residential structures shown on Figure 6-16. Comparison of the two figures shows that the earthquake scenarios will do more damage to commercial structures than to residential buildings, consistent with the findings presented in the paragraphs above. Note that the same color scheme was used for both figures, with darker shades of a given color indicating a higher percentage of building damage.

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Figures 6-16 and 6-17 in particular show that a 7.4 magnitude earthquake on the Santa Monica fault is the worst-case earthquake scenario for the city of Santa Monica, with a relatively high percentage of damaged residential structures and a significantly higher percentage of damaged commercial structures. The Palos Verdes fault has the potential to cause substantial damage in the city too, followed by the Newport-Inglewood fault in terms of severity. Although not modeled for this study, an earthquake on the section of the Compton buried thrust fault that underlies the city may result in damage similar to that modeled from an earthquake on the Palos Verdes fault. The data presented here shows that the San Andreas fault earthquake scenario, although most likely, is anticipated to result in only minor damage in Santa Monica. This does not consider, however, the impacts that rupture of the San Andreas fault is expected have on the southern California region as a whole. For a description of the anticipated damage to southern California as a result of the ShakeOut scenario, the reader is encouraged to read the report by Jones et al. (20089F 10).

10 https://pubs.usgs.gov/publication/ofr20081150
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Figure 6-16: Total Residential Building Related Losses as a Percentage of Total Residential Building Exposure Local Hazard Mitigation Plan Section 6
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Figure 6-17: Total Commercial Building Related Losses as a Percentage of Total Commercial Building Exposure Local Hazard Mitigation Plan Section 6
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Total Economic Losses: Sum of Building-Related and Lifeline-Related Losses HazUS breaks down building-related losses into two categories: 1) direct building losses and 2) business interruption losses. Direct building losses (capital stock losses in Table 6-9) are the estimated costs to repair or replace the structural and non-structural damage caused to a building and its contents, and in the case of businesses, damage to inventory. Income losses, or businessinterruption losses, are related to the inability to operate a business because of the damage it sustained during the earthquake. Business interruption loss estimates also include the temporary living expenses for those people displaced from their homes because of the earthquake. Income losses, however, do not include losses related to the inability to operate the business because of lifeline outages or damage to the transportation network limiting access to a business.

Table 6-9 shows the building-related economic loss estimates (in $ millions) calculated by HazUS for each of the four earthquake scenarios considered for this study. The models estimate that total building-related losses in the city of Santa Monica will range from about $31 million for an earthquake on the San Andreas fault, to $3,414 million for an earthquake on the Santa Monica fault. Between 14 and 23 percent of these estimated losses are related to business interruption in the city, with loss in wages and relocation expenses amounting to about 60 percent of the total income losses. An earthquake on the Santa Monica fault is estimated to be at least 2.3 times costlier than the Palos Verdes fault earthquake scenario, and about 3.4 times costlier than an earthquake on the Newport-Inglewood fault.

As a function of dollar loss, damage to residential occupancies (single-family and other residential structures combined) accounts for between about 42 and 45 percent of the total building-related losses estimated for each earthquake scenario. The estimated losses for commercial occupancies also account for between 39 and 45 percent of the total building-related economic losses.

The total economic losses estimated for the city of Santa Monica include the building-related losses presented in Table 6-9, combined with the lifeline-related losses estimated for the area (presented in Table 6-10). The lifeline-related losses are the costs of repairing or replacing the damaged transportation and utility systems. The transportation system includes highways, railways, light rail, ferry facilities, port facilities, and the airport. The utility system includes potable and waste water, natural gas, oil systems, electrical power and communication. The losses to the transportation and lifeline systems do not include losses related to business interruption as a result of lifeline outages.

Based on the data presented in Table 6-10, an earthquake on the Santa Monica fault is anticipated to cause slight damage to the bridges and tunnels that are part of the highway system, with a loss ratio of about 6.25 percent of the inventory value. Combined damage to the light rail and airport facilities in the city is estimated to cost about $9.94 million; this amounts to a 32.6 percent loss ratio. Significantly, the airport runways are not damaged by this earthquake scenario. Economic losses related to the utility system are dominated by damage to the potable water and waste water distribution lines. Although the estimated losses to the electrical power system are shown as zero, other data provided by HazUS and discussed later in this report suggest that the electrical power system is damaged extensively by a Santa Monica fault earthquake scenario. The loss estimates to the transportation and lifeline systems as a result of other earthquake scenarios are only a small fraction of that estimated for the Santa Monica earthquake. Examining Santa Monica only, the HazUS results for the Newport-Inglewood fault scenario report no damage to the lifelines system; these results are suspect, however, with numbers similar to those generated by either the Palos Verdes or San Andreas earthquake scenarios considered more likely.

Local Hazard Mitigation Plan Section 6
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Table 6-9: Building-Related Economic Loss Estimates (in Million $) Area SingleFamily
Other
Residential Commercial Industrial Others Total
Santa Monica Fault Earthquake Scenario
Income
Losses
Wage 0.00 23.0956 146.1667 2.8905 6.3086 178.4614
Capital-Related 0.00 9.8164 135.3765 1.7034 2.1290 149.0253 Rental 8.2771 54.051 63.4010 1.4165 3.4982 130.6438
Relocation 30.8467 30.9302 96.2093 6.6907 29.4593 194.1362 Subtotal 39.1238 117.8932 441.1535 12.7011 41.3951 652.2667 Capital
Stock
Losses
Structural 66.9854 85.3777 182.4707 27.2748 26.4824 388.5910 Non-Structural 341.3432 548.1378 610.6316 95.1211 113.8058 1,709.0395 Content 107.6299 130.0402 271.5938 60.6893 54.3210 624.2742 Inventory 0.00 0.00 30.6030 8.9174 0.3749 39.8953
Subtotal 515.9585 763.5557 1,095.2991 192.0026 194.9841 2,761.80 Total 555.08 881.45 1,536.45 204.70 236.38 3,414.07
Newport-Inglewood Fault Earthquake Scenario
Income
Losses
Wage 0.00 4.5963 40.9619 0.8456 1.8698 48.2736
Capital-Related 0.00 1.9534 37.5366 0.5007 0.6259 40.6166 Rental 2.2646 13.6755 19.4119 0.4982 1.0025 36.8527
Relocation 7.9477 8.1903 28.8340 2.6097 8.3059 55.8876
Subtotal 10.2123 28.4155 126.7444 4.4542 11.8041 181.6305 Capital
Stock
Losses
Structural 21.0865 22.4195 46.1067 7.9588 6.7911 104.3626 Non-Structural 119.1254 169.2796 163.0665 27.5616 32.8862 511.9193 Content 40.1606 45.1788 81.0080 18.3410 18.9053 203.5937 Inventory 0.00 0.00 9.0198 2.6685 0.1231 11.8114
Subtotal 180.3725 236.8779 299.2010 56.5299 58.7057 831.6870 Total 190.58 265.29 425.95 60.98 70.51 1,013.32
Palos Verdes Fault Earthquake Scenario*
Income
Losses
Wage 0 16 64 1 77 158
Capital-Related 0 7 67 1 8 83
Rental 3 23 15 0 2 43
Relocation 10 13 23 1 7 54
Subtotal 13 59 169 3 94 338
Capital
Stock
Losses
Structural 28 40 44 4 10 126
Non-Structural 114 265 185 17 43 624
Content 47 97 170 20 39 373
Inventory 0 0 11 1 0 12
Subtotal 189 402 410 42 92 1,135
Total 202 461 579 45 186 1,473
San Andreas Fault Earthquake Scenario
Income
Losses
Wage 0.000 0.0703 0.9804 0.0237 0.0583 1.1327
Capital-Related 0.000 0.0298 0.9242 0.0140 0.0194 0.9874 Rental 0.0385 0.3332 0.6060 0.0183 0.0232 1.0192
Relocation 0.0845 0.1690 0.6860 0.0949 0.1690 1.2034
Subtotal 0.1230 0.6023 3.1966 0.1509 0.2699 4.3427
Capital
Stock
Losses
Structural 0.6458 0.6506 1.2045 0.2548 0.1660 2.9217
Non-Structural 3.6565 5.2220 5.7821 1.1030 1.2480 17.0116 Content 0.9811 1.2065 2.8418 0.6886 0.6849 6.4029
Inventory 0.000 0.000 0.3052 0.0993 0.0041 0.4086
Subtotal 5.28349 7.0791 10.1336 2.1457 2.1030 26.7448
Total 5.41 7.68 13.33 2.30 2.37 31.09
• The loss estimates for the Palos Verdes fault scenario were generated using the combined Tsunami and Earthquake HazUS module; the results provided were rounded to the closest whole integer.

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Table 6-10: Estimated Transportation and Utility System Economic Losses System
Components Inventory Value
(millions of
dollars)
Economic Loss (millions of dollars) Per Earthquake Scenario Transportation
Santa Monica
Fault
NewportInglewood FaultPalos Verdes
Fault
San Andreas
Fault
Highway 1,190.5 7.6831 1.8057 0.95 0.158
Railway 376 0 0 0 0
Light Rail 512 5.3539 3.4285 0 0.6613
Ferry 0 0 0 0 0
Port 7.62 0 1.3635 1.86 0.2419
Airport 24.76 4.5818 2.9553 0 0.5615
Sub-Total 2,110.60 17.62 9.55 2.80 1.48
Lifelines
Potable Water 7.7958 1.5901 0 0.23 0.3329
Waste Water 4.6775 0.7988 0 0.11 0.1672
Natural Gas 86.2719 0.2737 0 0.04 0.0573
Oil Systems 0 0 0 0 0
Electrical Power 0 0 0 0 0
Communication 0 0 0 0 0
Sub-Total 98.75 2.66 0 0.38 0.56
Total Building-Related Losses
(from Table 6-9) 3,414.07 1,013.22 1,473 31.09
Total Economic Losses 3,434.35 1,022.87 1,476.18 33.13
Casualties
Table 6-11 provides a summary of the casualties estimated for the earthquake scenarios on the Santa Monica, Palos Verdes, Newport-Inglewood and San Andreas faults. In all cases, the models indicate that the highest number of casualties are expected to occur as a result of an earthquake during the day, during maximum occupancy of commercial, industrial and educational facilities.

These numbers reflect the higher damage, as a percentage, to construction types other than woodframed, and to occupancy types other than residential, as described in the section above. More specifically, steel, concrete, precast, reinforced masonry and unreinforced masonry construction types are more often used for commercial, industrial, and educational purposes, and since these types of construction are expected to not perform well overall, the highest number of casualties are associated with the time of the day when these types of buildings are more likely to be occupied.

An earthquake on the Santa Monica fault occurring around 2 PM is estimated to result in more than 1,000 people being hurt enough to need medical treatment, although with no need for hospitalization (Level 1). The same earthquake scenario is estimated to result in approximately 275 people hurt with non-life-threatening injuries that do require hospital care (Level 2), and 44 people hurt with life-threatening injuries (Level 3). The number of injured people requiring hospitalization could overwhelm the local hospitals, assuming that when the earthquake occurs, 50 to 80 percent of the 531 total hospital beds available in the study area are already occupied by in-care patients.

Approximately 85 people in the city are estimated to die as a result of a Santa Monica fault earthquake in response to building damage alone. The next-most damaging earthquake in the city in terms of casualties is an event occurring during peak commute time, which HazUS models at 5 PM. The least number of casualties are attributed to an earthquake in the early morning hours (2 AM), when most people are at home.

Earthquakes on the Palos Verdes and Newport-Inglewood faults are estimated to result in similar numbers of casualties if only earthquake damage is considered (an earthquake on the offshore Local Hazard Mitigation Plan Section 6
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2025 Earthquakes Page 6-72
section of the Palos Verdes in Santa Monica Bay could generate a tsunami that could result in significant casualties along the coastline; tsunami-generated casualties are not included in Table 6-11). The earthquake models for the Newport-Inglewood and Palos Verdes faults estimate Level 1 casualties (lowest acuity) of about 167 to 186 people for a 2 PM event, with total estimated deaths of between 7 and 14 people. As with the Santa Monica earthquake scenario, the number of casualties is expected to be less if the earthquake strikes during maximum commuting time, and even less if it occurs at night, during maximum residential occupancy.

Given the low number of damaged structures anticipated as a result of an earthquake on the San Andreas fault, the total number of casualties resulting from building damage is low, regardless of the time of day when the San Andreas fault earthquake strikes. In all cases, casualties are estimated by HazUS as a result of only structural and non-structural damage to buildings. Thus, the estimated casualty numbers do not include injuries or deaths that might occur during or after the earthquake not related to building damage, such as “heart attacks, car accidents, mechanical failures due to power outages, incidents during post-earthquake search and rescue, postearthquake cleanup and construction, electrocution, tsunami, landslides, liquefaction, fault failure, dam failures, fires, or hazardous materials releases.”10F 11 It also does not account for injury or death
related to disruptions in access to prescription drugs or regular medical care, like dialysis.

Therefore, all of the numbers presented in Table 6-11 could be slightly higher to account for these other potential causes of injury or even death.

11 FEMA P-366, 2023, HazUS Estimated Annualized Earthquake Losses for the United States; available from https://www.fema.gov/sites/default/files/documents/fema_p-366-hazus-estimated-annualized-earthquake-lossesunited-states.pdf; accessed in March and April 2024.

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Table 6-11: Estimated Casualties*
1B
Earthquake Scenario Santa Monica Fault (M 7.4) Palos Verdes Fault (M 7.7) Time Sectors Level 1 Level 2 Level 3 Level 4 Level 1 Level 2 Level 3 Level 4 2 A.M.
(max.
residential
occupancy)
Commercial 11 3 1 1 2 1 0 0
Commuting 0 0 0 0 0 0 0 0
Educational 0 0 0 0 0 0 0 0
Hotels 1 0 0 0 0 0 0 0
Industrial 7 2 0 0 1 0 0 0
Other Residential 212 54 8 16 36 8 1 2
Single-Family 48 7 0 1 8 1 0 0
Total 279 66 9 18 48 10 1 3
2 P.M.
(max
educational,
industrial,
and
commercial)
Commercial 636 178 28 56 140 36 5 11
Commuting 0 0 0 0 0 0 0 0
Educational 231 63 10 20 22 6 1 2
Hotels 0 0 0 0 0 0 0
Industrial 50 14 2 8 2 0 1
Other Residential 69 18 3 5 13 3 0 1
Single-Family 16 2 0 0 3 0 0 0
Total 1,002 275 44 85 186 47 7 14
5 P.M.
(peak
commute
time)
Commercial 385 108 17 33 81 21 3 6
Commuting 2 2 4 1 0 0 1 0
Educational 72 19 3 6 1 0 0 0
Hotels 0 0 0 0 0 0 0 0
Industrial 31 9 2 3 5 1 0 0
Other Residential 82 21 3 6 14 3 0 1
Single-Family 19 3 0 0 3 0 0 0
Total 591 162 29 49 104 26 4 8
Time Sectors Newport-Inglewood Fault (M 7.4) San Andreas Fault (M 7.8 ShakeOut) 2 A.M.
(max.
residential
occupancy)
Commercial 2 1 0 0 0 0 0 0
Commuting 0 0 0 0 0 0 0 0
Educational 0 0 0 0 0 0 0 0
Hotels 0 0 0 0 0 0 0 0
Industrial 1 0 0 0 0 0 0 0
Other Residential 37 6 1 1 1 0 0 0
Single-Family 15 1 0 0 0 0 0 0
Total 55 8 1 1 1 0 0 0
2 P.M.
(max
educational,
industrial,
and
commercial)
Commercial 99 19 3 5 2 0 0 0
Commuting 0 0 0 0 0 0 0 0
Educational 43 8 1 2 1 0 0 0
Hotels 0 0 0 0 0 0 0 0
Industrial 8 2 0 0 0 0 0 0
Other Residential 12 2 0 0 0 0 0 0
Single-Family 5 0 0 0 0 0 0 0
Total 167 31 4 7 3 0 0 0
5 P.M.
(peak
commute
time)
Commercial 59 12 2 3 1 0 0 0
Commuting 0 0 1 0 0 0 0 0
Educational 15 3 0 1 0 0 0 0
Hotels 0 0 0 0 0 0 0 0
Industrial 5 1 0 0 0 0 0 0
Other Residential 14 2 0 0 1 0 0 0
Single-Family 6 0 0 0 0 0 0 0
Total 99 18 3 4 2 0 0 0
*Based on a 2020 estimated population of 93,043, with values rounded to whole numbers.

Level 1: Medical treatment without hospitalization Level 2: Hospitalization but not life threatening Level 3: Hospitalization and life threatening Level 4: Fatalities due to scenario event Local Hazard Mitigation Plan Section 6
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Damage to Critical and Essential Facilities
HazUS breaks critical facilities into two groups: (1) essential facilities, and (2) high potential loss (HPL) facilities. Essential facilities are those parts of a community's infrastructure that must remain operational after an earthquake, such as hospitals, emergency operation centers, fire and police stations, schools, and communication centers. HPL or high-risk facilities are those that if severely damaged, may result in a disaster far beyond the facilities themselves. Examples include power plants, dams and flood control structures, and industrial plants that use or store explosives, extremely hazardous materials or petroleum products in large quantities. No HPL facilities were modeled in Santa Monica proper, but damage to HLP facilities outside City limits could potentially impact section of Santa Monica and its residents. The impact that failure of Stone Canyon Dam would have on the eastern portion of the city is described in detail in Section 8.

The critical facilities that were considered in the earthquake scenarios for the Santa Monica area include 27 schools, two (2) emergency operations centers (the main EOC near City Hall, and a secondary EOC at the airport, two (2) hospitals, five (5) police stations and four (4) fire stations.

The EOC at the Main Library was not considered in the analysis. The expected damages to these essential facilities as a result of the earthquake scenarios conducted for this study are summarized in Table 6-12 below.

Table 6-12: Estimated Damage to Critical Facilities as a Result of the Earthquake Scenarios Earthquake
Scenario Facility Type Total Number
Number of Facilities
With at Least
Moderate
Damage >50%
With Complete
Damage >50%
With
Functionality
>50% on Day 1
Santa
Monica
Hospitals 2 1 0 0
Schools 27 16 0 5
EOCs 2 0 0 1
Police Stations 1 0 0 1
Fire Stations 5 2 0 0
Palos Verdes
Hospitals 2 0 0 2
Schools 27 2 0 25
EOCs 2 0 0 2
Police Stations 1 0 0 1
Fire Stations 5 1 0 4
NewportInglewood
Hospitals 2 0 0 2
Schools 27 1 0 18
EOCs 2 0 0 2
Police Stations 1 0 0 1
Fire Stations 5 0 0 5
San Andreas
Hospitals 2 0 0 2
Schools 27 0 0 27
EOCs 2 0 0 2
Police Stations 1 0 0 1
Fire Stations 5 0 0 5
The earthquake scenarios indicate that an earthquake on the San Andreas fault is not likely to cause damage to any of the critical and essential facilities in the study area, with all facilities expected to be more than 50 percent functional immediately after the earthquake. An earthquake on the Santa Monica fault, on the other hand, is likely to cause at least moderate damage to one of the hospitals, with neither hospital expected to be more than 50 percent functional on the first day Local Hazard Mitigation Plan Section 6
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after the earthquake. Two (2) of the fire stations are also expected to have at least moderate damage. Furthermore, on the first day after the earthquake, none of the fire stations is expected to be more than 50 percent functional. Although both EOCs are expected to perform fairly well, only one of the EOCs is expected to be more than 50 percent functional immediately after the earthquake. The HazUS model also estimates that about 16 of the 27 schools in the city will experience at least moderate damage, with only five (5) schools being more than 50 percent functional on the first day after the event.

Both hospitals and the two EOC locations are expected to be more than 50 percent functional following either the Palos Verdes or Newport-Inglewood earthquake scenario. The models also suggest that 25 of the 27 schools considered in the analyses will be more than 50 percent functional on the day after a Palos Verdes earthquake scenario; in the case of the NewportInglewood fault scenario, the estimate is 18 out of 27 schools. The models also suggest that at least one school will experience at least moderate damage in response to an earthquake on the Palos Verdes fault, whereas the Newport-Inglewood earthquake scenario is anticipated to result in at least moderate damage to one school and one of the fire stations.

Other critical facilities not considered in the HazUS analyses that should be considered in both emergency preparedness and emergency response operations given their potential impact on the community include: (1) High-occupancy facilities such as high-rise buildings, large assembly facilities, and large multi-family residential complexes because of the potential for a large number of casualties or crowd-control problems; (2) dependent care facilities such as preschools, rehabilitation centers, group care homes, nursing homes, and other facilities that house populations with special evacuation considerations; and (3) economic facilities such as banks, archiving and vital, record-keeping facilities, and large industrial or commercial centers that should remain operational to avoid severe economic impacts.

Utility Systems Damage
Damage to pipelines considers two states: leaks and breaks. Leaks generally occur in response to seismic wave propagation, and occur as a result of the joints separating, or crushing of the bell at the bell-spigot connection. Breaks typically occur in response to ground deformation, such as fault rupture, liquefaction, or landsliding. Breaks take a lot longer to fix than a leak, with a crew of four utility workers estimated to take four hours to fix a leak in a pipe 20 inches or less in diameter, and the same crew estimated to take eight hours to fix a break in the same pipe. Leaks in pipes more than 20 inches in diameter are estimated to take a four-person crew ten hours to fix, whereas a break is estimated to take the same crew 20 hours to fix. HazUS uses these estimates to calculate how many days it will take to repair the damaged pipes and restore service. The estimated number of breaks and leaks to the potable and waste water systems and the natural gas system as a result of three earthquake scenarios is summarized in Table 6-13 below.

According to the models, the Santa Monica fault earthquake scenario will result in more than 350 leaks in the potable water distribution lines and 88 breaks within Santa Monica city limits.

Similarly, the waste water system will experience about 178 leaks and 44 breaks. The Palos Verdes and San Andreas fault earthquake scenarios are expected to cause leaks and breaks to both the potable and waste water systems, although in lesser numbers than the Santa Monica earthquake scenario. Although not analyzed, the Newport-Inglewood fault scenario is expected to result in damage similar to that modeled for the Palos Verdes fault. Damage to the local pipelines as a result of the San Andreas fault earthquake scenario returned a relatively high number of leaks and Local Hazard Mitigation Plan Section 6
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breaks that are probably related to the long-period seismic waves anticipated in the Los Angeles basin region, including Santa Monica. All of these earthquakes will also result in leaks and breaks of the water mains and sewer lines throughout the wider geographic region. The San Andreas fault ShakeOut earthquake, in particular, is anticipated to cause significant damage to the southern California water distribution system, with broken pipes, damage to the aqueducts bringing water into the region, and impaired infrastructure. This in turn is expected to result in widespread disruptions to the water supply and other associated problems.

Table 6-13: Estimated Damage to Pipelines as a Result of Three Earthquake Scenarios System
Total
Length of
Pipelines
(km)
Santa Monica Fault Palos Verdes Fault San Andreas Fault # of Leaks # of Breaks # of Leaks # of Breaks # of Leaks # of Breaks Potable Water 390 353 88 51 13 74 18
Waste Water 234 178 44 25 6 37 9
Natural Gas 24 5 1 2 1 1 0
Oil Systems 0 0 0 0 0 0 0
It is noteworthy to mention that the HazUS inventory does not include any oil pipelines through the city, but that there is in fact one petroleum pipeline that extends in a northerly direction across the eastern portion of the city. This pipeline crosses the mapped traces of the Santa Monica fault, so if this fault breaks in an earthquake, rupture to that pipeline should be expected. This is discussed further in the section on hazardous materials.

The results of the analyses regarding the functionality of the potable water and electric power systems in the city for three of the earthquakes discussed herein are presented in Table 6-14. The HazUS inventory for the Santa Monica area does not include specifics regarding the various lifeline systems in the city, therefore, the models estimated damage to the potable water and electric power using empirical relationships based on the number of households served in the area, combined with, in the case of the water distribution system, the number of breaks and leaks anticipated in the system. The high number of leaks and breaks in the potable water system as a result of the Santa Monica earthquake scenario means that a large number of households (nearly 72 percent) will likely not have potable water one day after the earthquake. Repairing the damage is anticipated to take several days; as a result, even on the third day after the earthquake, nearly 5,000 households are expected to not have water in their faucets. Therefore, residents should be advised to have drinking water stored as part of their earthquake emergency kits. These drinking water reserves should be enough to provide 1 gallon of water for all members of the household (including pets) for at least 3 days, and preferably one week.

Although the models anticipate that both the Palos Verdes and San Andreas earthquake scenarios will not result in any households in the city being without potable water immediately after the earthquake, given that both leaks and breaks in this system are anticipated, it is appropriate to expect that some neighborhoods may be without potable water for a few days also. Thus, the recommendation to have enough drinking water stored in bottles to last at least a few days is appropriate.

Local Hazard Mitigation Plan Section 6
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Table 6-14: Expected Performance of Potable Water and Electricity Services Scenario Utility Number of Households without Service*
Day 1 Day 3 Day 7 Day 30 Day 90
Santa Monica Potable Water 33,955 4,916 0 0 0
Electric Power 36,011 23,284 9,566 796 47
Palos Verdes Potable Water 0 0 0 0 0
Electric Power 3,866 1,958 554 24 6
San Andreas Potable Water 0 0 0 0 0
Electric Power 0 0 0 0 0
*Based on Total Number of Households = 47,422
The models also suggest that the Santa Monica earthquake scenario will cause considerable damage to the electric power system, with nearly 76 percent of Santa Monica households expected to be without electricity on the first day after the earthquake. Repair of the system is expected to take days to weeks, with nearly 10,000 households modeled to be without electricity one week after the earthquake, and nearly 50 households still without electricity three months out.

The Palos Verdes earthquake event is expected to cause approximately one-tenth the damage to the electric power system, which means that nearly 3,900 households are estimated to be without electric power on the first day after the earthquake. Repair of this system is anticipated to take much longer, so that seven days after the earthquake, still more than local 500 households are estimated to be without power.

Fire Following Earthquake
History shows that earthquake-induced fires have the potential to be the worst-case firesuppression scenarios for a community because an earthquake typically causes multiple ignitions distributed over a broad geographic area, with the potential to severely tax the local fire suppression agencies. Furthermore, if fire fighters are involved with search and rescue operations, they are less available to fight fires. Fire suppression efforts can also be limited by a water distribution system that has been impaired by the earthquake. Thus, many factors affect the severity of fires following an earthquake, including ignition sources, types and density of fuel, weather conditions, functionality of the water systems, and the ability of firefighters to suppress the fires. The principal causes of earthquake-related fires are open flames, electrical malfunctions, gas leaks, and chemical spills. Downed power lines may ignite fires if the lines do not automatically de-energize. Unanchored gas heaters and water heaters have in the past been common problems, as these readily tip over during strong ground shaking; State law however now requires new and replaced gas-fired water heaters to be attached to a wall or other support. This is expected to result in fewer overturned water heaters in future earthquakes.

The major urban conflagrations of yesteryear in major cities were often the result of closely built, congested areas of attached buildings with no fire sprinklers, no adequate fire separations, no Fire Code enforcement, and narrow streets. In the past, fire apparatus and water supplies were also inadequate in many large cities, and many fire departments were comprised of volunteers. Many of these conditions no longer apply to the cities of today. Nevertheless, major earthquakes can result in fires and the loss of water supply, as it occurred in San Francisco in 1906, and in Kobe, Japan in 1995. A large portion of the structural damage caused by the great San Francisco earthquake of 1906 was the result of fires rather than ground shaking.

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The moderately sized, M6.7 Northridge earthquake of 1994 caused 15,021 natural gas leaks that resulted in three street fires, 51 structure fires (23 of these caused total ruin) and the destruction, by fire, of 172 mobile homes. The 51 structure fires were caused by overturned water heaters (20), other overturned or damaged gas appliances (8), broken interior gas lines (8), broken gas meter set assemblies (2), street fires due to breaks in gas mains (7), and other unknown causes (8). The mobile home fires were primarily the result of failure of the supports leading to breakage of the gas risers, and breakage of the interior gas lines due to overturned water heaters and other appliances (Savage, 1995). The Southern California Gas Company reported 35 breaks in its natural gas transmission lines and 717 breaks in its distribution lines. About 74 percent of the leaks were corrosion related. In one incident, the earthquake severed a 22-inch gas transmission line and a motorist ignited the gas while attempting to restart his stalled vehicle. Response to this fire was impeded by the earthquake’s rupture of a water main; as a result, five nearby homes were destroyed. Elsewhere, one mobile home fire started when a ruptured transmission line was ignited by a downed power line. In many of the destroyed mobile homes, fires erupted when inadequate bracing allowed the houses to slip off their foundations, severing gas lines and igniting fires.

HazUS uses a Monte Carlo simulation model to estimate the number of ignitions and the amount of burnt area as a result of an earthquake. For the earthquake scenarios ran for Santa Monica, HazUS estimates between three and five ignitions immediately following an earthquake, with the San Andreas and Newport-Inglewood fault earthquake scenarios both triggering three (3) ignitions, and the Santa Monica fault triggering five (5) ignitions (Table 6-15). The burnt area resulting from these ignitions will vary depending on wind conditions; the scenarios assumed normal wind conditions of about 10 miles per hour (mph) or less. The models estimated that between 0.12 and

0.35 percent of the total area covered by the city (equivalent to between 0.01 and 0.03 square miles) would be burnt by the fires following the earthquake. If Santa Ana wind conditions are present at the time of the earthquake, the burnt areas can be expected to be larger.

Table 6-15: Fires Following an Earthquake
Earthquake Scenario No. of
Ignitions
Approximate
Burn Area
(% of city)
No. of Displaced
Individuals
Building Value
Destroyed
(Millions of $)
Santa Monica 5 0.35 407 80
Newport-Inglewood 3 0.12 80 21
San Andreas 3 0.12 78 20
Debris Generation
The models estimate that between 3 and 537 thousand tons of debris will be generated by the earthquake scenarios considered in this study (see Table 6-16). Of the total amount, brick and wood comprise between 29 and 50 percent of the total, with the remainder consisting of reinforced concrete and steel. If the debris tonnage is converted to an estimated number of truckloads, it will require 120 to 21,480 truckloads (assuming 25 tons per truck) to remove the debris generated by the earthquakes modeled. The largest numbers are associated with the Santa Monica earthquake scenario, and the smallest numbers with the San Andreas fault earthquake scenario. The Palos Verdes and Newport-Inglewood earthquake scenarios returned very similar estimates, although the Newport-Inglewood earthquake scenario has a higher percentage of brick/wood debris compared to the Palos Verdes fault earthquake scenario.

Local Hazard Mitigation Plan Section 6
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2025 Earthquakes Page 6-79
Table 6-16: Debris Generated by Earthquake Scenarios
Earthquake Scenario Brick/Wood
(1000s Tons)
%
Total
Reinforced
Concrete/Steel
(1000s Tons)
%
Total
Total
(1000s
Tons)
No. of
Truckloads
Santa Monica 156 29 381 71 537 21,480
Palos Verdes 32.5 29 79.5 71 112 4,480
Newport-Inglewood 44 35 81 65 125 5,000
San Andreas 1.5 50 1.5 50 3 120

SEISMIC HAZARD MITIGATION ACTIVITIES
Existing Mitigation Activities
Existing mitigation activities include current mitigation programs and activities that are being implemented by county, regional, State, or Federal agencies or organizations.

California Earthquake Mitigation Legislation
California is painfully aware of the threats it faces from earthquakes. Since the 1800s, Californians have been killed, injured, and lost property as a result of earthquakes. As the State’s population continues to grow, and urban areas become even more densely built up, the risk will continue to increase. In response to this concern, for decades now the Legislature has passed laws to strengthen the built environment and protect the citizens. Table 6-17 provides a sampling of some of the 200 plus laws in the State’s codes.

Table 6-17: Partial List of the Over 200 California Laws on Earthquake Safety Government Code Section
8870-8870.95 Creates Seismic Safety Commission.

Government Code Section
8876.1-8876.10
Established the California Center for Earthquake Engineering Research.

Public Resources Code
Section 2800-2804.6
Authorized a prototype earthquake prediction system along the central San Andreas fault near the city of Parkfield.

Public Resources Code
Section 2810-2815
Continued the Southern California Earthquake Preparedness Project and the Bay Area Regional Earthquake Preparedness Project.

Health and Safety Code
Section 16100-16110
The Seismic Safety Commission and State Architect will develop a state policy on acceptable levels of earthquake risk for new and existing state-owned buildings.

Government Code Section
8871-8871.5
Established the California Earthquake Hazards Reduction Act of 1986.

Health and Safety Code
Section 130000-130025 Defined earthquake performance standards for hospitals.

Public Resources Code
Section 2805-2808 Established the California Earthquake Education Project.

Government Code Section
8899.10-8899.16 Established the Earthquake Research Evaluation Conference.

Public Resources Code
Section 2621-2630 2621. Established the Alquist-Priolo Earthquake Fault Zoning Act.

Government Code Section
8878.50-8878.52 8878.50.

Created the Earthquake Safety and Public Buildings Rehabilitation Bond Act of 1990.

Education Code Section
35295-35297 35295.

Established emergency procedure systems in kindergarten through grade 12 in all the public or private schools.

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Health and Safety Code
Section 19160-19169
Established standards for seismic retrofitting of unreinforced masonry buildings.

Health and Safety Code
Section 1596.80-1596.879
Required all child day care facilities to include an Earthquake Preparedness Checklist as an attachment to their disaster plan.

City of Santa Monica Codes
Implementation of earthquake mitigation policy most often takes place at the local government level. The City of Santa Monica Building and Safety Department enforces building codes pertaining to earthquake hazards. The City has adopted the provisions of the 2023 California Building Code, a modification to the 2022 Uniform Building Code with more restrictive amendments based upon the local geographic, topographic and climatic conditions. The City of Santa Monica, along with dozens of other local jurisdictions, have worked together to make these amendments to the California Building Code consistent with the rest of southern California.

The City of Santa Monica Planning Department enforces the zoning and land use regulations relating to seismic hazards. Generally, these codes and regulations seek to discourage development in areas that could be prone to seismic hazards, such as surface fault rupture, unstable slope areas, and areas susceptible to liquefaction; and where development is permitted, that the applicable construction standards are met. Developers in hazard-prone areas (other than surface fault rupture) may be required to retain a qualified professional engineer to evaluate the level of risk on the site and recommend appropriate mitigation measures. The Building and Safety Department is responsible for the enforcement of building, plumbing, electrical and mechanical codes. It also enforces ordinances and codes to ensure that buildings are safe by providing inspection services and plan check for new construction. The Building and Safety Department strives to protect the residents of a city by enforcing construction regulations.

Information about the City’s departments, codes and policies, handouts, bulletins, forms required for permit applications, fees, etc., are available from the City’s website. Look for the link to the City’s departments, and then refer to the Community Development Department, which oversees the City Planning Division and the Building and Safety Division and is responsible for the review of all development plans, permit issuance, zoning, and development-related applications (such as Conditional Use Permits) as well as the drafting and maintenance of City plans and codes. The Planning and Building and Safety Departments are listed and described under that link.

Community Development staff is also available to help at Santa Monica’s City Hall.

Businesses/Private Sector
Natural hazards have a devastating impact on businesses. In fact, according to the Institute for Business and Home Safety (IBHS), approximately 25 percent of all businesses do not reopen following a major disaster. Business owners and homeowners alike can protect their investment by identifying the risks associated with the natural and man-made disasters that their area is susceptible to (which this Plan covers), and then creating and implementing an action plan that defines the steps to take should a disaster strike. To help business owners with this effort, the IBHS has developed “Open for Business,” a disaster planning toolkit to help guide businesses in preparing for and dealing with the adverse effects of natural hazards (available from https://ibhs.org/BusinessDisasterRecovery/). The kit integrates protection from natural disasters into the business' risk reduction measures to safeguard employees, customers, and the investment itself.

The guide helps businesses secure human and physical resources during disasters, and helps to develop strategies to maintain business continuity before, during, and after a disaster occurs. The Local Hazard Mitigation Plan Section 6
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U.S. Small Business Administration also provides helpful information and checklists that can be used for this purpose (http://www.sba.gov/content/disaster-planning).

Hospitals
The Alfred E. Alquist Hospital Seismic Safety Act (“Hospital Act”) was enacted in 1973 in response to the moderate Magnitude 6.6 Sylmar Earthquake in 1971 when four major hospital campuses were severely damaged and evacuated. Two hospital buildings collapsed, killing forty seven people. Three others were killed in another hospital that nearly collapsed.

In approving the Act, the Legislature noted that “Hospitals, that house patients who have less than the capacity of normally healthy persons to protect themselves, and that must be reasonably capable of providing services to the public after a disaster, shall be designed and constructed to resist, insofar as practical, the forces generated by earthquakes, gravity and winds.” (Health and Safety Code Section 129680).

When the Hospital Act was passed in 1973, the State anticipated that, based on the regular and timely replacement of aging hospital facilities, the majority of hospital buildings would be in compliance with the Act’s standards within 25 years. However, hospital buildings were not, and are not, being replaced at that anticipated rate. In fact, the great majority of the State’s urgent care facilities are now more than 40 years old.

The moderate magnitude 6.7 Northridge earthquake in 1994 caused $3 billion in hospital-related damage and evacuations. Twelve hospital buildings constructed before the Act were cited (red tagged) as unsafe for occupancy after the earthquake. Those hospitals that had been built in accordance with the 1973 Hospital Act were successful in resisting structural damage. However, nonstructural damage (for example, plumbing and ceiling systems) was still extensive in those post-1973 buildings. Santa Monica Hospital Medical Center suffered about $15 million in immediate damage, forcing the tower building of the medical facility to close for nine months.

Saint John’s Hospital also suffered significant damage: its north wing, with 185 beds, the hospital nursery, and the neo-natal intensive care unit, were all immediately evacuated and moved to other parts of the facility. The hospital stayed open for three days before it was forced to shut down completely because of the extensive damage. On October 3, 1994, Saint John’s was again fully operational minus its north wing, which was demolished. The cost of repairs totaled $32 million.

Senate Bill 1953 (SB 1953), enacted in 1994 after the Northridge earthquake, expanded the scope of the 1973 Hospital Act. Under SB 1953, all hospitals were required by January 1, 2008 to survive earthquakes without collapsing or posing the threat of significant loss of life (life safety level). Provisions were made to allow this deadline to be extended to January 1, 2013 if compliance by the 2008 deadline would result in diminished capacity of healthcare services to the community. Subsequent amendments have provided for additional extensions, with the final date by which all hospitals must comply with the provisions of the act being January 1, 2020. To grant an extension to a hospital, the Office of Statewide Health Planning and Development (OSHPD) must consider the structural integrity of the building, the loss of essential healthcare services to the community if the hospital closed, and the financial hardship that the hospital would experience in complying with the provisions of the Act. The 1994 Act further mandates that all existing hospitals be seismically evaluated, and retrofitted, if needed, by 2030, so that they are in substantial compliance with the Act (which requires that the hospital buildings be reasonably capable of providing services to the public after disasters). SB 1953 applies to all urgent care facilities Local Hazard Mitigation Plan Section 6
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(including those built prior to the 1973 Hospital Act) and affects approximately 2,500 buildings on 475 campuses statewide.

SB 1953 directed OSHPD, in consultation with the Hospital Building Safety Board, to develop emergency regulations including “…earthquake performance categories with subgradations for risk to life, structural soundness, building contents, and nonstructural systems that are critical to providing basic services to hospital inpatients and the public after a disaster” (Health and Safety Code Section 130005).

In 2001, recognizing the continuing need to assess the adequacy of policies and the application of advances in technical knowledge and understanding, the California Seismic Safety Commission created an Ad Hoc Committee to re-examine the compliance with the Alquist Hospital Seismic Safety Act. The formation of the Committee was also prompted by the evaluations of hospital buildings reported to OSHPD that at the time revealed that a large percentage (40 percent) of California’s operating hospitals were in the highest category of collapse risk.” Both hospitals in Santa Monica have been seismically strengthened and/or replaced, and have been assigned high marks for structural performance. For the most recent information on these rankings for hospitals in the state refer to https://hcai.ca.gov/facilities/building-safety/seismic-compliance-andsafety/seismic-performance-ratings/#SPC.

Earthquake Education
Earthquake research and education activities are conducted at several major universities in the southern California region, including (but not limited to) CalTech, University of Southern California (USC), University of California at Berkeley, Los Angeles (UCLA), Santa Barbara (UCSB), Irvine (UCI), and San Diego (UCSD) locations, and the California State University system, including locations at Northridge (CSUN), Long Beach (CSULB), Fullerton (CSUF), San Diego (SDSU), and San Bernardino (CSUSB).

The local clearinghouse for earthquake information is the Statewide California Earthquake Center (SCEC. SCEC is currently transitioning to become the Statewide California Earthquake Center, is a community of scientists and specialists who actively coordinate research on earthquake hazards associated with the entire transform boundary between the North America and Pacific tectonic plates, and communicate earthquake information to the public. SCEC is a National Science Foundation (NSF) Science and Technology Center and is co-funded by the United States Geological Survey (USGS).

Los Angeles County, along with 13 other southern California counties, sponsors the Emergency Survival Program (ESP), an educational program for learning how to prepare for earthquakes and other disasters (https://lacounty.gov/emergency/). Survival guides in 15 different languages can be downloaded from https://ready.lacounty.gov/emergency-survival-guide/. Many school districts have very active emergency preparedness programs that include earthquake drills and periodic disaster response team exercises. Many schools also participate in the Great ShakeOut earthquake scenario drills sponsored by the Southern California Earthquake Center, typically in October of every year. For additional information about resources and how to participate in these drills, refer to www.shakeout.org.

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Earthquake Resource Directory
Local and Regional Resources
Statewide California Earthquake Center (SCEC)
SCEC compiles new information about earthquakes in southern California, integrates this information into a comprehensive and predictive understanding of earthquake phenomena, and communicates this understanding to end-users and the general public in order to increase earthquake awareness, reduce economic losses, and save lives.

University of California
3651 Trousdale Parkway, Suite 169, Los Angeles, CA 90089-0742 Ph: 213-740-5843
www.scec.org
Earthquake Country Alliance (ECA)
Public-private-grassroots partnership of people and organizations that work together to improve earthquake and tsunami preparedness, mitigation and resiliency. This organization is administered by SCEC (see above).

Ph: 213-740-1560
http://www.earthquakecountry.org
State Resources
California Department of Transportation (Caltrans) – District 7 CalTrans is responsible for the design, construction, maintenance, and operation of the California State Highway System, as well as that portion of the Interstate Highway System within the state's boundaries. Alone and in partnership with Amtrak, Caltrans is also involved in the support of intercity passenger rail service in California.
100 S. Main Street, 10th Floor, Los Angeles, CA 90012
Ph: 213-897-1200
http://dot.ca.gov/caltrans-near-me/district-7
California Resources Agency
The California Resources Agency restores, protects and manages the state's natural, historical and cultural resources for current and future generations using solutions based on science, collaboration and respect for all the communities and interests involved. 715 P Street, 20th Floor, Sacramento, CA 95814
Ph: 916-653-5656
https://resources.ca.gov/
California Geological Survey (CGS)
The CGS develops and disseminates technical information and advice on California’s geology, geologic hazards, and mineral resources. In addition to providing technical and mapping services on landslide, earthquake and burned watersheds, the CGS is the state’s scientific representative on tsunami preparedness, mitigation, and response programs. 715 P Street, Sacramento, CA 95814
Ph: 916-445-1825
www.conservation.ca.gov/cgs
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California Geological Survey: Southern California Regional Office Junipero Serra Building
320 W. 4th Street, Suite 850, Los Angeles, CA 90013
Ph: 213-239-0877
California Department of Conservation
The Department of Conservation provides services and information that promote environmental health, economic vitality, informed land-use decisions and sound management of our state's natural resources.
715 P Street, Sacramento, CA 95814
Ph: 916-322-1080
www.conservation.ca.gov
California Seismic Safety Commission
The Seismic Safety Commission investigates earthquakes, researches earthquake-related issues and reports, and recommends to the Governor and Legislature, policies and programs needed to reduce earthquake risk. Some of the duties of the Commission include managing California’s Earthquake Hazards Reduction Program, reviewing seismic activities funded by the State, providing a consistent policy direction for earthquake-related programs for all agencies at all government levels, proposing and reviewing earthquake-related legislation, conducting public hearings on seismic safety issues, recommending earthquake safety programs to governmental agencies and the private sector, and investigating and evaluating earthquake damage and reconstruction efforts following earthquakes
3650 Schriever Avenue, Mather, CA 95655
Ph: 916-263-5506
https://ssc.ca.gov
California Governor’s Office of Emergency Services (Cal OES) Cal OES coordinates overall state agency response to major disasters in support of local government. The office is responsible for assuring the state's readiness to respond to and recover from natural, manmade, and war-caused emergencies, and for assisting local governments in their emergency preparedness, response and recovery efforts.
3650 Schriever Avenue, Mather, CA 95655
Ph: 916 845- 8910
www.caloes.ca.gov/
Federal and National Resources
Building Seismic Safety Council (BSSC)
The BSSC develops and promotes building earthquake risk mitigation regulatory provisions for the nation. Provides a forum that fosters improved seismic safety provisions for the use by the building community in the planning, design, construction, regulation and utilization of buildings. 2121 K Street NW, Suite 800
Washington, DC 20037
Ph: 202-289-7800
www.nibs.org/bssc
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Federal Emergency Management Agency (FEMA), Region 9
An agency within the U.S. Department of Homeland Security, FEMA is tasked with responding to, planning for, recovering from and mitigating against disasters. FEMA provides extensive resources to help communities, businesses, and residents prepare for disasters. It is also a major source of funding through grants for hazard mitigation and hazard preparedness. coordinates the federal response agency support of state and local jurisdictions during disaster events. FEMA also provides reimbursement to jurisdictions for their disaster response costs and facilitates programs to implement mitigation improvements post-disaster.
1111 Broadway, Suite 1200, Oakland, CA 94607-4052
Ph: 510-627-7100
www.fema.gov
United States Geological Survey (USGS)
The USGS provides scientific information to describe and understand the Earth; minimize loss of life and property from natural disasters; manage water, biological, energy, and mineral resources; and enhance and protect our quality of life.
12201 Sunrise Valley Drive, Reston, VA 20192
Ph: 650-853-8300, 1-800-ASK-USGS
Pasadena Field Office
525 South Wilson Avenue, Pasadena, CA 91106-3212
Ph: 626-583-7811
http://www.usgs.gov/
Publications
“Land Use Planning for Earthquake Hazard Mitigation: Handbook for Planners” by Wolfe, M.R., Bolton, P.A., Heikkala, S.G., Greene, M.M., and May, P.J., 1986: Special Publication 14, Natural Hazards Research and Applications Information Center, University of Colorado, Institute of Behavioral Science, National Science Foundation. This older but still applicable handbook provides techniques that planners and others can utilize to help mitigate seismic hazards. It provides information on the effects of earthquakes, sources on risk assessment, and effects of earthquakes on the built environment. The handbook also gives examples on application and implementation of planning techniques to be used by local communities.

Available online from several sites including https://digitalcommons.usf.edu/fmhi_pub/82/. “Local Mitigation Planning Handbook,” FEMA (March 2013). This handbook was prepared by FEMA to help local governments in the preparation of their local hazard mitigation plans.

According to FEMA, the Handbook is a companion to the Local Mitigation Plan Review Guide released by FEMA in 2011. While the Plan Review Guide is intended to help State and Federal officials review and approve local hazard mitigation plans, the Handbook is intended to help local officials develop these plans. The Handbook is available from https://www.fema.gov/sites/default/files/2020-06/fema-local-mitigation-planninghandbook_03-2013.pdf. “Public Assistance Debris Management Guide,” FEMA (July 2000). The Debris Management Guide was developed to assist local officials in planning, mobilizing, organizing. and controlling large-scale debris clearance, removal, and disposal operations, Debris management is generally associated with post-disaster recovery. While it should be compliant with local and county emergency operations plans, developing strategies to ensure strong debris management Local Hazard Mitigation Plan Section 6
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is a way to integrate debris management within mitigation activities. The “Public Assistance Debris Management Guide” is available in hard copy or on the FEMA website. “A Safer, More Resilient California: The State Plan for Earthquake Research,” California Seismic Safety Commission (2004). This is a 5-year statewide earthquake research plan that contains identifies research activities, and provides strategies to receive federal funding to implement the plan. For additional information and to review many more publications issued by the CSSC, refer to their website at http://www.seismic.ca.gov/pub.html. “Staying Safe Where the Earth Shakes” are undated regional publications prepared by the Earthquake Country Alliance, the California Earthquake Authority and the California Governor’s Office of Emergency Services and available from the Earthquake Country Alliance website. The Southern California Coast regional guide that includes Los Angeles, Orange and Ventura counties with an emphasis on the coastal region, and thus includes the hazard of tsunami, is available from https://www.earthquakecountry.org/southerncaliforniacoast/.

Pamphlets available for other California regions are also available from www.earthquakecountry.org.
“Putting Down Roots in Earthquake Country,” Southern California Earthquake Center, 2011. This is an updated version of a classic booklet that discusses the earthquake risk in California and provides homeowners with specific information on how to earthquake-proof their homes and be prepared for an earthquake. The document is available online from https://www.earthquakecountry.org/booklets/. A Spanish version of the pamphlet is also available from the same site.
“7 Steps to an Earthquake Resilient Business – A Supplemental Guide to Putting down Roots in Earthquake Country,” Southern California Earthquake Center, 2008. This booklet provides information helpful to business owners to earthquake-proof their place of business, keeping their employees safe, and prevent work stoppages or business closure. This document is also available from www.earthquakecountry.org/roots/.

Refer to the References section (Appendix I) for a listing of the reports referenced in this section and other resources.

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SECTION 7: TSUNAMIS
WHY ARE TSUNAMIS A THREAT TO THE CITY OF SANTA MONICA?
As discussed in more detail later in this section, the historical record, albeit short, includes several tsunamis that have been observed and/or measured in the Santa Monica Bay region, including along the Santa Monica coastline. Most of these tsunamis have amounted to small waves about 1 foot (0.30 m) or less in height, but a few have exceeded 3 feet (1 m). For the most part, these tsunamis have been generated by distant earthquakes. Extensive and constant monitoring of the ocean basins now provide warnings up to hours in advance of the arrival of a tsunami for distant earthquake sources; this in turn allows for evacuation efforts that help to significantly reduce loss of life.

However, as discussed in Section 6, several offshore Holocene-active faults extend across the Santa Monica Bay (see Figure 6-1). Surface rupture during an earthquake on any of these faults could generate a local tsunami that could impact Santa Monica and other coastal communities. An earthquake-generated landslide along the continental slope or in the canyons offshore, in the bay (Figure 7-1), could also generate a tsunami. These local tsunamis would reach the coastline in a short period of time, measured in minutes rather than hours, limiting evacuation efforts, with the potential to result in significant loss of life, injuries, and damage to coastal structures and infrastructure. All coastal structures, including residential and commercial buildings on the seaward side of Pacific Coast Highway, in addition to the Santa Monica Pier and other tourist attractions, could be damaged. This could have a significant impact on the local economy.

Figure 7-1: Bathymetric Map of Santa Monica Bay Showing Continental Slope and Offshore Canyons that Could Be a Source of Tsunami-Generating Landslides Local Hazard Mitigation Plan Section 7
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TSUNAMI BASICS – DEFINITIONS AND CAUSES
What is a Tsunami?
A tsunami (pronounced (t)soo-NAH-mee, a Japanese composite word meaning “harbor wave”) is a series of sea waves caused by a sudden displacement of the ocean water. Tsunamis are characterized by their length, speed, low period, and low observable amplitude: the waves can be up to 125 miles (200 km) long from one crest to the next, they travel in the deep ocean at speeds of typically about 600 mi/hr (950 km/hr), and have periods of between 5 minutes and up to a few hours (with most tsunami periods ranging between 10 and 60 minutes). Their height in the open ocean is very small, a few meters at most, so they pass under ships and boats undetected (Garrison, 2002).

However, as a tsunami approaches a coastline and enters shallow water, its speed decreases to about 20 to 30 mi/hr (which is still much too fast for a person to outrun), and the height of the waves increases as the water piles up, with the potential to reach heights of 30 m (100 ft) or more. The effect of these waves can be amplified where a bay, harbor or lagoon funnels the waves as they move inland. Although 30-foot-plus waves are obviously extremely damaging, even tsunamis only 1 to 3 feet high can be very destructive and cause many deaths and injuries.

Terminology (see Figure 7-2) related to observations or measurements of the reach of a tsunami on normally dry land include “water height or tsunami height,” “runup, wave runup, or uprush,” and “inundation.” Tsunami height is the maximum elevation, typically in meters, of the wave above a reference sea level, such as mean sea level, at the shoreline. Runup is the maximum elevation, also typically reported in meters relative to the same reference level, of ground wetted by the tsunami.

Inundation is the horizontal distance of the inland reach of the tsunami.

Figure 7-2: Diagram Showing Terms Commonly Used in Tsunami Studies Source: From Lekkas et al., 20130F
1 , their Figure 2.

It is important to note that a tsunami wave may first arrive at a coast as a trough, with water receding and exposing the sea floor over a much larger area than is typical during extremely low tide. Public education efforts need to stress that this is an important warning sign and the public needs to head 1 Lekkas, E.L., Andreadakis, E., Kostaki, I., and Kapourani, E., 2013, A proposal for a new integrated tsunami intensity scale (IT IS-2012): Bulletin of the Seismological Society of America, Vol. 103, No. 2B, pp. 1493-1502, doi:10.1785/0120120099; accessed December 2, 2024.

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inland or to higher ground as soon as possible. The receding water will rush back in, likely in a few minutes, typically as a bore, with the fast-rising flood potentially causing significant damage and loss of life if people are trapped within the flooded zone.

What Causes a Tsunami?
The most frequent causes of tsunamis are underwater earthquakes and submarine landslides that cause a sudden disturbance or displacement of the sea floor and the column of water above.

However, tsunamis can also be caused by underwater volcanic explosions, oceanic meteor impacts, glacier calvings, underwater nuclear explosions, and even meteorological conditions. Tsunamis caused by weather events or meteorological conditions are called meteotsunamis.

Earthquake-generated tsunamis have been studied more extensively than any other type. The earthquakes that often generate tsunamis typically, though not always, occur along subducting plate boundaries, where one tectonic plate is being subducted under another. Subduction zones are typically characterized by deep ocean trenches, with a zone of volcanoes paralleling the plate boundary (referred to as a volcanic arc). The volcanoes are the surface manifestation of the subducted crust being heated as a result of increased temperatures and pressures with increasing burial depth. The heated crust heats and melts the overlying mantle, forming magma. The magma then rises through the mantle to the surface, forming volcanoes.

The margins of nearly the entire Pacific Ocean basin, with the exception of the coastline along Canada and most of California and Baja California, are defined by subducting plates and associated volcanoes, in a zone referred to as the Ring of Fire, Rim of Fire, or the Circum-Pacific Belt (Figure 7-3). The belt runs along the north edge of the Aleutian Bay in Alaska, the western coast of Washington, Oregon and northern California, the western coast of Mexico and Central America, and parallels the coastline of Ecuador, Peru and Chile. On the west side of the Pacific Ocean basin, subducting plates occur along the east side of Russia, Japan, Taiwan and the Philippines, through Papua New Guinea and south of the Solomon Islands and Vanuatu, to the north and east of Tonga and the North Island in New Zealand. Earthquakes generated along any of these subduction plates have the potential to generate tsunamis.

Other regions in the world, including some parts of the Indian and Atlantic Oceans and the Mediterranean Sea, can also generate tsunami-inducing earthquakes. For example, the devastating 2004 earthquake and tsunami that occurred in the Indian Ocean, off the west coast of Sumatra, and the 1755 Lisbon earthquake and tsunami are thought to have originated on a fault under the Atlantic Ocean, offshore of the Iberian Peninsula.

Early researchers found that there is a correlation between the depth and size of the earthquake and the size of the associated tsunami: the larger the earthquake and the shallower its epicenter, the larger the resulting tsunami (Imamura, 1949; Iida, 1963, as reported in McCulloch, 1985). The size of the tsunami is also related to the volume of displaced sea floor (Iida, 1963). Tsunamis can travel across an entire ocean basin (called teletsunamis1F
2 ), or they can be local.

2 A teletsunami originates from a distant source, defined as more than 1,000 km away or at least three hours travel from the area of interest, sometimes travelling across an ocean.

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Figure 7-3: World Map Showing Approximate Location of Ring of Fire (pink shaded areas), Subduction Trenches (blue lines), Active Volcanoes (yellow triangles), and Large Earthquakes Between 1900 and 2013 (red, green and blue dots). Cascadia Subduction Zone also shown.

Source: Modified and combined from two open-source maps available from https://en.wikipedia.org/wiki/Ring_of_Fire, accessed in May 2024.

Tsunamis can also be caused by underwater or water-adjacent volcanic eruptions; these are called volcanic tsunamis or volcanogenic tsunamis. These tsunamis can be caused by a variety of phenomena associated with volcanic eruptions, including earthquakes, collapse of the volcano’s caldera, landslides or avalanches along the sides of the volcano consisting of either older material or new hot lava, pyroclastic flows or lahars, and shock waves or airwaves associated with underwater eruptions. Most of these tsunamis are fairly local, but they can be devastating to communities adjacent to the source. For example, the waves generated by the 1883 eruption of Krakatou (Latter, 1981) were nearly 135 ft (41m) high, destroying coastal towns and villages along the western coast of Java and the southern coast of Sumatra, with a death toll of nearly 35,000.

More recently, an undersea volcano called Hunga Tonga-Hunga Ha’apai in the Kingdom of Tonga erupted violently on January 15, 2022. The explosions triggered tsunamis in the adjacent Tongan islands that were almost 65 ft (20m) high, destroying hundreds of buildings, displacing 1,500 people and killing four. The volcanic explosion also created atmospheric waves that caused pressure changes in the ocean water, causing meteotsunamis. The atmospheric waves travel faster than a typical tsunami, with the fastest of these, referred to as Lamb waves, travelling at the speed of sound.

As a result, the first of the teletsunamis observed in Japan arrived more than 2 hours earlier than expected. Many tide gauges around the world, including the Indian Ocean, the Mediterranean, and along the west coast of the United States also recorded the arrival of meteotsunamis related to this volcanic eruption, typically followed by regular tsunami waves hours later. These waves ranged in Local Hazard Mitigation Plan Section 7
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height between a few inches to about 8 feet (2.5 m)2F
3 . Tsunami runups related to this volcanic
eruption were observed in Santa Monica.

Meteotsunamis are typically generated by air pressure disturbances associated with fast-moving weather systems, like squall lines. The development, direction and magnitude of these meteotsunamis are controlled by the intensity, direction, and speed of the air pressure disturbance as it travels over a body of water deep enough to enhance wave magnification. Unlike the volcanicrelated meteotsunamis described in the previous paragraph, most weather-related tsunamis tend to be regional, and do not affect entire ocean basins. Moreover, certain parts of the world appear to be prone to meteotsunamis due to a combination of factors, including local weather patterns, the depth and shape of the ocean floor near the coast (bathymetry), and coastal elevation and features (topography), although the record is probably not long enough to be certain of this. In the United States, locally generated meteotsunamis have been observed in Hawaii, Alaska, California, the Gulf of Mexico, Florida, from Pennsylvania to southern New England, Maine, and in Lake Erie. Most meteotsunamis are too small to be noticed, with maximum water heights of less than 1 foot, but large meteotsunamis can generate dangerous waves, flooding, and strong currents that can cause damage, injuries, and even death. For example, a meteotsunami in Daytona Beach, Florida on July 3, 1992 with a reported maximum water height of 3 m (9.8 ft) injured 75 people (Churchill et al., 1995).

HISTORICAL, LOCALLY SIGNIFICANT TSUNAMIS
As described above, most of the Pacific Ocean Basin is bordered by prominent submarine trenches related to the ongoing subduction of tectonic plates. It has been estimated that between 80 and 90 percent of all tsunamis worldwide are generated in the subduction zones along the Pacific Rim.

Review of the data presented in Table 7-1, and the sources used to prepare this table, show that tsunami-generated runups that have been observed along the southern California coastline have originated from all around the Pacific Ocean, including the Aleutian Islands and Bay of Alaska, the western coast of Central and South America, offshore eastern Russia and Japan, and southeastern Asia. Some portions of southern California are partly protected from tsunamis by the Channel Islands, which deflect east- and northeast-trending waves, and by Point Arguello, which deflects waves coming in from the continental area of Alaska. However, tsunami waves up to about 6 feet (2 meters) high sourced from the Pacific Ocean Basin have been recorded in the Santa Monica Bay region.

Although the historical record for southern California is short, over 150 tsunamis have been recorded in southern California since the early 1800s. Most of these were minor and barely noticeable, but at least 40 made an impression, causing damage to harbors, boats and other coastal structures, and a few caused loss of life (Table 7-1; only tsunamis with reported maximum water height equal to or greater than 0.1 meter are included therein). The earliest tidal gauges in southern California date from 1906 in San Diego, 1923 in Los Angeles, 1924 in La Jolla, and 1932 in Santa Monica3F 4 ; thus,
wave heights for tsunami events prior to these dates were estimated based on historical accounts, sometimes written down one or two generations after the fact.

Table 7-1 also shows that tsunamis generated by local earthquakes and/or landslides, and

3 https://www.usgs.gov/centers/pcmsc/news/depth-surprising-tsunamis-caused-explosive-eruption-tonga; accessed in April 2024. 4
https://tidesandcurrents.noaa.gov/stations.html?type=Historic+Water+Levels#California; accessed April 24, 2024.

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meteotsunamis have historically posed a risk to southern California. Because the water height measurements in many early cases are based on passed-down recollections, there are uncertainties in the numbers reported. However, the data suggest that the highest sea waves historically recorded in the southern California area caused by a locally generated tsunami, the result of an earthquake and/or offshore landslide, is either 11.2 feet (3.4 m) in Gaviota (in 1812), or 19.7 ft (6 m) in Santa Monica (in 1930). The records for Santa Monica, in particular, are questionable, with other sources indicating water height values of 2 ft (0.6 m4F
5 ) and 10 feet (3.05 m5F
6 ) for the same event. The largest
historically reported meteotsunami along the southern California coastline likely occurred in Newport Beach on August 21, 1934, where waves 30 to 39 feet (9-12 m) high were reported.

Text associated with tsunamis that are inferred or known to have been observed in Santa Monica is in bold on Table 7-1. Local or world-wide significant tsunamis described further in the following sub-sections are shown in bold across the table entry.

Table 7-1: Significant Historical Tsunami Record for Southern California – 1812 to January 2024
Date Source Water Height
March 26, 1806 Possibly an earthquake offshore
southern California
Unknown, but boats reportedly beached in
Santa Barbara
December 21, 1812 Possibly an earthquake and/or
landslide in Santa Barbara Channel
Gaviota (El Refugio): 3.4 m (11.2 ft)
Santa Barbara: ~2-3 m (6.6-9.8 ft)
Ventura: ~2-3 m (6.6-9.8 ft)
November 1853 Earthquake in Kuril Islands Unknown; possibly observed in San Diego May 31, 1854 Possibly an earthquake offshore
southern California; possibly same as
July or December events
Unknown; observed in Santa Barbara
July 24, 1854 Unknown; possible meteorological
origin
San Diego: ~0.3 m (~1 ft)
December 23-24,
1854
Earthquake offshore Japan San Diego: 0.1 m (0.3 ft)
July 11, 1855 Possible offshore landslide caused by
earthquake in Los Angeles
Unknown; two unusually large waves
surged on shore at San Juan Capistrano
May 27, 1862 Estimated M 5.9 earthquake near San
Diego
San Diego: 1.2 m (4 ft)
April 3, 1868 Earthquake in Hawaii San Diego: 0.1 m (0.3 ft) August 13, 1868 Earthquake offshore Chile San Diego: 0.3 – 0.8 m (0.6-2.6 ft) San Pedro: 0.76 - 1.8 m (2.5 - 5.9 ft)
Wilmington: 0.76 - 1.8 m (2.5 - 5.9 ft)
April 16, 1877 Central and southern California;
possible meteotsunami or small
offshore earthquake
Cayucos: 1.8 m (5.9 ft) (mild tremor(s) felt
there that morning)
Seal Beach: 1.8 m (5.9 ft)
May 10, 1877 Earthquake offshore Chile Gaviota: 1.83 m (6 ft) San Pedro: 2.07 m (6.8 ft)
Wilmington: 1.68 m (5.5 ft)

5 Legg, M.R., Borrero, J.C., and Synolakis, C.E., 2003, Evaluation of Tsunami Risk to Southern California Coastal Cities: Earthquake Engineering Research Institute Report PF2002-11; available from https://www.eeri.org/images/archived/wp-content/uploads/FinlRept.pdf. 6 https://www.ngdc.noaa.gov/hazel/view/hazards/tsunami/eventdata?regionCode=88&maxYear=2024&maxEventValidity=4&area=CA&minYear=1600&country=USA&minEvent Validity=0; accessed April 2024.

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Date Source Water Height
Seal Beach: 1.52 m (5 ft)
November 22, 1878 Central California landslide At least 9 runups recorded in Avila Beach, Pismo Beach, Morro Bay, Port Harford,
Surf and other localities: 1 m (3.05 ft);
damage to wharfs reported.

Wilmington: 2 m (6 ft) high wave; record
deemed questionable.

August 10, 1879 Possible undersea landslide caused
by earthquake in San Fernando area?
Unknown; tsunami reported at Santa
Monica following shaking felt therein
December 25, 1899 Possible underwater landslide
generated by earthquake in San
Jacinto area or meteotsunami?
Unknown; large wave reported along
southern California coast
February 26, 1902 Earthquake offshore El SalvadorGuatemalaUnknown; large wave reported in San Diego; record considered questionable
December 11, 1913 Unknown disturbance offshore Santa
Barbara; submarine landslide,
earthquake or meteotsunami
Waves 4.6 m (15 ft) high, strong surf;
dislodged the wreckage of an old ship
thought to be >100 years old, and the
bones of a mastodon that washed onshore
May 1, 1917 Earthquake in Kermadec Islands,
New Zealand
Unknown; large waves reported in La Jolla
San Diego: 0.1 m (0.3 ft)
November 11, 1922 Earthquake offshore Chile San Diego: 0.2 m (0.7 ft) February 3, 1923 Earthquake offshore Kamchatka,
Russia
San Diego: 0.2 m (0.7 ft); strong “tides”
reported in Los Angeles and Santa Cruz
April; 13, 1923 Aftershock offshore Kamchatka,
Russia
Los Angeles: 1.8 m (5.9 ft)
San Diego: 0.1 m (0.3 ft)
October 4, 1925 Unknown; possible meteorological
origin or submarine volcanic event
Long Beach: 0.34 m (1.1 ft)
January 1, 1927 Southern California; possibly a
submarine landslide caused by an
earthquake in Imperial Valley
Unknown; large waves reported along
southern California coast
Long Beach: oscillations of 0.34 m (1.1 ft)
November 4, 1927 Central and southern California;
offshore earthquake off Lompoc,
Santa Barbara County
La Jolla: 0.2 – 0.3 m (0.7 – 1 ft)
Surf: 1.8 m (5.9 ft)
Port San Luis: 1.5 m (4.9 ft)
June 17, 1928 Earthquake in Oaxaca, Mexico La Jolla: 0.1 m (0.3 ft) August 31, 1930 Southern California; offshore
earthquake in Santa Monica Bay
Tsunamis reported in Redondo Beach,
Santa Monica and Venice. Max. water
height in Santa Monica & Venice: between

0.6 m (2 ft) and 6 m (20 ft) depending on
source, with preferred value of 3.05 m (10
ft)
June 3, 1932 Earthquake offshore Central Mexico Long Beach: 0.1 m (0.3 ft) Santa Barbara: 0.1 m (0.3 ft)
March 2, 1933 Earthquake offshore Sanriku, Japan Los Angeles: 0.1 - 0.2 m (0.3 - 0.7 ft) Long Beach: 0.1 m (0.3 ft)
Santa Monica: 0.07 - <2 m (0.2 - 6.6 ft)
March 11, 1933 Earthquake offshore Newport Beach,
California (Long Beach earthquake)
but marigrams show wave activity
starting before earthquake and
lasting for >36 hours after,
suggesting meteorological source
Long Beach: 0.1 m (0.3 ft)
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Date Source Water Height
August 21, 1934 Unknown; possibly caused by
earthquake near Balboa, or of
meteorological origin
(meteotsunami or rogue waves)
Newport Beach: 3 m rise (10 ft)
9-12 m (30 –39 ft) waves
Damage in Long Beach
November 10, 1938 Earthquake offshore Alaska San Diego: 0.1 m (0.3 ft) Santa Monica: 0.05 m (0.15 ft)
Crescent City: 0.18 m (0.6 ft)
December 7, 1944 Earthquake offshore Kii Peninsula,
Japan
La Jolla: 0.1 m (0.3 ft)
Santa Monica: 0.07 m (0.23 ft)
April 1, 1946 Earthquake offshore Unimak Island,
Alaska
Avila Beach: 1.3 m (4.3 ft)
Long Beach: 0.2 m (0.66 ft)
Los Angeles: 0.4 m (1.3 ft)
San Pedro: 0.38 m (1.25 ft)
December 20, 1946 Earthquake offshore Honshu, Japan Los Angeles: 0.1 m (0.3 ft) March 4, 1952 Earthquake offshore Hokaido Island,
Japan
Long Beach: 0.1 m (0.3 ft)
Los Angeles: 0.13 m (0.4 ft)
November 4, 1952 Earthquake offshore Kamchatka
Peninsula, Russia
La Jolla: 0.19 m (0.62 ft)
Los Angeles: 0.38 m (1.25 ft)
Santa Monica: 0.48 m (1.6 ft)
March 9, 1957 Earthquake offshore Andreanof
(Aleutian) Islands
San Diego: 0.2 – 1.0 m (0.7–3.3 ft)
Los Angeles: 0.1 – 0.5 m (0.3 – 1.6 ft)
Santa Monica: 0.46 m (1.5 ft)
May 22-23, 1960 Earthquake offshore Valdivia, Chile Alamitos Bay: 0.61 m (2.0 ft) Long Beach: 0.88 m (2.9 ft)
Los Angeles: 0.76 – 0.95 m (2.5 – 3.2 ft)
Santa Monica: 1.4 m (4.6 ft)
October 13, 1963 Earthquake offshore Kuril Islands,
Russia
La Jolla: 0.1 m (0.3 ft)
Los Angeles: 0.1 m (0.3 ft)
March 28, 1964 Earthquake offshore Gulf of Alaska
(Prince William Sound)
Alamitos Bay: 0.43 m (1.4 ft)
Crescent City: 4.8 m (15.75 ft)
Los Angeles: 0.49 m (1.6 ft)
Santa Monica: 1.03 m (3.4 ft)
San Diego: 0.53 – 0.99 m (1.74 – 3.25 ft)
February 4, 1965 Earthquake offshore Rat (Aleutian)
Islands, Alaska
Los Angeles: 0.1 m (0.3 ft)
Santa Monica: 0.08 m (0.26 ft)
San Diego: 0.1 m (0.3 ft)
October 17, 1966 Earthquake offshore Peru Long Beach: 0.1 m (0.3 ft) Los Angeles: 0.1 m (0.3 ft)
Newport Bay: 0.1 m (0.3 ft)
May 16, 1968 Earthquake offshore Honshu Island,
Japan
Crescent City: 0.61 m (2.0 ft)
Santa Monica: 0.2 m (0.7 ft)
Los Angeles: 0.1 m (0.3 ft)
Long Beach: 0.1 m (0.3 ft)
July 26, 1971 Earthquake offshore Papua New
Guinea in Bismarck Sea
Long Beach: 0.1 m (0.3 ft)
Los Angeles: 0.05 m (0.2 ft)
November 29, 1975 Submarine landslide generated by
earthquake offshore Hawaii Island
La Jolla: 0.15 m (0.5 ft)
Los Angeles: 0.15 m (0.5 ft)
Catalina Island: 1.4 m (4.6 ft)
June 22, 1977 Earthquake in the Tonga Trench,
South Pacific
Los Angeles: 0.05 m (0.2 ft)
Long Beach: 0.12 m (0.4 ft)
San Diego: 0.08 m (0.26 ft)
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Date Source Water Height
1979 and 1989
Unknown source affecting Santa
Monica Bay; most likely associated
with offshore earthquakes in the bay
Local oscillations; no water heights
reported.

July 30, 1995 Earthquake offshore North Chile
Santa Monica: 0.13 m (0.43 ft)
Los Angeles: 0.05 m (0.2 ft)
San Diego: 0.05 m (0.2 ft)
June 23, 2001 Earthquake offshore South Peru
Santa Monica: 0.1 m (0.3 ft)
Los Angeles: 0.05 m (0.15 ft)
La Jolla: 0.05 m (0.15 ft)
December 26, 2004 Earthquake offshore west coast of
Sumatra
Los Angeles: 0.13-0.27 m (0.43-0.9 ft)
La Jolla: 0.06-0.12 m (0.2-0.4 ft)
San Diego: 0.16 m (0.52 ft)
Santa Monica: 0.19-0.41 m (0.62-1.35 ft)
May 3, 2006 Earthquake offshore Tonga Islands
La Jolla: 0.04 m (0.13 ft)
Los Angeles: 0.08 m (0.26 ft)
Santa Monica: 0.10 m (0.3 ft)
November 15, 2006 Earthquake offshore South Kuril
Islands, Russia
Crescent City: 0.88 m (2.9 ft); $20 M in
damages to Inner Harbor
Los Angeles: 0.11 m (0.36 ft)
La Jolla: 0.1 m (0.3 ft)
Santa Monica: 0.15 m (0.5 ft)
April 1, 2007 Earthquake offshore Solomon Islands San Diego: 0.1 m (0.3 ft) Santa Monica: 0.11 m (0.36 ft)
September 29, 2009 Earthquake offshore Samoa Islands Los Angeles: 0.13 m (0.43 ft) Santa Monica: 0.15 m (0.49 ft)
February 27, 2010 Earthquake offshore Maule, Chile La Jolla: 0.6 m (2.0 ft) Dana Point: 0.7 m (2.3 ft)
Los Angeles: 0.42 m (1.38 ft)
Santa Monica: 0.64 m (2.1 ft)
Damaged boats and docks in southern and
central California; damage to harbor
structures estimated at >$1M
March 11, 2011 Earthquake offshore Honshu, Japan Santa Barbara: 1.02 m (3.3 ft) Port Hueneme/Ventura harbors: 1-1.5 m
(3-4.5 ft)
Santa Monica: 0.85 m (2.8 ft); wave front
travelled at least 2.5 km (1.6 mi) up
Ballona Creek channel; barges broke loose
from their moorings in Alamitos Bay.

Los Angeles/Long Beach: 0.5-0.7 m (1.7-

2.3 ft)
La Jolla: 0.5 m (1.4 ft),
San Diego Harbor: 0.5 m (1.7 ft). One
dock was submerged and boat sank near
Shelter Island.

April 1, 2014 Earthquake offshore North Chile Santa Barbara: 0.21 m (0.69 ft) Santa Monica: 0.12 m (0.39 ft)
September 16, 2015 Earthquake offshore Central Chile Santa Monica: 0.21 m (0.69 ft) Santa Barbara: 0.34 m (1.12 ft)
Los Angeles: 0.16 m (0.52 ft)
January 15, 2022 Hunga-Tonga volcanic eruption Ventura: 1.2 m (3.94 ft) Local Hazard Mitigation Plan Section 7
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Date Source Water Height
Santa Monica: 0.61 m (2 ft)
Los Angeles: 0.43 m (1.4 ft)
San Diego: 0.42 m (1.38 ft)
Santa Barbara: 0.64 m (2.01 ft)
Sources: Compiled from Lander and Lockridge (1989), McCulloch (1985), Ewing (2011); https://en.wikipedia.org/wiki/List_of_tsunamis (accessed February 2023); https://www.ngdc.noaa.gov/hazel/view/hazards/tsunami/event-search (accessed February 2023 & April 2024).

Santa Barbara Tsunami of 1812
The second of two strong earthquakes in the Santa Barbara area on December 21st, 1812 produced a tsunami that caused damage in Santa Barbara and Ventura counties and was reported along the coast of southern California. The most likely source for the earthquakes is a fault zone in the Santa Barbara Channel, although onshore faults east of Santa Barbara cannot be ruled out.

While some historical accounts suggest the tsunami produced a maximum one-mile runup and wave heights of 50 ft (15 m) at Gaviota, 30 to 35 ft (9 to 10.5 m) at Santa Barbara and 11.2 ft (3.4 m) at Ventura, contemporary records from the missions at Santa Barbara and Ventura do not mention tsunami runup or damage to nearby coastal communities (Lander and Lockridge, 1989). The mission records describe only a disturbed ocean and fear of tsunami, suggesting that the accounts of high waves, most of which were recorded years after the event, may have been exaggerated (Lander and Lockridge, 1989). For example, an account of “an old trader” printed in the San Francisco Bulletin 52 years after 1812, reported a 1-mile runup in Gaviota. From this account, a 50 ft (15 m) wave height was derived using topographic maps.

Accounts collected by Trask (1856), 44 years after the event, reported that waves damaged the lower part of the town of Santa Barbara, half a mile inland. Trask (1856) also recorded reports of a ship damaged by a tsunami wave near San Buenaventura (present day Ventura). This may be the same vessel reported by Los Angeles Star in 1857 to have been swept up a canyon at El Refugio Bay, near Gaviota. A third-hand account of tsunami damage to the mission in Ventura, located 15 ft (4.5 m) above sea level, is not corroborated by the mission records (Grauzinis et al., unpublished report).

Grauzinis et al. (unpublished, based on data from Soloviev and Go, 1975; McCulloch, 1985; Marine Advisors, 1965; Iida et al., 1967; Wood, 1916; Heck, 1947; Toppozada et al., 1981), concluded that the most reliable historical data support a tsunami height of less than 10 ft (3 m) at Santa Barbara and Ventura, 11.2 ft (3.4 m) at El Refugio, and lower elsewhere in southern California. This is roughly consistent with analysis of predicted tidal data for the region by Long (1988) who suggested a wave height of 6.6 ft (2 m) at Santa Barbara and Ventura.

Tsunami of May 10, 1877
A magnitude 8.3 earthquake off the coast of northern Chile generated a tsunami that was detected along the California coastline from Sausalito south to at least Seal Beach (referred to at the time as Anaheim Landing). At Gaviota, the water level rose 12 feet6F 7 , then fell, then rose and fell three more
times in a span of 20 minutes. At Anaheim Landing, “swift currents carried 2 sailors in a rowboat upstream beyond their destination, then were suddenly rushed back again. . .” Water rushed in and out of the beach every five minutes, covering the beach with 6 to 8 feet of water, and leaving sharks

7 Maximum water height is considered half of this value, i.e., 6 feet.

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and fish on dry land when the water retreated, before being washed off again when the water rushed back in (Lander et al., 1993).

Tsunami of April 13, 1923
A magnitude 7.2 earthquake that is considered an aftershock of a magnitude 8.4 earthquake that occurred in the same region off Kamchatka, in Russia, on February 3, 1923, generated a tsunami that was observed in the Los Angeles harbor, with swells that ebbed and flowed for four hours, rising and falling at least twelve times during that time period. The swirling currents caused havoc at the harbor, causing several ships to collide with other ships or with docks, and causing several smaller fishing boats to break their moorings. Damage at the port was estimated at about $5,000 (equivalent to about $90,750 in 2024). Note that tsunami runups associated with this smaller earthquake were larger than those reported two months earlier.

Earthquake and Tsunami of August 31, 1930
A magnitude 5.2 earthquake occurred in Santa Monica Bay, causing building damage in Venice, Los Angeles, Chatsworth, Hollywood, and Pasadena, among other localities. The earthquake was felt throughout the Los Angeles Basin, as far north as Kern County, San Diego County in the south, and Palm Springs in the east. Ground cracks, small landslides and rockslides were reported along the edge of the Palisades Bluff in Santa Monica. A tsunami variously reported as between 0.6 m (2 feet) and 6 m (20 feet) high was observed in Santa Monica7F 8 . One man was overtaken by the waves
and killed south of Redondo Beach, and “scores” of swimmers were rescued from the surf by lifeguards at Santa Monica.

Possible Tsunami of August 21, 1934
On August 21, 1934, large destructive waves were reported along the coast of southern California from Malibu to Laguna Beach. The true source of the waves is not known; however, several causative events have been suggested. Although official records show no large earthquakes in the area on the day of the waves, a small, magnitude 3 tremor was reported in the Balboa (Newport Beach) region before the waves struck. Submarine landsliding, volcanic activity, and unusual meteorological conditions (meteotsunami or rogue waves?) have also been suggested as possible explanations for the waves. A runup of 890 ft (270 m) inland, 10 ft (3 m) above mean high tide level was recorded at Newport Beach, which flooded part of that city to a depth of 3.3 ft (1 m). Four people were injured near the channel entrance to Newport Bay, at the western pier. Many houses were destroyed, including a two-story home in Balboa that was detached from its foundation. Part of the pavement on Balboa Peninsula was washed away, temporarily isolating the residents of this area from the mainland. Thousands of tons of debris were tossed onshore. The waves also flooded a moorage in Balboa Island and collapsed part of the breakwater in Long Beach (Lander and Lockridge, 1989).

Damage was estimated at $75,000.00 ($1.74 million in 2024). In a later publication, Lander et al., 1993) considered this event to have been triggered by remote storms and aggravated by high tides.

Aleutian Islands Tsunami of March 9, 1957
A magnitude 8.6 earthquake in the Aleutian Islands on March 9, 1957 generated a small tsunami in the San Diego area that damaged two ships in San Diego Harbor and caused minor damage at La

8 https://www.ngdc.noaa.gov/hazel/view/hazards/tsunami/runup-data?sourceMaxEventValidity=4&sourceMaxYear =2024&sourceArea=CA&sourceMinYear=1800&sourceMinEventValidity=2&sourceCountry=USA and https://www.ngdc.noaa.gov/hazel/view/hazards/earthquake/event-more-info/6619; accessed April 2024.

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Jolla (McMulloch, 1985; Iida et al., 1967; Salsman, 1959; Joy, 1968). A wave height of up to 3.3 ft (1 m) was reported at Shelter Island, off the San Diego coast, although the tide gauge there recorded only a 0.7 ft (0.2 m) wave. No reports of damage were recorded in the Los Angeles area. Tsunamis generated by this earthquake caused extensive damage in Oahu and Kauai, Hawaii, and near the source at Scotch Cap on Umnak Island.

Chilean Tsunami of May 23, 1960
On May 22, 1960, a moment magnitude (Mw) 9.5 earthquake off the coast of Chile produced a tsunami that damaged coastal communities in southern California between Crescent City and San Diego. A wave height of 4.6 ft (1.4 m) was recorded in Santa Monica and the tidal gauge in San Diego was carried away by the tsunami waves (Lander and Lockridge, 1989). Significant damage was recorded in the Los Angeles and Long Beach Harbors, where 30 small craft were sunk and over 300 were set adrift. Over 340 boat slips, valued at the time at $300,000, were also damaged in the area. At Santa Monica, eight small boats were swept away and a surge of 300 ft (90 m) flooded a parking lot along the Pacific Coast Highway. Damage of $20,000 was reported in the Santa Barbara area. At San Diego, two passenger ferries were knocked off course by the waves; the first ferry was pushed against a dock in Coronado, destroying 260 ft (80 m) of the dock, and the second was rammed into a flotilla of anchored destroyers. The waves also rammed a 100-ton dredge into the Mission Bay Bridge, knocking out a 70-ft (21-m) section and sinking a barge at Seaforth Landing (Lander and Lockridge, 1989; Iida et al., 1967; Talley and Cloud, 1962; Joy, 1968).

Good Friday (March 28) Earthquake Tsunami of 1964
On March 28, 1964, a Mw 9.2 earthquake in the Gulf of Alaska produced the largest and still, to date, the most damaging historical tsunami to ever hit the West Coast. The tsunami killed 16 people in northern California and Oregon and caused $8 million ($80.6 million in 2024 dollars) in damage in California. Although damage was primarily focused in coastal areas north of San Francisco, southern California experienced hundreds of thousands of dollars in losses. In Los Angeles Harbor, the wave damaged six small-boat slips, pilings, and the Union Oil Company fuel dock. It scoured the harbor sides, causing an estimated $175,000 to $275,000 in damage. The tsunami also destroyed eight docks in the Long Beach Harbor at a loss of $100,000 (Spaeth and Berkman, 1972). A wave amplitude of 3.3 ft (1 m) was recorded in Santa Monica, and eight surges of water were reported in Marina del Rey between 1:45 AM and 7:00 AM. In Santa Monica, one 16-foot outboard boat was sunk, and in Marina del Rey, several boats, a fueling station and floating anchorage were damaged, for an estimated $100,000 (>$1 million in 2024) in losses. Minor damage was reported elsewhere along the southern California coast.

Boxing Day (December 26) Earthquake and Tsunami of 2004 On December 26, 2004, a magnitude 9.1 to 9.3 earthquake off the northwest coast of Sumatra, Indonesia caused tsunamis in the Indian Ocean that resulted in nearly 228,000 confirmed fatalities in the region, with about 170,000 of those in Indonesia alone. The earthquake and resulting tsunamis displaced more than 1.1 million people in ten countries in South Asia and East Africa, making it one of the most devastating natural events in recorded history. The staggering loss significantly increased the world’s awareness of tsunamis as a potentially devastating natural hazard. The highest wave recorded as a result of this earthquake had a height of 167 ft (50.9 m). The tsunami was felt in coastal communities around the globe, in both the Pacific and Atlantic Oceans, making it a “global tsunami.” In Santa Monica, waves as high as 1.35 ft (0.4m) in height were recorded.

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As a result of this event, an international tsunami monitoring system was developed and deployed in the Indian Ocean with earthquake sensors on the ocean floor that trigger early warnings. This event also led coastal communities worldwide, including many cities in southern California, to identify evacuation routes, and develop training programs to teach people how to recognize a possible tsunami, with emphasis on evacuation and disaster response.

Chilean Earthquake and Tsunami of February 26-27, 2010
Approximately 4 hours after the February 26 (at 10:34 PM PDT) magnitude 8.8 earthquake off the coast of Chile occurred, the West Coast Alaska Tsunami Warning Center (WCATWC) issued a Tsunami Advisory for the entire California coast. The tsunami arrived in southern California approximately 10 hours after the advisory was issued, on the 27th, with highest wave amplitudes reported one to six hours after the first wave arrival. The forecast estimated a maximum tsunami amplitude of 3.87 ft in Santa Monica, but the actual observed amplitude was 2.01 ft. The highest tide gauge readings were reported in Pismo Beach (2.95 to 3.94 ft), Oxnard (3.28 ft), Santa Barbara (2.99 ft), and Santa Cruz (2.95 ft). Minor damage to docks and marine infrastructure was reported in Marina del Rey, Two Harbors (Catalina), Los Angeles, and Oceanside. Moderate damage to docks, concrete piers and boats was reported in North Shelter Island (San Diego Bay). No damage was reported in Newport Beach, Huntington Beach, Seal Beach, Long Beach, and La Jolla (Wilson et al., 2011). The tsunami advisory was cancelled by the WCATWC at 8:03 PM PDT.

Tohoku, Japan Earthquake and Tsunami of March 11, 2011
The magnitude 9.1 earthquake that occurred offshore along the northeast coast of Japan triggered a tsunami that caused widespread damage in northern Japan. The tsunami breached existing sea walls and caused the meltdown of three reactors at the Fukushima nuclear power plant, which in turn caused the discharge of radioactive materials into the surrounding area. Waves as high as 133 ft (40.5 meters) were recorded in Miyako, and in the Sendai area, the runup traveled inland 6 miles (10 km). In total, more than 22,000 deaths were reported, and four years later, in 2015, a survey showed that more than 228,000 people were still displaced.

The tsunami traveled throughout the Pacific Ocean region, arriving at the west coast of the United States about 10 hours after the initial shock, with tsunami activity lasting as much as 24 hours after the first wave arrived. Evacuation orders were issued for some US coastal areas, but at least five people in Oregon and California were swept out to sea (four were rescued), and several boats were torn off their moorings8F
9 . About $100 million in damages were reported for California ports and harbors, including Moss Landing, Ventura, and San Diego. Specifically, the tsunami waves destroyed a dock and damaged 13 boats in Mission Bay, a boat was sunk and a dock was damaged in south Shelter Island (San Diego), a pylon was damaged when hit by a boat in Dana Point, a boat was pulled off its moorings in Huntington Beach, and minor damage to docks and boats was reported in Los Angeles and Long Beach (Wilson et al., 2011). A Tsunami Advisory was issued for Santa Monica for over 24 hours and emergency response personnel closed the Santa Monica beach, adjacent parking lots and the Santa Monica Pier during the advisory. Maximum wave heights in Santa Monica were recorded at 2.8 ft.

9 https://www.bbc.com/news/world-us-canada-12714558, accessed in 2023 and April 2024.

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Tonga Volcanic Eruption and Tsunami of January 15, 2022 On January 15, 2022, violent eruptions from volcanic vents below the ocean surface generated a plume of gas, steam and ash that rose 36 miles (58 km) into the atmosphere; the plume eventually covered all of the Tongan islands. The explosions generated two different types of tsunamis: “classic” tsunamis produced by “the displacement of large volumes of water, and meteotsunamis caused by fast-moving pressure disturbances in the atmosphere.”9F 10 The tsunamis triggered by the eruption
reached up to 65 feet above sea level, destroyed hundreds of structures, displaced more than 1,500 people, and caused four deaths in the Tongan Islands. The tsunami waves propagated across the Pacific Ocean, causing damage in Fiji, Japan, New Zealand, Russia, Hawaii, Chile, and California.

In Peru, it caused an oil spill and two deaths. The relatively low number of deaths is attributed to an improvement in global tsunami preparedness.

The first tsunamis arrived hours earlier than expected in many shores around the Pacific Ocean, travelling at an unprecedented speed of more than 700 mph, and radiating in all directions.

Eventually these waves were observed all around the globe, circling the world approximately threeand-a-half times. These were determined to be meteotsunamis. The “classic” tsunamis arrived later.

In California, tsunamis associated with this volcanic eruption were reported in several areas, with water heights of between about 1.4 and nearly 4 ft recorded in some areas (see Table 7-1). These tsunamis are now thought to have been caused by pyroclastic flows emanating from the caldera and flowing down the underwater slopes of the volcano10F

11. A Tsunami Advisory that lasted approximately
19 hours was issued for the California coastline, including Santa Monica. Across Los Angeles County, people were directed to stay off the beach and out of the ocean.

TSUNAMI HAZARD ASSESSMENT FOR SANTA MONICA
Hazard Identification
Identifying hazardous locations is an essential step towards implementing more informed mitigation activities. Recognition that the southern California region is susceptible to tsunamis dates to at least the early 1960s, following the 1964 Alaska earthquake and tsunami. Early researchers looked at the long-term worldwide historical data and combined that information with mathematical models to estimate distantly generated, 100-year and 500-year probability teletsunami runup elevations for the west coast of the United States (Houston and Garcia, 1974, 1978; Garcia and Houston, 1975; Houston et al., 1975; Houston, 1980; as presented in McCulloch, 1985). The 100-year (R100) and 500-year (R500) teletsunami runup heights first predicted for the Santa Monica area were approximately 4.9 to 6.6 ft (1.5 to 2.0 m) and 8.2 to 10.8 ft (2.5 to 3.3 m), respectively (Houston, 1980, based on Figure 208 in McCulloch, 1985).

The 100- and 500-year risk levels selected were used by the Federal Insurance Administration in the 1980s to calculate flood-insurance rates, similar to storm flooding. As with flooding, the 100- and 500-year designations do not mean that these tsunamis occur only once every 100 or 500 years, but rather, these terms describe the tsunami that has a 1 percent (for 100-year) or 0.2 percent (for 500- year) probability of occurring in any one year. The 100-year and 500-year tsunami runup elevations were thought to have the potential to cause significant damage to harbors and upland areas, whereas

10 https://www.usgs.gov/centers/pcmsc/news/depth-surprising-tsunamis-caused-explosive-eruption-tonga; accessed December 3, 2024. 11 Ibid.

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smaller 50-year events could cause damage to boats and harbor facilities, with onshore damage restricted to very low-lying areas. Houston (1980) did not have the technology available to quantify the effect that estuaries, the offshore zone where water is 15 to 30 ft (5 to 10 m) deep, and the shoreline have on tsunami runup. Furthermore, Houston’s (1980) predicted heights were based on mean sea level elevation data, and thus did not consider the maximum credible heights possible if a teletsunami coincides with peak high tide, or with storm-induced high water.

Then, in the early 1990s, McCarthy et al. (1993) reviewed the historical tsunami record for California and suggested that the tsunami hazard in the southern California region from the Palos Verdes Peninsula south to San Diego, is moderate. However, as discussed previously, the southern California historical record is very short. Given that the recurrence interval for many of the faults in the world is in the order of hundreds to thousands of years, it is possible that southern California has been impacted by tsunamis for which there are no records. There are also several faults immediately offshore of the southern California area that are either thrust faults, or have some component of vertical motion, and are thus capable of triggering a local tsunami if they rupture in an earthquake.

Similarly, several submarine landslides and landslide-susceptible areas have been mapped offshore, within 2 to 9 miles (3.5 to 15 km) of the coastline (Field and Edwards, 1980; McCulloch, 1985; Clarke et al., 1985), and movement on any of these landslide-susceptible areas could also generate a local tsunami.

Synolakis et al. (1997) reviewed the McCarthy et al. (1993) study and other data, and concluded that not only do the early methods used to estimate tsunami runups yielded results that were in some areas more than 50% lower than anticipated but, as discussed above, the early models did not consider near-shore tsunami events. Synolakis et al. (1997) argued that near-shore tsunamis should be considered worst-case scenarios, as these have the potential to cause high runups that would impact the coastline with almost no warning. For example, Borrero et al. (2001) found that an earthquake in the Santa Barbara channel could generate a 6.6-ft (2-m) runup, whereas an earthquake-induced submarine landslide could generate a runup of as much as 66 ft (20 m). These analyses prompted the California Seismic Safety Commission (2005) to issue a report on tsunami threats where they indicated that both teletsunamis and locally generated tsunamis pose a significant threat to life and property in California.

In response to these concerns, in 2009 the University of Southern California Tsunami Research Group published tsunami inundation maps for a large section of coastal California. The maps showed the worst-case scenario based on an analysis of both local and distant tsunami sources and their impact on 33 coastal populated areas along the California coastline (Wilson et al., 2008).

Sources used to develop these maps included surface-rupturing earthquakes on offshore faults based on studies by Legg et al. (2003), submarine landslides off the Palos Verdes peninsula, and earthquakes in the Aleutians and Chile subduction zones. These models highlighted the concern with local tsunami sources; specifically, that the time it takes between the local, tsunami-generating earthquake and the arrival of the first waves along the coastline is estimated at 10 to 20 minutes.

This does not allow much time for broadcasting warnings and evacuation orders, so people in lowlying coastal areas need to be educated about these risks and to recognize that if they feel strong ground shaking they should evacuate to higher ground immediately.

The 2016 edition of the American Society of Civil Engineers (ASCE) building standard (ASCE 7-16), entitled “Minimum Design Loads and Associated Criteria for Buildings and Other Structures,” was the first edition of this building standard to contain a chapter on tsunami loads and design impacts.

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The standard included tsunami design maps for Hawai’i, Alaska, Washington, Oregon and California developed by Thio et al. (2010, 2017) and Wei et al. (2017). The tsunami design zones were mapped using a horizontal resolution of 60 meters (~200 ft), and considered several probabilities of occurrence in 50 years ranging from 2% to 50% (equivalent to annual frequencies of exceedance of between 1/2475 and 1/72 per year). The maps were based on subduction zone sources around the Pacific Ocean, and did not consider local tsunami sources such as crustal faults or offshore landslides. The ASCE 7-16 and 7-22 tsunami design zone maps used in the California Building Code are based on what is referred to as the Maximum Considered Tsunami, with ~2,500-year return period (approximately equal to a 2% probability of being exceeded in 50 years). The tsunami design zone, 3-ft inundation depth, and runup elevation limits for Santa Monica and any other area along the Pacific Coast of North America can be viewed online, here. The tsunami design zone or tsunami hazard zone is the area susceptible to being flooded by the Maximum Considered Tsunami. Updated tsunami hazard maps are expected to be issued as part of the ASCE-7-28 building standard.

On March 22, 2021, the California Geological Survey (CGS, also referred to as the State) issued new tsunami inundation maps for several coastline areas in Los Angeles and Orange counties, including Santa Monica (Figure 7-4) that were prepared by AECOM (Thio, 201911F 12). The tsunami hazard area
maps were created using a probabilistic tsunami hazard analysis approach similar to that used for the ASCE-7 standard that considered the tsunami waves generated by a variety of tsunami sources around the Pacific Ocean using several average return periods ranging from 72- to 3,000-year annual return probabilities. The maps issued to the public are based on a 975-year average return period (equivalent to a 5% probability of exceedance in 50 years), a smaller event than that used for the ASCE-7-16/22 maps. The probabilistic tsunami model used computes wave evolution as a result of changes in bathymetry and topography, with the maps from the CGS relying on 10-meter (~30-ft) resolution topographic data rather than the 30- to 90-meter (~100- to ~300-ft) resolution used for the older 2009 maps. The inundation limits developed were then smoothed to create a realisticlooking inundation boundary, modified to consider levees or other features not recognized by the topographic data that would limit the reach of the inundation zone, and matched to geographic features or city streets. The final inundation boundaries presented in the published maps also rely on input from local emergency managers, first responders and other experts and stakeholders that were asked for advice regarding the “placement of the final hazard area in places that would help the public and government safely evacuate during a tsunami event” (CGS, 2021). For a more indepth description of the process refer to Thio et al. (2010), Thio (2019) and Graehl et al. (2019)12F

13. The tsunami hazard zone and evacuation area defined by the CGS for the Santa Monica area (red shaded region on Figure 7-4) extends approximately 700 to 1,500 ft (215 to 460 m) inland from the coast, and places all structures and parking lots west of the Pacific Coast Highway (PCH) and Appian Way within the area susceptible to tsunami inundation. These structures include the Santa Monica Pier and buildings associated with it, in addition to the residential and commercial buildings on the west side of the roads mentioned above. North of the Pier, the inundation limits are defined by the base of the sea bluffs, as the height of the bluffs provide a natural constraint to the inland

12 https://docftppub.blob.core.windows.net/temp/202305042119/AECOMProbabilisticTsunamiHazardMapsForCalifornia-Phase2.pdf?sv=2020-02-10&st=2024-12- 06T23%3A51%3A30Z&se=2024-12-
20T23%3A51%3A30Z&sr=b&sp=r&sig=/Q6BXk7HEm2w7qOEr2iOjOECgP3DIfeSXRe1kZxStks%3D, accessed in December 2024. 13 https://www.conservation.ca.gov/cgs/Documents/Tsunami/Tsunami-inundation-map-methodology-2019.pdf, accessed in December 2024.

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advancement of the runup. Significant damage to the base of the bluffs and PCH can be expected.

In the southwestern portion of the city, the inundation limits extend locally to Neilson Way. This places several multi-family residential structures within the hazard zone. In addition to the Pier, Cityowned or managed facilities located in the inundation hazard zone include Beach Maintenance, the Santa Monica Urban Runoff Recycling Facility, and the Annenberg Community Beach House.

Note that the 2021 tsunami hazard zone identified by the CGS extends offshore past the beach and into the inner shelf area of Santa Monica Bay. Swimmers, surfers, and mariners within this area are at risk because a tsunami causes strong currents that can sweep people and objects out to sea.

Floating debris can also cause extensive damage. A common occurrence as a tsunami travels over the continental slope and approaches the shore is the formation of a trough, with the sea receding from the shore. Beach goers that observe this phenomenon should immediately run inland and to higher ground, before the tsunami runup occurs. A tsunami typically causes strong surges and rapid changes in sea level that may persist for hours after the first wave arrives. Furthermore, the first wave may not be the largest wave to hit the shoreline, so people should avoid the beach for, at a minimum, several hours after the tsunami first hits the area. Computer simulations for the northern and central California coastlines showing the complicated tsunami wave pattern that may develop are available at the link provided.

As indicated above, the State-issued tsunami hazard map presented on Figure 7-4 is based on a tsunami event with an approximately 975-year return period, which is a smaller tsunami event than the ~2,500-year-return-period event used in the ASCE-16/22 standard (and the 2022 edition of the California Building Code [CBC], which is based on the ASCE standard). As a result, the maps in the CBC in general show a wider inundation zone that extends east of PCH north of the California Incline, and along a stretch between approximately Arizona Avenue and just north of the Pier.

However, because there are no structures east of PCH along the northern portion of the city (just the bluffs), the anticipated losses in this section of the Santa Monica are the same for both events. In the southern portion of the city, the CBC map includes in the inundation zone several buildings on the east side of Barnard Way, between Strand Street and Hill Street, and between Barnard Way and Neilson Way from Hill Street south to the city boundary. However, in the southwestern portion of the city, the State (CGS, 2021; Figure 7-4) map is locally more conservative than the CBC map because the inundation limits were modified to match city streets, locally extending the inundation zone inland to Neilson Way.

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Vulnerability and Risk
A vulnerability assessment for tsunamis can help predict how different types of property or infrastructure and population groups will be affected by the hazard of tsunamis. As mentioned in previous sections, some of the most important factors defining the hazard of tsunamis include the source (offshore fault or submarine landslide, volcanic eruption, or some other cause), the location and distance of the source relative to the city’s coastline, the slope of the continental shelf, and whether or not there are canyons, bays, or other bathymetric features that could funnel the tsunami inland or cause complicated (and dangerous) wave patterns and currents. The wide, relatively shallow continental shelf in Santa Monica Bay, in front of Santa Monica, helps to slow down incoming tele-tsunami waves and decrease their wavelength, which in turn can increase the runup height. Depending on the location of the tele-tsunami source relative to Santa Monica Bay, the offshore Channel Islands could shield the bay from the waves, or could interfere with the pattern of the waves, intensifying their effect as they approach the coastline. All of these conditions complicate the development of tsunami models used to estimate the impact that a tsunami may have on the region. Modelers typically compare the analytical results developed from these models with the historical record to evaluate the reliability of their results (Arcas, 2015).

The data presented in Table 7-1 suggests that teletsunamis sourced in Chile, Central America, Alaska and Japan have created significant historical runup inundations in Santa Monica and other nearby coastal areas. The tsunami hazard maps issued by both the CGS and the ASCE (see the Hazard Identification section above) are based on teletsunamis sourced out of Alaska and the Cascadia Subduction Zone, as these analyses showed that these sources pose the greatest risk to the California coastline. This certainly seems to be the case given the widespread geographic reach of a tsunami generated from a distant source, with the entire California coastal zone at risk of being inundated, and the fact that both of these subducting areas are seismically active and have a high probability of generating a significant tsunami-generating earthquake during the time-frame considered for the probabilistic analyses. Significantly, the most recent tsunami hazard maps that have been prepared for California do not include a tsunami-generating earthquake in the West Melanesian Trench, a slow-moving to potentially currently inactive subducting trench north of Papua New Guinea. A tsunami forecast model for Santa Monica (Arcas, 2015) considered tsunamis sourced from 20 different locations around the Pacific Ocean basin, and found that a tsunami generated along the West Melanesian Trench could generate maximum runups of more than 7.5 meters (~25 ft) in Santa Monica. These runup levels are three times higher than the second-largest runups modeled, which originated along the New Zealand-Kermadec-Tonga trench.

However, the highest water levels that have been reported historically in the Santa Monica Bay (based on Table 7-1) appear to have been associated with nearby tsunami sources, including earthquakes generated by faults in the bay or in the Santa Barbara Channel, and possible offshore submarine landslides mobilized by earthquakes on faults in the southern California region. These local tsunami sources are particularly concerning because a tsunami could reach the coastline in minutes rather than the hours it takes for a teletsunami to reach the area, and a tsunami warning may not be issued in time to evacuate the inundation hazard zone. Moreover, depending on the volume of water displaced by the fault rupture or submarine landslide, the tsunami-generated water heights (runups) could be higher than the modeled runups used to generate the tsunami hazard zone maps discussed in the previous section, meaning that an area larger than that delineated in the tsunami hazard zone could be impacted. Finally, the time of day when the tsunami hits the coast would also have an impact on the number of people exposed. All of these unknowns make it difficult to conduct a reliable analysis of possible tsunami losses. Nevertheless, as discussed in the section Local Hazard Mitigation Plan Section 7
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below, this report summarizes the results of three different analyses that provide generalized information on the impact that a tsunami could have on the coastal area of Santa Monica.

Risk Assessments - Methodology
For this report, we relied on three different sources or methods to obtain estimates of the impact that a tsunami would have on the city of Santa Monica. These include:

1. Loss analyses conducted and reported by the CGS (Becker et al., 2023) using FEMA’s HazUS Version 6.0 Tsunami Model and the tsunami hazard zone shown on Figure 7-4, which is based on a tsunami with a 975-year return period generated by an earthquake in the subduction zone along Alaska’s Aleutian Islands. This teletsunami is estimated to take approximately 320 minutes (5 hours and 20 minutes) to reach the coastline along Los Angeles County, providing ample time for evacuation orders to be issued. Significantly, these analyses were conducted at the County level, so Santa Monica-specific results are not available. The results provided apply to all of Los Angeles County.

2. Loss analyses conducted for this study also using FEMA’s Version 6.0 HazUS Tsunami Model and a similar inundation hazard zone as above, but with the tsunami generated by an earthquake on the offshore section of the Palos Verdes fault in Santa Monica Bay. The tsunami generated by this hypothetical earthquake would take only a few minutes to reach the coastline, so advanced warnings of the incoming tsunami are unlikely. This analysis was conducted specifically for the City of Santa Monica.

3. Economic analysis conducted by the City of the properties in Santa Monica within the tsunami inundation zone identified by the CGS (2021) and shown on Figure 7-4, using the damage assessment software Crisis Track. The software allows calculation of the minimum and maximum cost for residential and commercial properties within the specified hazard zone. Different damage levels based on FEMA’s Preliminary Damage Assessment 4-Point Terminology were applied to generate a range of loss amounts based on the most recent tax assessment values (2023) available at the time of the analysis rather than market valuations.

As discussed further below, all three of these methods have limitations, so the results provided should be considered carefully and judiciously, recognizing that the true impact of a tsunami on Santa Monica is likely to have severe, long-term implications on the economy of the City that were not fully captured here. Similarly, because it is difficult to estimate the total number of people that may be in the city at the time of a tsunami, especially one sourced nearby, the casualty estimates may significantly underestimate the total actual counts should a tsunami impact the area.

Risk Assessments - Results
County-Level Loss Estimates Published by the California Geological Survey The County-level HazUS results published by the CGS (Becker et al., 2023) indicate that if all individuals within the tsunami inundation zone heed the evacuation orders within 15 minutes of receiving the notification, no casualties would be reported in Los Angeles County. Casualties are defined as the projected sum of fatalities and injuries resulting from the number of people exposed to the tsunami, with the population estimates based on 2020 Census data and advanced demographic at-risk population profile data from Esri. If, on the other hand, people take 30 minutes to depart after receiving orders to evacuate (referred to as “evacuation departure delay time”), an estimated 197 casualties are expected if the tsunami occurs during daylight hours. If the tsunami occurs at night, an estimated 117 casualties are estimated. A larger number of casualties can be Local Hazard Mitigation Plan Section 7
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expected for longer evacuation departure delay times. The daytime Los Angeles County population count within the tsunami hazard zone used in the analyses was 85,162. This figure includes residential, commercial and institutional populations within the tsunami hazard zone. The figure however does not account for day visitors to the coastal areas, so higher casualty numbers are likely.

For the casualty analyses, 99% of the population exposed to a flood height of 2 meters (~6.6 ft) are expected to perish, with only 1% being injured. For flood heights of less than 2 meters, the fatality and injury rates are each estimated at 50%.

As will be discussed in more detail in the next sub-section, the HazUS Tsunami Model estimates building damage counts, classifying damaged buildings into four categories as follows:

1. Buildings that incur losses that amount to less than 5% of their value are considered to be Affected.

2. Buildings that incur losses that amount to between 5 and 30% of their total value are considered to have Minor damage.

3. Major damage means that the losses incurred amount to 30 to 50% of the building’s value, and the

4. Destroyed category captures all buildings with damage that exceeds 50% of their value.

These categories generally correspond to the tagging nomenclature used in the post-disaster inspection of buildings (green, yellow and red), hence the colors assigned to each column in the table below. The loss value includes both structural and non-structural elements, but does not include the value of the land. Total building damage counts and residential building damage counts for Los Angeles County are presented in Table 7-2 below.

Table 7-2: Total Building and Total Residential Building Damage Counts for Los Angeles County Based on a HazUS Tsunami Analysis of the CGS (2021) Tsunami Hazard Maps for California Damage Category
Building Type
Affected
(Green Tag)
Minor
(Green Tag)
Major
(Yellow Tag)
Destroyed
(Red Tag)
Total Buildings 7,348 996 9 34
Residential Buildings 6,394 875 1 26
Source: Extracted from Becker et al. (2023)
Total statewide losses as a result of this tsunami scenario are estimated at $12.6 billion. Losses in Los Angeles County alone amount to about $600 million, with approximately half of that associated with residential structures. It is difficult to estimate what percentage of this total amount would pertain specifically to losses in Santa Monica, but a conservative estimate would be about 10 to 20% of the total, which amounts to between about $60 and $120 million.

Loss Estimates Due to a Local Tsunami Using HazUS
As discussed in far more detail in Section 6, HazUS is a GIS-based loss estimation methodology funded by and available through FEMA. The software includes several components or modules, with tsunami being one of them. The tsunami modeled for this study was sourced in the Santa Monica Bay, as a result of fault rupture on the offshore section of the Palos Verdes fault, and was assumed to generate a 6-meter- (~20-ft-) high runup that would essentially inundate the same area identified in the State-issued tsunami hazard map. The principal difference between this analysis and that conducted by the State and described in the sub-section above is that the tsunami generated in the Local Hazard Mitigation Plan Section 7
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bay would arrive at the coastline in 10 minutes, with maximum inundation occurring in 15 minutes.

An official tsunami warning alert within this time could potentially be issued within 5 to 10 minutes of the earthquake, but that really would limit the amount of time people would have to evacuate. In this case, as directed in tsunami education, the strong shaking generated by the earthquake itself should be considered the tsunami warning, with people evacuating to higher ground immediately after shaking subsides.

As discussed above, the tsunami module estimates the number of people that will be injured and killed by the hypothetical tsunami event. To do this, three community tsunami preparedness levels are considered: “Good,” “Fair” and “Poor.” The “Good” level represents a well-prepared community that has invested on education programs, signage showing evacuation routes, and/ or is otherwise prepared because of prior experience (a “Tsunami Ready” community), whereas a community with a “Poor” level has had very little experience and has not prepared its local population to respond to the hazard of tsunamis. Given the large number of out-of-town beachgoers (visiting population) in Santa Monica that may not be aware of the tsunami hazard and may not know to respond immediately upon feeling shaking, different preparedness levels are considered appropriate for the analysis.

The casualty estimates also consider two different times of the day when the tsunami occurs: 2:00 AM and 2:00 PM. These times represent the periods of the day when different sectors of the community are at their peak occupancy loads. Specifically, during the 2:00 AM scenario the residential load is considered to be at its maximum, whereas the 2:00 PM scenario time considers the educational, commercial and industrial sector loads to be at their maximum. The population estimates use Census 2020 data, and do not include the significant seasonal and daytime increase in population due to regional and out-of-town visitors. Therefore, the casualty estimates are likely to undercount the total number of casualties. Pedestrian travel time using standard walking speeds and reduction factors based on the local demographics, such as population 65 years old and older, and estimated number of individuals in the evacuation zone with mobility limitations, are considered in the analysis. The estimated travel times are then combined with the estimated tsunami travel time, warning time and community reaction time to estimate the number of potential casualties. The casualty results are summarized in Table 7-3 below.

Table 7-3: Estimated Tsunami Casualties in Santa Monica by Time of Day, Exposed Population and Community Preparedness Level (for a Local Tsunami Scenario) Day Casualties (2:00 PM)
Exposed Population
Community Preparedness Level
Good Fair Poor
Under
65 65
and
Over
Total
Population Fatalities Injuries Total Casualties Fatalities Injuries Total Casualties Fatalities Injuries Total Casualties 1,857 166 2,023 281 281 561 931 545 1,476 1,392 324 1,716 Night Casualties (2:00 AM)
Exposed Population
Community Preparedness Level
Good Fair Poor
Under
65 65
and
Over
Total
Population Fatalities Injuries Total Casualties Fatalities Injuries Total Casualties Fatalities Injuries Total Casualties 729 119 848 80 80 160 376 237 613 572 145 717
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The estimates calculated by HazUS indicate that about 2,023 people would be within the tsunami inundation zone during the day, including residential, commercial and institutional occupancies.

These numbers are based on Census (2020) data, and thus do not include visitors to the beach, pier, and adjacent roads and businesses, nor motorists on PCH and other roads adjacent to the beach.

Given that on a regular summer day, thousands of people visit the beach and the pier, the population counts likely grossly underestimate the number of people that would have to evacuate the area upon feeling strong ground shaking, and thus the total number of casualties. The population count for the nighttime scenario is based on maximum residential loads and estimates that 848 people within the tsunami hazard area would have to evacuate. These numbers are expected to be much closer to the true values on any given night in the area.

Depending on the level of preparedness considered, the number of casualties during a daytime tsunami event are estimated at between 516 and 1,716 (with these numbers again representing only the Census-counted individuals within the tsunami hazard area). The number of fatalities ranges between about 281 (for a community preparedness level of “good”) and 1,392 (for a community preparedness level of “poor”). Total number of casualties as a result of a nighttime tsunami event ranges between 160 and 717, with the total number of fatalities ranging between 80 and 572.

The HazUS tsunami model estimates that a total of three (3) structures would be completely destroyed, with total building losses estimated at $15 million. Note that this estimate is lower than the estimates obtained using other methods. Nearly 78% of this value pertains to loss of contents, and 7% accounts for business-interruption losses including loss of income and inventory, and relocation expenses. Residential occupancies are estimated to account for about 41% of the estimated $15 million in total losses.

As discussed in both Section 6 (Earthquakes) and Section 9 (Landslides), an earthquake on the offshore section of the Palos Verdes fault, which was used as a source for the local tsunami scenario described above, could result in damage of the slopes along PCH, which in turn would impact the use of PCH as an evacuation route. A sudden tsunami runup onto PCH is also likely to severely impact traffic patterns and result in additional casualties, with closures or limited access on PCH anticipated to result in major disruptions to emergency services, including the mobilization of people in need of emergency services to the local hospitals Loss Estimates Using Crisis Track®
Crisis Track®, a damage assessment software used by local governments, including the City of Santa Monica, was also used to develop estimates of the monetary impact that a tsunami event would have on Santa Monica. This software also classifies monetary losses based on the nomenclature used in post-disaster inspection of structures, as described on page 7-20. The software calculates the economic losses to residential and commercial structures within a user-designated zone. For this analysis, the City used the tsunami hazard area on Figure 7-4. The results of this analysis are summarized on Table 7-4 below.

Table 7-4: Estimated Economic Losses to Residential and Commercial Buildings in Santa Monica Using Crisis Track® Software
Damage Category Affected
(Green Tag)
Minor
(Green Tag)
Major
(Yellow Tag)
Destroyed
(Red Tag)
Total Cost $18,254,688.90 $45,636,722.25 $109,528,133.40 $182,546,889.00 Local Hazard Mitigation Plan Section 7
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The estimated losses calculated using this approach are also expected to be on the low end as the software does not compute damage to public infrastructure such as roadways, utilities, and beaches, and the tax assessment valuations used are likely to under-report the true value of the structures. It is also important to note that this analysis does not consider the structural characteristics of individual buildings or their location within the tsunami inundation zone; instead, the same damage level is considered for the entire inventory of structures, with total loss estimates calculated for each of the four different damage levels assumed for the entire study area. Given that different damage levels can be expected depending on the structure type and distance from the coastline, the true cost of the anticipated damage to the residential and commercial structures impacted by the tsunami is likely to be somewhere between the minimum and maximum values considered (between about $18 million and $182.5 million).

TSUNAMI MITIGATION ACTIVITIES
Tsunamis, like earthquakes, cannot be avoided, and their occurrence cannot be predicted. However, extensive instrumentation of the ocean floors in the Pacific Ocean following the 1948 Alaska earthquake, and increased instrumentation following the 2004 Indian Ocean tsunami now means that immediately after an earthquake, tsunami warning centers can detect whether or not a tsunami has been generated, and if so, predict where and when the tsunami is likely to reach different shorelines. These messages are transmitted to local agencies as needed, allowing local emergency managers to implement tsunami evacuation warnings or orders, as appropriate. The Tsunami Center at the National Oceanic and Atmospheric Administration (NOAA) uses a four-category system to convey information about the potential for a tsunami to impact a region following an earthquake, as summarized on Table 7-5 below.

Table 7-5: Tsunami Alerts and Appropriate Responses
Alert Type Potential Hazards Appropriate Action
Warning
DANGER! TSUNAMI IMMINENT!
Dangerous coastal flooding and
powerful currents are possible. Wave
heights > 3 ft or unknown.

Get to high ground or inland
IMMEDIATELY. Follow tsunami
evacuation signage.

Advisory

STRONG CURRENTS AND
DANGEROUS WAVES in or very near
coastal water. Wave heights of 1 -3 ft.

Stay out of the water, and away from
beaches and waterways.

Watch
TSUNAMI IS POSSIBLE. Alert level
may change once more information is
known.

Be prepared to take action. Stay tuned
to local radio / TV / NOAA “alert”
weather radios
Information
Statement
NO TSUNAMI IMPACT EXPECTED.

Alert level may change once more
information is known.

No action needed.

Sources: https://www.weather.gov/safety/tsunami-alerts and https://www.tsunami.noaa.gov/pmeltheme/forecast-warning Some communities, like Japan, which has been impacted by tsunamis dozens of times, have built seawalls designed to stop or reduce the impact of tsunamis. The difficulty in the design of these Local Hazard Mitigation Plan Section 7
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structures, however, is to predict the maximum wave height likely to be experienced in a given area, and build walls that are high enough. Japan has a long historical record of tsunamis, with markers that show the limits of previous, historical tsunami runups, and yet, the 2011 tsunami was larger than expected, and overtopped the existing seawalls. That in turn trapped water on the landward side of the seawall, adding to the overall damage and loss of life. Some communities, especially since the 2004 Indian Ocean tsunami, have recognized the importance of protecting and enhancing mangroves, sand bars, and other natural systems that slow down the waves and protect inland areas from debris carried by the tsunami. Avoiding the construction of buildings and infrastructure within the hazard area, building instead on areas inland or elevated above the projected inundation levels, is recommended and now often considered in site planning, but these mandates are generally not retroactive and do not apply to structures already built within the hazard zone. Note that the most recent California Building Code, which is based on the ASCE-7/16-22 standard, requires that tsunami loads be considered for new structures built within the tsunami hazard area. Building types that perform best in tsunamis include concrete, masonry and heavy steel frames if built to modern building code standards. In Japan some buildings within the tsunami hazard zone that have been built to withstand tsunami loads have been used as vertical evacuation areas, with people converging in the upper floor or roof, which has been built to be above the anticipated water level.

Other communities have invested significant resources in education and outreach efforts to prepare their population for future tsunamis. These programs may include some or all of these items:
• preparing evacuation plans that identify roads and pathways to be used to reach higher ground in the event of an evacuation
• installing signs showing the tsunami evacuation routes and the intended destination areas outside of the potential tsunami inundation zone
• deploying alarm systems and/or reverse 911 communication systems that can be used to provide evacuation instructions and other pertinent important information • preparing and disseminating educational pamphlets or other materials that describe when and what to do if an evacuation order is issued
• conducting regular drills to test the preparation level of first responders, identify bottlenecks and procedures that could be improved, and increase the awareness of residents and workers in the tsunami hazard zone
These efforts work best when disaster mitigation is community-based, with local residents involved in the planning process and emergency drills. These efforts allow community members to define specific concerns and problems, and identify potential solutions. World Tsunami Awareness Day is on November 5th. Many communities organize educational drills and other activities on that day to raise awareness of the dangerous effects of tsunamis and to highlight the importance of having tsunami evacuation plans in place. Communities that implement a series of disaster-preparedness activities can become certified as a TsunamiReady community. The guidelines that need to be met to obtain this certification are described here.

As discussed in the previous sections, the California Geological Survey (CGS) in collaboration with the Governor’s Office of Emergency Services (Cal OES) and other partners, have prepared maps showing the tsunami hazard zones in the State. The map that covers the Santa Monica area is included here as Figure 7-4. Together with these products, these agencies have also prepared and provided several resources designed for emergency managers in areas susceptible to tsunami hazards. These resources are described in the paragraphs below. Information on these products was Local Hazard Mitigation Plan Section 7
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extracted from this website. For additional information and links to download the Special Reports discussed below, refer to the link provided.

1. Maritime Tsunami Response Playbooks (CGS Special Report 241; Wilson et al., 2016): This report and the individual “playbook” plans were developed in response to the impact that the 2010 Chile and 2011 Japan tsunamis had on the California coastline and the harbors and ports therein. Authorities in charge of these facilities were interested in more detailed information on the estimated hazard and impact that a future tsunami would have on these maritime communities. In response, the California Tsunami Program developed and provided harbor-specific maps and other products detailing the estimated currents, wave heights, and damage potential, the minimum offshore safe depth for vessels to evacuate to in advance of an incoming tsunami, and specific recommendations for each harbor to follow depending on the estimated size and damage potential for tsunamis of different sizes.

The “playbooks” provide harbor and marina officials with a variety of response maps for different-sized tsunamis and different source origin locations, with guidance regarding actions that can be taken based on the anticipated in-harbor hazards (such as strong currents and eddies, damage potential to docks, piles and other infrastructure) and the areas offshore where boats can sit out the tsunami without being damaged (remember than in open, deep seas tsunamis are almost imperceptible). The decisions taken, including tsunami evacuations and response activities, are made by each maritime community, but these decisions can be guided by the instructions recommended in their Playbook Plan.

The California Tsunami Program, together with FEMA and other partners involved in the preparation of these documents also envisioned that the Playbooks would be used not only for tsunami response, but for pre-disaster planning and preparedness, including the identification of mitigation measures and infrastructure improvements that would make their facilities more disaster-resilient. Funding to make some of the identified improvements could then be sought, including, possibly, through additions to their Local Hazard Mitigation Plans.

2. Tsunami Emergency Response Playbooks and FASTER Tsunami Height Calculation:

Background Information and Guidance for Use (CGS Special Report 236; Wilson and Miller, 2014): These products are similar to the playbooks described above, but are meant to help communities susceptible to tsunami hazards prepare for an incoming tsunami by providing implementation actions tailored to that individual tsunami event. As with the maritime tsunami response playbooks, these products were also developed in response to the impacts to the California coast from both the 2010 Chile and 2011 Japan tsunamis.

Typical tsunami evacuation plans at that time identified the evacuation area for a worst-case tsunami event, which typically included areas at elevations below 30 to 50 feet. In some communities, this amounted to a large evacuation area that was not applicable to most tsunami events and required significant resources to implement. The alternative was to not issue an evacuation order at all, placing individuals near the coastline in danger when the tsunami arrived.

To address these concerns, the California Tsunami Program developed the approach referred to by the acronym FASTER, which allows emergency managers to estimate tsunami flood conditions, including runup and flood elevation, at given locations for specific tsunami Local Hazard Mitigation Plan Section 7
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events, which in turn helps determine which areas should be evacuated. Input variables used by FASTER include:

FA = Forecasted amplitude (i.e., wave heights), which are calculated and provided by the National Tsunami Warning Center in the first few hours after a tsunami is generated Storm = Storm surge or existing ocean conditions. These values are predicted and issued by the regional NOAA Weather Forecast offices
T = Maximum Tidal height anticipated in the first 5 hours of a tsunami hitting the coastline. These values are also issued by NOAA
E = Forecast modeling Error potential which, based on an analysis of past historical events (Wilson et al., 2012), has been calculated to be about 30% of the forecast amplitude
R = Site-specific amplified Runup potential, a value calculated from existing State tsunami modeling.

The City of Santa Monica has a Tsunami Playbook prepared by the CGS that emergency managers in the City use to estimate runup elevations and evacuation areas when a teletsunami warning is issued. Public safety personnel are prepared to use the local Tsunami Playbook is to implement evacuations during a tsunami event. In addition, the Playbook is used to educate public safety personnel on potential tsunami impacts, design local tsunami evacuation plans and maps, and assist in the design of realistic disaster drills. The City’s tsunami hazard zone, evacuation routes, and evacuation sites are reviewed regularly by Office of Emergency Management, Santa Monica Police, and Santa Monica Department of Transportation personnel and are publicly available online.

The City of Santa Monica is designated a TsunamiReady Community by the National Weather Service based on established program criteria. It regularly participates in statewide tsunami drills, testing notification procedures and use of the Playbook. The City has pre-scripted alerts and warnings that can be quickly distributed to the public using overhead speakers on the Santa Monica Pier, reverse 911 communications, or even Wireless Emergency Alerts (WEA) to anyone who is within the hazard area. The City participates in National Tsunami Preparedness Week and World Tsunami Awareness Day by educating the public and staff on tsunami inundation zones and actions to take during a tsunami event. The City includes tsunami education in its general public outreach activities, including a designated tsunami preparedness website that provides direct access to tsunami hazard maps, meanings of tsunami terminology, and general preparedness information.

TSUNAMI RESOURCE DIRECTORY
City Resources
City of Santa Monica Office of Emergency Management
City agency that administers the implementation of disaster planning, training and community-based preparedness programs to ensure that Santa Monica is a disaster-resilient community. City-prepared information on tsunami preparedness is available from this link.

County Resources
Los Angeles County Emergency Response Website
When a disaster occurs, this website is activated to provide information in real-time about the incident, evacuation instructions, shelter locations and other important information.

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Los Angeles County Office of Emergency Management (OEM) This agency is responsible for comprehensively planning for, responding to and recovering from large-scale emergencies and disasters that impact Los Angeles County. OEM’s work is accomplished in partnership and collaboration with first response agencies, as well as non-profit, private sector and government partners.
500 W. Temple Street, Room 358,
Los Angeles, CA, 90012
Ph: (213) 974-1311
State Resources
California Geological Survey, Southern California Regional Office 320 W. 4th Street, Suite 850
Los Angeles, CA 90013
Ph: 213-239-0877
Fax: 213-239-0894
California Geological Survey, Headquarters
801 K Street, MS 12-30
Sacramento, CA 95814
Ph: 916-445-1923
Fax: 916-445-5718
Governor’s Office of Emergency Services (Cal OES)
Website provides information on disaster planning, preparedness and prevention; disaster response; getting assistance following a disaster, and recovery operations. 3650 Schriever Avenue
Mather, CA 95655
Ph: 916-845-8510; Fax: 916-845-8511
www.caloes.ca.gov
California Tsunami Program (CTP)
Collaboration between Cal OES and the CGS Tsunami Unit that also partners with the National Weather Service and other agencies at the local level (including counties, cities, and community workgroups) to lead California’s efforts to assess the tsunami hazard and risks of California’s coastal communities. Their website is the gateway to a variety of tsunami products, including maps, preparedness guides, educational materials, technical reports and data, and publications. New tsunami maps for California can be accessed from this site.

TsunamiZone.org
A consortium of state and Federal partners including NOAA, the National Tsunami Hazard Mitigation Program, the USGS, Cal OES and the Statewide California Earthquake Center (SCEC), with support from the CGS, FEMA, the University of Southern California and the Earthquake Country Alliance. The TsunamiZone website provides information on how to understand the warnings, how to better prepare for earthquakes and tsunamis, including materials on preparedness drills, and registering as a tsunami-ready individual, family, business, school, group, or organization.

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Federal Resources and Programs
National Oceanic and Atmospheric Administration –
National Tsunami Hazard Mitigation Program
U.S. Tsunami Warning System
Lists tsunami warnings, advisories, watches or threats in real-time, around the world. 910 S. Felton Street
Palmer, AK 99645 USA
Ph: 907-745-4212
Federal Emergency Management Agency (FEMA) – Region IX
For information on disaster declarations and disaster assistance, and the use of HazUS. 1111 Broadway, Suite 1200
Oakland, CA 94607-4052
Ph: 510-627-7100
Fax: 510-627-7112
Southern California Field Office Ph: 626-851-7900
Tsunami Page at Ready.gov
A website of the U.S Government that provides information on what to do before, during and after a tsunami.

Publications
The resources presented in the previous section include links to many publications and educational materials.

For the References cited in this section, refer to Appendix I.

There are hundreds of publications on tsunamis, and many more are published every year. The modeling of tsunamis is evolving rapidly as computer power increases and better bathymetric models are developed. The publications below set the stage for many of the more recent models that were used to develop the CGS Tsunami Hazard Maps and the ASCE-7-16/22 and ASCE-7-28 tsunami section.

Ross, S., and Jones, L. (editors), 2013, The SAFRR (Science Application for Risk Reduction) Tsunami Scenario: U.S. Geological Survey Open-File Report 2013-1170 and California Geological Survey Special Report 229; available from https://pubs.usgs.gov/of/2013/1170/. Similar to the ShakeOut San Andreas fault earthquake scenario, this report addresses a hypothetical tsunami scenario generated by an earthquake offshore the Alaska Peninsula, and its impacts on the California coast.

Wilson, R., Hemphill-Haley, E., Jaffe, B., Richmond, B., Peters, R., Graehl, N., Kelsey, H., Leeper, R., Watt, S., McGann, M., Hoirup, D., Chague-Goff, C., Goff, J., Caldwell, D., and Loofbourrow, C., 2014, Chapter C: The search for geologic evidence of distant-source tsunamis using new field data in California, in Ross, S.L., and Jones, L.M., editors, The SAFRR (Science Application for Risk Reduction) tsunami scenario: U.S. Geological Survey Open-File Report 2013–1170–C, 122 p., doi:10.3133/ofr20131170c.

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Wood, N.J., Ratliff, J. and Peters, J., 2012, Community Exposure to Tsunami Hazards in California:

U.S. Geological Survey Scientific Investigations Report 2012-5222; available at https://pubs.usgs.gov/sir/2012/5222/.

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SECTION 8: FLOOD HAZARDS
WHY ARE FLOODS A THREAT TO THE CITY OF SANTA MONICA?
Floods are natural and recurring events that only become hazardous when human activities and structures encroach onto floodplains, modifying the landscape and building structures in the areas meant to convey excess water during floods. Flooding is one of the most destructive natural hazards in the world, and average annual flood losses have increased steadily over the last decades as development in floodplains has increased. Thus, in 2005, the U.S. Department of Homeland Security reported that flood damage costs nationwide exceeded $5 billion per year, while in 2022, the National Centers for Environmental Information0F
1 , a division of the National Oceanic and
Atmospheric Administration (NOAA), reported that flooding has an average cost of $4.6 billion per event, and is the fourth most costly natural hazard in the US (cyclones, drought and wildfires occupy the first three rankings. In short, flooding poses a threat to life and safety, and can cause severe damage to public and private property.

Under the National Flood Insurance Program, a flood is:
a) a general and temporary condition or partial or complete inundation of normally dry land areas from:
(1) the overflow of inland or tidal waters,
(2) the unusual and rapid accumulation or runoff of surface waters from any source, or (3) mudslides (i.e., mudflows) which are caused by flooding and are akin to a river of liquid and flowing mud on the surfaces of normally dry land areas, or b) the collapse or subsidence of land along the shore of a lake or other body of water as a result of erosion or undermining caused by waves or currents of water exceeding anticipated cyclical levels or suddenly caused by an unusually high water level in a natural body of water, accompanied by a severe storm, or by an unanticipated force of nature, such as flash flood or abnormal tidal surge, or by some similarly unusual and unforeseeable event which results in flooding.

This very broad definition of flooding is used in this document to address the potential for partial or complete inundation of normally dry areas in Santa Monica as a result of storms and catastrophic failure of reservoirs. Inundation resulting from tsunamis is discussed in Section 7, and mudflows are discussed in Section 9.

Like most of southern California, the city of Santa Monica and surrounding areas are subject to unpredictable seasonal rainfall. Most years, the scant winter rains are barely sufficient to turn the hills green for a few weeks, but every few years, the region is subjected to periods of intense and sustained precipitation that results in flooding. The variability in annual rainfall amounts is illustrated on Figure 8-1, which shows the annual rainfall measured in the city of Los Angeles, California for the period between 1877 and 2022. Notable historical flood events reported in southern California occurred in 1862, 1884, 1916, 1938, 1969, 1978, 1980, 1983, 1988, 1992, 1995, 1998, 2005, 2019, and 2023.

1 https://www.ncei.noaa.gov/access/monitoring/dyk/billions-calculations, accessed Nov.-Dec. 2023.

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Figure 8-1: Annual Rainfall in the City of Los Angeles, California for the Period Between 1877 and 2023 Local Hazard Mitigation Plan Section 8
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Notice that whereas in the 19th and first half of the 20th century the significant flood events appear to have been spaced about 20 to 30 years apart, in the last decades damaging flood events have been spaced between two and 14 years apart. The list of flood events provided in Table 8-1 below suggests that frequency of flooding in the region has not changed much in the past approximately 250 years, however the increased occupation of low-lying areas has likely increased exposure, resulting in an increase in losses to property and life as a result of flooding. Specifically, as the population in the region has increased, there has been an increased pressure to build on flood-prone areas and upstream of already developed areas. Increased development in turn increases the amount of impervious surfaces, such as asphalt and concrete. Water that used to be absorbed into the ground now becomes runoff to downstream areas. If storm drain systems are not designed or improved to convey these increased flows, areas that may not have flooded in the past are now subject to flooding or may be subject to flooding in the future. This is especially true for developments at the base of the mountains and downstream from canyons that have the potential to convey mudflows.

The city of Santa Monica lies at the western edge of the Los Angeles basin and the greater floodplain of the Los Angeles and San Gabriel rivers. When Spanish explorers first encountered the basin in the late 1700s, the Los Angeles River floodplain was heavily vegetated and dotted with vast marshes, shallow lakes, and small ponds. The rivers, which collected runoff from the surrounding mountains, were rarely confined to a distinct channel and often radically changed their courses spreading storm water and sediment loads across the basin. Then, in the late 1800s and early 1900s, as the region became increasingly urbanized, several of the marshes were drained to increase acreage for agricultural purposes. During wet winters, however, the basin was subject to several episodes of severe flooding. In response, natural channels were dredged and major rivers in the basin were confined to artificial channels to control flooding and sedimentation. These efforts included channelization of the Los Angeles River and lining with concrete or grout the channels of Santa Monica Canyon, Rustic Canyon, and the eastern, upstream portion of Ballona Creek, east of Centinela Avenue. West of Centinela, the banks of Ballona Creek were paved or lined with boulders, but its bottom was left unpaved.

Today, the 50-plus miles-long Los Angeles River begins in the foothills of the San Gabriel and northern Santa Monica Mountains, then flows east across the San Fernando Valley, where it is joined by major tributaries draining the western San Gabriel Mountains. It then turns south, passing through the Hollywood Hills and the Glendale Narrows, onto the broad coastal plain where it receives water from several more tributaries. The river then flows south, eventually reaching the Long Beach Harbor and draining a total watershed of about 834 square miles. Prior to 1825, the Los Angeles River used to flow westward across the northern portion of the Los Angeles Basin, through the Ballona Gap, and into what has been referred as both the Ballona Lagoon or Ballona Lake (Dark et al., 2011), in the area now occupied by Marina del Rey. Given that the river has now been channelized and the region has been extensively developed, the likelihood of the Los Angeles River changing course again and flowing through the Ballona Gap area is very low to nil.

Much of the work described above regarding the channelization of the Los Angeles River and other drainages was conducted by the U.S. Army Corps of Engineers and Los Angeles County. Both of these agencies continue to maintain numerous regional flood control structures within the Los Angeles River system, including channels, levees, flood-control basins, and debris basins. Today, this flood-control effort is also tied to water conservation: it is now the policy of both the County and cities therein, including Santa Monica, to capture as much storm runoff as feasible and store the water in either water treatment plants to use as reclaimed water, or return it to the underlying aquifers Local Hazard Mitigation Plan Section 8
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is no flow data for these channels. Flows in
Ballona Creek, however, are measured,
with the majority of the flow in this
channel occurring between the months of
November and March, reflective of the
“Mediterranean climate” typical for coastal
southern California.

The closest U.S. Geological Survey stream
gaging station near the city of Santa
Monica is on Ballona Creek in Culver City
(Site Number 111035001F
2 ), approximately
3 miles to the southeast of Santa Monica
Airport. Daily measurements were made at
this station for 50 water years between
1928 and 1978. These records are used
here to illustrate and discuss the significant
variability in daily discharge on Ballona
Creek, and thus in the region. A tile plot
that shows the monthly discharges for the entire 50-year period is shown on Figure 8-2. Darker blues indicate higher discharge amounts, whereas white cells indicate little to no discharges. This figure illustrates the statement made above that most flows occur between November and March, and that there is significant variability in discharge from year to year. 2 https://maps.waterdata.usgs.gov/mapper/ accessed on December 18, 2023. via soft-bottom channels and spreading grounds. In the densely urbanized Santa Monica area, like most of the Los Angeles Basin, runoff is now largely controlled by streets, retention basins, storm drains, and flood control channels.

HISTORY OF FLOODING IN THE CITY OF SANTA MONICA
Floods of consequence in and around the city of Santa Monica are typically flash floods, caused by short-duration but heavy rainfall events. These flood events generally have high peak volumes and high velocities. This type of flooding occurs in response to the local geology and geography and the built environment (human-made structures). When a storm approaches, runoff from the Santa Monica Mountains to the north and the Baldwin Hills to the southeast collects rapidly into the natural and artificial channels in the built environment. The flood control channels then direct the runoff directly into the Pacific Ocean. A few of these high-discharge flood events have impacted the city of Santa Monica historically. For example, heavy rains in 1938 resulted in flooded streets, flooded sumps and depressions, and mudflows in the vicinity of Santa Monica Canyon. More recently, as a result of tropical storm Hilary in August 2023, the Pico-Kenter storm drain overflowed its top and flooded some areas of the beach.

There are no major streams in the city limits of Santa Monica; channels of the Santa Monica and Rustic Canyons roughly define the
northern boundary of the city but are not Figure 8-2: Tile Plot Showing Monthly Discharges in located in the city proper. These streams Ballona Creek at Site 11103500 for Years 1928-1978 are not currently being monitored, so there
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Historic Flooding in Los Angeles County
There are several rivers in the southern California region, but the river with the best-recorded history is the Los Angeles River. The flood history of the Los Angeles River is generally indicative of the flood history of much of southern California. Records show that the Los Angeles River floods, on average, about once every six to seven years. But averages are deceiving, for the Los Angeles Basin goes through periods of drought and then periods of above-average rainfall. For example, between 1868 and 1884, a period of 16 years, there were no major floods, but this was followed by a series of wet years with floods in 1885, 1886, 1889 and 1891. A similar cluster of wet years was recorded in the 1990s, and more recently, flooding events were recorded in 2005, 2014, 2017, 2022, and 2023. The history of floods in Los Angeles County is summarized on Table 8-1 below. The entries in bold text are flood events that likely impacted the Santa Monica coastal region. FEMA began to record disaster declarations in 1953; flood events in Los Angeles County that prompted a disaster declaration are identified in the table.

Table 8-1: Historical Floods in Los Angeles County
Date Comments
1770-1771 Great flooding on the Los Angeles River recorded by Father Juan Crespi. River overflowed its channel.
1771-1772 Flooding recorded by Spanish Mission Fathers. San Gabriel Mission crops destroyed. 1775-1776 Due to heavy flooding, San Gabriel Mission was moved about 6 miles back from the river. 1779-1780 Flooding recorded by Spanish Mission fathers. Flows filled riverbed and flooded the lowlands where wheat and barley had been planted.
1811 Flooding reported, although records are sparse.
1815 Flooding washes away the original Plaza in Los Angeles. River changes course at Alameda and 4th Street to cut west and join Ballona Creek. From there it emptied into Santa Monica Bay.
1822 A great flood on the Los Angeles River “covered all the lowlands and reached a greater height than was ever known before.”
1824-1825 Los Angeles River changed its course back from the Ballona wetlands to San Pedro. Before this storm, the river would spread over the entire area, filling depressions at the surface and forming lakes, ponds and marshes, rarely discharging its waters into the sea. The 1825 floods cut a riverway to the ocean, draining the marshlands and causing the forests to disappear. 1832 Heavy flooding caused the drainage near Compton to change so that many lakes and ponds that “had been permanent, became dry a few years thereafter.” Drainage of these ponds and lakes completed the destruction of the forests that used to cover a large part of southern Los Angeles County.
1849-1860 Floods of various magnitudes occurred in 1849-1850, 1851-1852, and 1859-1860. 1861-1862 The “great flood” or the “Noachian deluge of California.” Fifty inches of rain fell during December and January, with rain falling every day for 43 days. The entire valley from Los Angeles to the ocean was a great lake. Part of the Los Angeles River split and drained into Ballona Creek. The San Gabriel River also overflowed its banks and started a new channel. 1867-1868 Floods spilled over the river channel and created a large, temporary lake out to Ballona Creek. The San Gabriel River broke out of its channel and washed out thousands of acres of land.
1884 Two periods of intense rainstorms separated by 6 to 8 days. The first storms caused little damage. The second washed out all but one of the bridges across the Los Angeles River, washed away many houses, and drowned several people. Parts of Los Angeles flooded 3 to 4 feet deep.

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Date Comments
1886-1887 A good part of Los Angeles was inundated. The levees were damaged and railway communication was impossible for 2 to 3 weeks.
1889 Flood on Christmas Day caused much damage; bridges and levees washed away; the old San Gabriel, new San Gabriel and Los Angeles Rivers joined near Downey and formed one body of water. Los Angeles River overtopped its channel. 1914 Heavy flooding in January and February. Great damage to Los Angeles harbor. 1916-1938 Flooding in 1916. Minor floods causing damage in certain areas reported in 1918, 1921- 1922, 1926, 1927, 1931, 1932, 1934, 1936, and 1937.
1934 Moderate to severe flooding starting January 1. Over 40 dead in La Cañada – Glendale area.

Debris flow killed 12 people who had taken shelter in the Montrose Legion Hall. 1938 Series of storms beginning December 1937. March floods exceeded all previous floods for which records were available. Large tracts inundated; bridges, highways and railroads severely damaged. 87 people killed, over $78 million (1938 dollars, approximately equivalent to $1.7 billion in 2025) in damages.
1941-1944
Los Angeles River floods five times.
1952 Moderate flooding.
1969 Disaster declaration due to recurrent precipitation during January and February that nearly approached the largest total since 1884. Nearly 40 people died as direct result of the floods in southern California, and more than 10,000 had to be evacuated. 1978 Two moderate floods prompted a disaster declaration issued in February. 1979 Los Angeles experiences severe flooding and mudslides. 1980 Disaster declaration issued in response to flooding. Flood tops banks of river in Long Beach.

Sepulveda Basin spillway almost opened.
1983 Disaster declaration issued in response to a coastal storm that caused flooding; six people died.
1992 Disaster declaration issued as a result of 15-year flood. Motorists trapped in Sepulveda basin, resulting in six deaths.
1994-1995 Disaster declarations issued as a result of heavy flooding throughout California. The total damages were estimated at $2 billion.
1997-1998 The 1997 floods caused extensive damage in 48 California counties, including Los Angeles County, prompting a Disaster declaration. Total damages estimated at $1.8 billion. The 1998 El Niño storms also caused damage, but this was less than it could have been because many had taken measures to reduce their risk following the 1997 storms. Still, a Disaster declaration was issued for the Feb. 2, 1998 storms.
2003-2004 Disaster declaration issued for Los Angeles County due to flash floods. The rains followed the extensive fires of 2003; in many areas, canyons chocked with ashes and debris caused debris flows that did substantial damage downstream.
2004-2005 The second-wettest year on record in the Los Angeles Basin; the rains caused extensive damage in some areas, triggering landslides and debris flows. In January 2005, flooding and landsliding caused 28 deaths, 8 injuries, affected 500, and caused $200 million in damages.

Between 17-23 February, flooding in Los Angeles County alone killed 9 people, affected 150, and caused $250 million in damages. The 2005 storms triggered two Disaster declarations for Los Angeles County.
2005-2006 Flooding due to intense precipitation between Dec. 31 and Jan. 18 killed 3 people, affected 3,600, and caused $245 million in damages in northern and southern California, and in Nevada.

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Date Comments
2010-2011 California winter storms caused flooding, debris flows and mudflows in several counties, especially along the foothills, and in areas previously burned by wildland fires. Major Disaster Declarations issued on March 10, 2010 and January 26, 2011. Another powerful winter storm caused flash floods in March 2011.

Oct. 2012 A cold and unstable air mass produced strong thunderstorms and flash floods across Los Angeles County. The strongest storms occurred across the San Fernando and San Gabriel Valleys. Water flooding 3 to 4 feet high was reported along all lanes of I-710 in the Bandini neighborhood of Commerce.

Feb.- Mar.
2014
Strong winter storm generated heavy showers and thunderstorms that produced flash flooding and debris flows across Los Angeles County. Flash flooding and debris flows were reported near the Colby fire burn area in the San Gabriel Valley near Glendora and Azusa.

Several homes were damaged. In Lakeview Terrace, two women were trapped when a wall of water swept across their driveway into the street.

Dec. 2014 Disaster declaration due to severe storms, flooding and mudslides impacting the Soboba Band of Luiseño Indians, with damage primarily impacting roads and bridges. Event occurred Dec. 4-6; declaration issued on Jan. 27, 2015. A strong thunderstorm on Dec. 16th over the city of Torrance produced intense rainfall and flash flooding. Streets were flooded and numerous cars were stranded.

Jan. 2017 Disaster declaration issued in response to severe winter storms, flooding and mudslides throughout several counties in California, including Los Angeles County. The event occurred January 18-23, 2017; declaration issued on March 16, 2017. The dollar amount obligated to assist the public exceeded $57 million. Between 2 and 6 inches of rain fell across the region, resulting in flash flooding across Los Angeles County. Flash flooding was reported at the intersection of Pacific Coast Highway with I-710.

Feb. 2017 Disaster declaration issued in response to severe winter storms, flooding and mudslides, with nearly all counties north of and including Santa Barbara affected. The event occurred February 1-23, 2017; disaster declaration was issued April 1, 2017. The dollar amount obligated by FEMA to assist the public was almost $428 million.

Dec. 2022
– Jan. 2023
Disaster declaration issued in response to severe winter storms, flooding and mudslides, with the event occurring Dec. 27, 2022 to Jan. 31, 2023. Declaration issued on Jan. 14, 2023. All coastal counties, with the exception of Orange County, were included in the declaration.

Jan. 2023 Disaster declaration issued in response to severe winter storms, flooding and mudslides impacting all coastal counties from Humboldt south to San Diego, and dozens of counties inland, including San Bernardino and Riverside counties in southern California. Los Angeles County included in the impacted area. Storms occurred Jan. 8-31, 2023, and declaration was issued Jan. 9, 2023.

Feb. 24-
25, 2023
Powerful winter storm brought 2-5 inches of rain to coastal areas and valleys, and 4-10 inches of rain to foothill and mountain areas. Heavy rain caused flash flooding, mudslides and debris flows in several areas in the region. These include flash flooding on I-5 near Burbank where a couple of vehicles were flooded. Flash floods also closed Highway 101 near Campo de Cahuenga, with all lanes experiencing significant flooding.

Mar. 2023 Disaster declarations issued in response to severe storms, flooding and mudslides impacting a large portion of California, including Los Angeles County. The declarations were issued on March 10th and April 3rd, and covered storm damage that began on March 9th.

Aug. 20,
2023
Heavy rain brought on by Tropical Storm Hilary; numerous reports of flash flooding, mudslides and debris flows throughout the region. Flooding impacted I-10 at various locations including Culver City, stranding numerous vehicles. Also in the Culver City area, small streams overflowed their banks and flooded several backyards. Heavy rains flooded the I-405 near Sherman Oaks, leaving numerous vehicles stranded on the flooded roadway.

Mud and debris flows reported on Piuma Road near Malibu Canyon Road.

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Date Comments
Feb. 4-8th,
2024
Slow-moving atmospheric river dropped between 6 and 12 inches of rain in the southern California region, from Santa Barbara south to San Diego, with areas in the Santa Monica Mountains reporting about 12 inches of rain in three days. A Disaster declaration was issued for the storms in early February, to address damage caused by the winter storms and their after-effects, including flooding, landslides and mudslides.

Sources: http://www.em-dat.net/disasters/ (accessed at various times in 2014-2017; https://www.emdat.be in 2023, but no longer available to consultants without an annual fee; http://www.fema.gov (accessed at various times in 2014, 2017, 2022, and 2023); https://www.ncdc.noaa.gov/stormevents/ accessed various times in Aug.-December 2023; https://www.fema.gov/data-visualization/disaster-declarations-states-and-counties accessed in March 2025
WHAT FACTORS CREATE FLOOD RISK?
Climate
Flooding occurs when climate, geology, and hydrology combine to create conditions where water flows outside of its usual course. In the city of Santa Monica, geography and climate may combine to create seasonal flooding conditions. The Santa Monica, Santa Susana, and Verdugo Mountains for the most part do not reach heights above three thousand feet. In contrast, the western San Gabriel Mountains reach elevations of more than seven thousand feet, and to the southwest, the San Bernardino Mountains reach elevations of more than 11,500 feet. These higher ridges, given their elevations, often trap east-moving winter storms approaching from the Pacific Ocean side. As a result, downtown Los Angeles and the ridges to the north (including the Santa Monica Mountains) typically average just about 13 to 15 inches of rain a year, whereas the northern part of the Los Angeles Basin receives about 23 to 24 inches per year, and the mountain peaks in the San Gabriel and San Bernardino Mountains receive more than 40 inches of precipitation annually. “Averages” however, are not particularly representative of rainfall in the southern California area, as illustrated with the following discussion about downtown Los Angeles: the average annual rainfall in Los Angeles for approximately the past 125 water years is 14.75 inches, but rainfall during this period ranged from only 3.21 inches in 2006-2007 to 38.18 inches in 1883-1884 (see Figure 8-1).

This makes the Los Angeles Basin a land of extremes in terms of annual precipitation.

Rainfall that falls on the mountains and hillsides moves rapidly downstream, often with severe consequences for anything in its path. The intensity of the runoff increases in hillside areas recently burned by wildfires, where vegetation has not yet formed a protective ground cover that helps keep the soil in place. Furthermore, the oils in many of the plants native to southern California, when burned, react with the soils, making them water repellant. As a result, less rainwater than usual infiltrates the ground, and instead makes its way downstream as runoff, carrying ashes and other burned debris with it. Several of the mudflow and debris flows that prompted the enactment of Disaster declarations in the past decades (see Table 8-1) occurred at the base of the San Gabriel Mountains, in areas burned by wildfires, such as the communities of Azusa, Glendora, and Burbank.

Runoff often also concentrates in major roadways, leading to flash floods in low-lying sections of freeways, including the interstates (I-5, I-10, I-405) and Highway 101.

There are three main types of storms that produce precipitation in southern California: winter storms, local thunderstorms, and summer tropical storms (or monsoons). These are briefly described below.

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• Winter Storms are characterized by heavy and sometimes prolonged precipitation over a large area. These storms usually occur between November and April and are responsible for most of the precipitation recorded in southern California. Most of these storms have been found to be associated with atmospheric rivers (ARs). The storms originate over the Pacific Ocean and move eastward (and inland), with the mountains forming a rain shadow, as described before. A significant portion of the moisture drops on the mountains as snow. If large storms are coupled with snowmelt from these mountains, large peak discharges can be expected in the main watersheds at the base of the mountains during the spring. Atmospheric rivers are discussed further in the following pages.

Some of the severe winter storm seasons that have historically impacted the southern California area have been related to El Niño events. El Niño is the name given to a phenomenon that starts every few years, typically in December or early January, in the southern Pacific off the western coast of South America, but whose impacts are felt worldwide. Briefly, warmer than usual waters in the southern Pacific are statistically linked with increased rainfall in both the southeastern and southwestern United States, droughts in Australia, western Africa and Indonesia, reduced number of hurricanes in the Atlantic Ocean, and increased number of hurricanes in the Eastern Pacific. Atmospheric rivers are also statistically linked to El Niño events, with an increase in both frequency and intensity of ARs in the western and southwestern United States during El Niño years.2F 3 Two of the
largest and most intense El Niño events on record occurred during the 1982-1983 and 1997- 1998 water years. These were also two of the worst storm seasons reported in southern California.
• Thunderstorms and Monsoons: Relatively infrequent sources of heavy rainfall in the southern California region are tropical storms (or monsoons related to hurricanes in the Pacific Ocean), with rainfall generally reported in the mountains, inland deserts and adjoining cities. These rains typically occur in the summer or early fall. The storms originate in the warm, southern waters off Central America, Mexico, or Baja California, in the Pacific Ocean, and move northward into southern California. During El Niño years, southern California can also experience increased thunderstorm activity and higher-than-average precipitation during the summer months. Thus, El Niño can result in increased rainfall both during the winter and summer months. This is reflected in Table 8-2 below. Occasional hurricanes and tropical storms that have dropped significant rainfall in the southern California area in the last 150 years or so are also listed in Table 8-2. Thunderstorms related to storms in the Gulf of Mexico, Gulf of California, or the southern Pacific Ocean can occur at any time but are usually more prevalent in the higher mountains and deserts during the summer. During La Niña years the jet stream tends to shift to the north, typically resulting in drier-than-average conditions in southern California. However, weather patterns are highly variable and occasionally during La Niña events, the area is still impacted by summer thunderstorms, as shown on Table 8-2. The summer and early fall storms that potentially impacted the Santa Monica area are highlighted with bold text. Storms that resulted in flooding and are listed in both Tables 8-1 and 8-2 are identified with an asterisk (*) after the date.

3 Kim, H-M., Zhou, Y., and Alexander, M.A., 2017, Changes in atmospheric river and moisture transport over the Northeast Pacific and western North America in response to ENSO diversity: Climate Dynamics, Vol. 52, pp. 7375-7388, doi:10.1007/200382-017-3598-9.

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Table 8-2: Tropical Storms That Affected Southern California Between 1858 and August 2023
Date Source of Storm; Area(s) Affected Rainfall
(inches)
Oct. 2-3, 1858
The only known historical hurricane that made a landfall in southern California; 75 mph winds estimated in San Diego, tropical storm along the coastline north to Long Beach; intense rain reported from San Diego to Santa Barbara.
>7
July 20-21, 1902 Deserts and southern mountains. El Niño of 1901-1902. up to 2 Aug. 18-19, 1906 Deserts and southern mountains. El Niño of 1905-1906. up to 5 Sept. 15, 1910 Mountains of Santa Barbara County. 2
Aug. 20-21,
1921* Deserts and southern mountains. La Niña of 1920-1921. up to 2 Sept. 30, 1921* Deserts. La Niña of 1920-1921. up to 4
Sept. 18, 1929 Southern mountains and deserts. up to 4
Sept. 28 – Oct. 1,
1932* Mountains and deserts, 15 fatalities. El Niño of 1932-1933. up to 7 Aug. 25, 1935 Southern valleys, mountains and deserts. up to 2 Sept. 4-7, 1939 Southern mountains, southern and eastern deserts. All storms in September 1939 associated with the El Niño of 1938-1939. up to 7 Sept. 11-12, 1939 Deserts, central and southern mountains. up to 4 Sept. 19-21, 1939 Deserts, central and southern mountains. up to 3 Sept. 25, 1939
Tropical cyclone that made a landfall in San Pedro, with sustained winds of 50 mph. Only known tropical cyclone to make a landfall in Southern California. 93 people died; 45 onshore and 48 offshore, at sea. Ten houses washed away in Belmont Shores.
5 Sept. 25, 1939 Surrounding mountains. 6 to 12
Sept. 9-10, 1945 Central and southern mountains. up to 2 Sept. 30 – Oct. 1,
1946 Southern mountains. up to 4
Aug. 27-29, 1951 Southern mountains and deserts; many roads washed out in the Imperial Valley. El Niño of 1951-1952. 2 to 5
Sept. 19-21,
1952* Central and southern mountains. El Niño of 1951-1952. up to 2 July 17-19, 1954 Deserts and southern mountains. El Niño of 1953-1954. up to 2 July 28-29, 1958 Deserts and southern mountains. El Niño of 1957-1958. up to 2 Sept. 9-10, 1960 Hurricane Estelle dissipated west of Central Baja California; southern mountains at and near Julian. 3.40
Sept. 17-19, 1963 Tropical storm Katherine made landfall in northern Baja California; impacted central and southern mountains. El Niño of 1963-1964. up to 7 Sept. 1-3, 1967 Hurricane Katrina in Baja California; impacted southern mountains and deserts. 2
Sept. 3, 1972
Remnants of Hurricane Hyacinth made landfall between Los Angeles and San Diego with 25-mph winds and rainfall in the central and southern mountains. El Niño of 1972-1973.
up to 1
Oct. 6, 1972
Hurricane Joanne made landfall in northern Baja California; maintained tropical storm strength into Arizona; rain in southeast deserts. El Niño of 1972-1973.
up to 2
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Date Source of Storm; Area(s) Affected Rainfall
(inches)
Sept. 10-11, 1976
As a result of the tropical storm Kathleen; impacted the central and southern mountains; sustained winds of 57 mph at Yuma. Killed 12 people in the U.S.; 70-80% of Ocotillo was destroyed; caused millions of dollars in damage. El Niño of 1976-1977.
6 to 12
Aug. 1977
Hurricane Doreen dissipated over the southern California coastal waters. Widespread flooding in Los Angeles and south; extensive crop damage. Also impacted the mountains. El Niño of 1977-1978. up to 2 in the
valleys, up to 8
in the
mountains
Oct. 6-7, 1977 Remnants of Hurricane Heather tracked into southern Arizona; impacted southern mountains and deserts. up to 2
Sept. 5-6, 1978 Remnants of Hurricane Norman impacted the mountains. El Niño of 1977-1978. 3
Sept. 24-26, 1982 Remnants of Hurricane Olivia; impacted the mountains. Strong El Niño of 1982-1983. up to 4
Sept. 20-21,
1983*
Hurricane Manuel dissipated off west coast of northern Baja California; impacted the southern mountains and deserts. Strong El Niño of 1982-1983.
up to 3
Oct. 7, 1983 Remnants of Hurricane Priscella scattered light rain across southern California. n/a
Sept. 10-11, 1984 Hurricane Marie dissipated off the west coast of northern Baja California; scattered rain in coastal areas.
n/a
Aug. 17-19, 1997
Tropical storm Ignacio dissipated near the south-central California coast with gale-force winds over coastal waters. Part of the strong El Niño of 1997-1998.
n/a
Sept. 1997
Hurricane Linda, the strongest storm recorded in the eastern Pacific with 180 mph winds, threatened to come ashore southern California as a subtropical storm. The storm turned away, but caused high surf, 18-foot high waves, showers and thunderstorms.
n/a
Sept. 25, 1997
Hurricane Nora crossed into southern California and Arizona from Baja California. Brought heavy rain to parts of the region, causing millions of dollars in damage to agriculture.
n/a
Sept. 10-19, 2004 Mid-level moisture from hurricane Javier spread over northern Mexico and southwestern United States. n/a
July 31, 20-22,
2006
Remnants of Tropical storm Emilia brought rain to southern California that helped extinguish the House Fire. n/a
Sept. 2007 Thunderstorms and showers; flooding watch in Santa Catalina Island; rain throughout the southern California area. n/a July 18-20, 2012 Remnants of Hurricane Fabio generated scattered showers and thunderstorms in the Los Angeles basin. n/a
Aug. 25-26, 2013
Moisture from the remnants of Tropical storm Ivo caused flash flooding and mudslides in San Bernardino County and Arizona. One motorist drowned in Needles. In the Antelope Valley, flash flooding was reported in the communities of Quartz Hill and Leona Valley. 3 to 4
Aug. 20, 2023* Tropical Storm Hilary swept north from Baja California, bringing winds and rains throughout the southern California area.

13.5 in San
Bernardino Co.
>7 in Los
Angeles Co.

Source: https://www.ncdc.noaa.gov/stormevents/ accessed various times in Aug.-December 2023 and January 2025.

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Much research in the last decade has focused on the study of a meteorological phenomenon called the Atmospheric River (AR). ARs are narrow streams of water vapor transported in the lower atmosphere that are now recognized as responsible for most of the very large storms on the west coast of the United States. Typically packing high wind speeds, ARs are about 400 to 500 kilometers wide and thousands of kilometers long, sometimes extending across whole ocean basins. When these sub-tropical jet streams carrying moisture-laden air directly from the tropics to the western U.S. collide with the mountain ranges in the west coast, the vapor is forced upwards, where it condenses and rains out, leading to significant flooding3F 4 . Because the jet stream passes over the
Hawaiian Islands, these events used to be called the “Pineapple Express.” The storms that occurred 1861-1862 (see Table 8-1) are now thought to have been a series of ARs.

These storms caused state-wide flooding that left the central coast impassible, the State capital underwater for three months, and the State bankrupt. Thousands of people are thought to have died, and one quarter of the cattle in California perished by drowning. These storms were significantly wetter than the 2022-2023 winter storms that caused extensive damage along the entire west coast.

A storm today similar to the 1861-1862 storms would be devastating as the region is now home to millions of people, and produces a large percentage of the U.S. crops. Geologists studying sediments offshore have found evidence that suggests that six “megastorms” have occurred in California in the last 1,800 years – all more severe than the 1861-1862 event. Other studies suggest that similar storms occur once every 100 to 200 years.4F
5 The U.S. Geological Survey’s (USGS) Multi Hazards Demonstration Project (MHDP) has been combining various science disciplines to test and improve the resiliency of communities to natural disasters. To do this, they develop disaster scenarios with feedback from scientists, engineers, and other experts, and in the process engage emergency planners, first responders, businesses, universities, insurance companies, government agencies and the public in preparing for a major natural disaster. The first project of the MHDP was the 2008 ShakeOut earthquake scenario on the San Andreas fault discussed in Section 6. Their second major project was a catastrophic winter storm scenario consisting of a series of hypothetical (but probable) Pacific storms striking the west coast of California for 23 days, which is a shorter period than the 1861-1862 storms.

Named the ARkStorm (for Atmospheric River 1,000 – the name is meant to convey the magnitude of the event, and not its return period), the hypothetical storm was projected to overwhelm the State’s flood protection system, which is normally designed to control the 100- to 200-year storm runoff. In 2011, when the study was first released, property damages and business disruptions from the ARkStorm scenario were estimated to be about $725 billion (Porter et al., 2011). Adjusted for inflation and increase in population, the cost of this hypothetical storm in 2023 was estimated at $1.7 trillion (Masters, 2023). Such a storm would be many times more destructive and costly than a magnitude 8.0 earthquake on the San Andreas fault. The products of the ARkStorm Scenario are intended to be used by emergency planners, policymakers and others to review disaster preparedness, conduct risk assessments and disaster drills, explore ways to adequately fund response and recovery, plan future hazards mapping, and educate the public. The December 2022/January 2023 ARs that impacted California, although significant and costly (responsible for 22 deaths and

4 Ralph, F.M., and Dettinger, M.D., 2011, Storms, floods, and the science of atmospheric rivers: Eos, Transactions of the American Geophysical Union, Vol. 92, No. 32. 5
Dettinger, M.D., and Ingram, B.L., 2013, Megastorms could drown massive portions of California: Scientific American, January 1, 2013, available from https://www.scientificamerican.com/article/megastorms-could-downmassive-portions-of-california/.

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estimated at more than $30 billion in losses), did not approach the rainfall and flooding levels expected from the ARkStorm scenario.

The scientists that ran the ARkStorm scenario described above have been conducting a revised study referred to as ARkStorm 2.0. This new study considers recent advances in the fields of climate, weather and hydrology, uses the latest in modeling tools, and takes into consideration the effects of climate change (Huang and Swain, 2022; Swain, 2022). ARkStorm 2.0 considers two megastorm scenarios: 1) an ARkHist scenario that is based on present-day climate using climatological data collected between 1995 and 2005, and 2) an ARkFuture scenario that considers a warmer climate.

The ARkHist event is anticipated to generate between about 10 and 15 inches of precipitation in Santa Monica in a 30-day simulated storm event, whereas the ARkFuture event would produce between about 15 and 20 inches of rain during a 30-day period in Santa Monica. The storms modeled in the ARkHist scenario produce less precipitation than the rainfall events that led to the 1861-1862 floods; the scenario is thought to have a recurrence interval of about 90 to 100 years.

Flooding in the Santa Monica area as a result of an ARkStorm event could result in flooded streets, overtopping of the concrete-lined channels in Rustic Canyon and Santa Monica Canyon, erosion and hillslope damage along the bluffs, and severe storm swells that may cause coastal inundation and erosion. The ARkFuture scenario is estimated to have a recurrence interval of about 400 years in the present climate. However, findings of the Fourth National Climate Assessment (USGCRP, 2017) include the conclusion that the frequency and severity of ARs impacting the west coast of the United States will increase with increasing temperature, as the warmer weather will result in an increase in evaporation and higher atmospheric water vapor. The economic and societal impacts of these new ARkStorm 2.0 models are being developed (Swain, 2022).

Geography and Geology
The mountains bordering the Los Angeles Basin are very steep and consist of rock types that are fairly impervious to water. Consequently, little precipitation infiltrates the ground; rainwater instead flows across the surface as runoff, collecting in the major drainages that drain into and pass through the basin. Specifically, when a major storm moves in, water collects rapidly and runs off quickly, making a steep, rapid descent from the mountains into man-made and natural channels within developed areas. Because of the steep terrain, scarcity of vegetation, and the constant shedding of debris from mountain slopes (primarily as dry ravel and rock falls), floodwaters often carry large amounts of mud, sand, and rock fragments. Sheet flow occurs when the capacities of the existing channels (either natural or man-made) are exceeded and water flows over and into the adjacent areas.

The greater Los Angeles Basin has been shaped by erosion and sedimentation for millennia. Most of the mountains that ring the basin are still being uplifted at a (geologically) fast rate in response to earthquakes on the region’s many active faults. The rapid uplift has resulted in steep, rugged slopes underlain by brittle, fractured rock, thereby creating a landscape that is very susceptible to erosion.

Over time, rivers and streams emanating from the mountain front have carried boulders, rocks, gravel, sand, and silt down these slopes to the valleys and coastal plain where sedimentary deposits are locally as much as twenty thousand feet thick. This sediment generally acts as a sponge, absorbing vast quantities of water received as precipitation in those years when heavy rains follow a dry period. But like a sponge that is near saturation, the same soil fills up rapidly when a heavy rain follows a period of relatively wet weather. So, in some years of heavy rain, flooding is minimal because the ground is relatively dry. The same amount of rain following a wet period, when the ground is already saturated, can cause extensive flooding.

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Built Environment
The greater Los Angeles Basin is essentially built out, with vast swaths of land covered by impervious surfaces such as concrete and asphalt. This leaves scant open land to absorb rainfall. This lack of open ground forces water to remain on the surface and accumulate rapidly. If it were not for the massive flood control system that has been built over the years, with its concrete-lined rivers and stream beds, flooding in the Los Angeles Basin would be a much more common occurrence. And the tendency is towards even less and less open land. In-fill building is becoming a much more common practice in many areas: This increase in impervious surfaces (including concrete walkways, and roofs) results in a direct increase in runoff.

Another potential reason for recurrent storm flooding in developed areas is “asphalt creep.” The street space between the curbs of a street is a part of the flood control system. Water leaves the adjacent properties and accumulates in the streets, where it is directed towards the underground portion of the flood control system. The carrying capacity of a given street is determined by the width of the street and the height of the curbs along the street. Often, when streets are being resurfaced, a one- to two-inch layer of asphalt is laid down over the existing asphalt. This added layer of asphalt subtracts from the rated capacity of the street to carry water. Thus, the original engineered capacity of the entire storm drain system is marginally reduced over time. Subsequent re-paving of the street will further reduce its engineered capacity.

When structures or fill are placed in the floodway or floodplain, water is displaced. Developments in these locations raise river levels by forcing the river to compensate for the flow space obstructed by the inserted structures and/or fill. When structures or materials are added to the floodway or floodplain and no fill is removed to compensate, serious problems can arise. Flood waters may be forced away from historic floodplain areas. As a result, other existing floodplain areas may experience floodwaters that rise above historic levels. Local governments must require engineer certification to ensure that proposed developments will not adversely affect the flood-carrying capacity of the Special Flood Hazard Area (SFHA). Displacement of only a few inches of water can mean the difference between no structural damage occurring in a flood event, and the inundation of many homes, businesses, and other facilities. Careful attention should be given to development that occurs within the floodway to ensure that structures are prepared to withstand base flood events.

In highly urbanized areas, increased paving can lead to an increase in volume and velocity of runoff after a rainfall event, exacerbating the potential flood hazards. Care should be taken in the development and implementation of storm water management systems to ensure that these runoff waters are dealt with effectively.

STORM FLOODING CHARACTERISTICS
There are several types of flooding that have historically affected the southern California region, including the Los Angeles Basin: coastal flooding, fluvial (riverine) flooding, pluvial (urban) flooding, and hillside debris flows. The first three types of flooding are covered below. Debris flows are discussed in Section 9 - Landslides.

Coastal Flooding
Coastal flooding, or the inundation of land areas along the coast, is the result of higher-than-average high tide and worsened by heavy rainfall and onshore winds (i.e., wind blowing landward from the Local Hazard Mitigation Plan Section 8
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ocean). Flooding is the costliest natural disaster in the United States, with coastal flooding due to coastal storms and hurricanes being responsible for a significant amount of the total loss.

Coastal communities face a range of unique flooding hazards including storm surge, waves, and erosion—all of which can cause extensive damage to homes, businesses, and infrastructure. As of 2018, about 40 percent of the United States’ population was estimated to live in a coastal county5F 6 ,
and that number is expected to grow. As the coastal population continues to increase, more people and properties will be at risk from coastal storms and flooding.

Coastal environments are dynamic and are constantly reshaped by the forces of nature. Land use changes and rising average temperatures and sea levels are anticipated to increase the risk of coastal flooding. Many communities are already being impacted more often by flooding from high tides.

This can lead to road closures, overwhelmed storm-water systems, and disrupted business districts.

By 2050, high tide flooding along some United States coastlines is estimated to occur, on average, 25 to 75 days per year.

Fluvial (Riverine) Flooding
Riverine flooding is the overbank flooding of rivers and streams. This process in a natural environment adds sediment and nutrients to the flooded area, cyclically enhancing the fertility of the soils, which is why floodplains have been the breadbaskets of civilizations through the ages.

However, large floods have the potential to cause significant damage to man-made structures and cause significant loss of life. Flooding in large river systems typically results from large-scale weather systems that generate prolonged rainfall over a wide geographic area, causing flooding in hundreds of smaller streams, which then drain into the major rivers.

Shallow area flooding is a special type of riverine flooding. FEMA defines shallow flood hazards as areas that are inundated by the 100-year flood with flood depths of only one to three feet. These areas are generally flooded by low-velocity sheet flows of water.

Pluvial (Urban) Flooding
Pluvial flooding occurs in direct response to intense and/or prolonged rainfall, with flooding occurring in areas that are not normally part of an existing body of water such as a stream or lake.

Urban flooding usually occurs as a result of land being converted from agricultural fields or woodlands to roads and parking lots, in the process losing its ability to absorb rainfall. Urbanization of a watershed changes the hydrologic systems of the basin. Heavy rainfall collects and flows faster on impervious concrete and asphalt surfaces. The water moves from the clouds to the ground, and either ponds at the surface, in level ground, resulting in what is referred to as a surface flood, or if there is some relief, into a flash flood, with floodwaters potentially rising very rapidly and peaking with violent force. The flooding of developed areas often occurs when the amount of water generated from rainfall and runoff exceeds the capability of the storm drain system to remove and transport it. Oversaturated ground due to two or more back-to-back storms can also contribute to this type of flooding, as the wet soil cannot absorb the water dropped by later storms.

6 https://coast.noaa.gov/states/fast-facts/economics-and-demographics.html; accessed on 01/31/2024.

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HOW FLOOD-PRONE AREAS ARE IDENTIFIED
The Federal Emergency Management Agency (FEMA) is mandated by the National Flood Insurance Act of 1968 and the Flood Disaster Protection Act of 1973 to evaluate flood hazards. To promote sound land use and floodplain development, FEMA prepares and provides Flood Insurance Rate Maps (FIRMs) for local and regional planners. Flood risk information presented on FIRMs is based on historic, meteorological, hydrologic, and hydraulic data, as well as topographic surveys, openspace conditions, flood control works, and existing development.

Rainfall-runoff and hydraulic models are utilized by the FIRM program to analyze flood potential, adequacy of flood protective measures, surface-water and groundwater interchange characteristics, and the variable efficiency of mobile (sand bed) flood channels. It is important to realize that FIRMs only identify potential flood areas based on the conditions at the time of the study, and do not consider the impacts of future development. To prepare FIRMs that illustrate the extent of flood hazards in a flood-prone community, FEMA conducts engineering studies referred to as Flood Insurance Studies (FISs). Using information gathered in these studies, FEMA engineers and cartographers delineate Special Flood Hazard Areas (SFHAs) on FIRMs. SFHAs are those areas subject to inundation by a “base flood,” which FEMA sets as a 100-year flood (see definitions below).

Flood Terminology
Floodplain
A floodplain is a land area adjacent to a river, stream, lake, estuary, or other water body that is subject to flooding. This area, if left undisturbed, acts to store excess floodwater. The floodplain is made up of two sections: the floodway and the flood fringe.

Floodway
Floodways are defined for regulatory purposes. Unlike floodplains, floodways do not reflect a recognizable geologic feature. For National Flood Insurance Program (NFIP) purposes, floodways are defined as the channel of a river or stream, and the overbank areas adjacent to the channel. The floodway carries the bulk of the floodwaters downstream and is usually the area where water velocities and forces are the greatest. NFIP regulations prohibit all development in the floodway, keeping the area free from development or other structures that would obstruct or divert flood flows onto other properties. The NFIP floodway definition is "the channel of a river or other watercourse and adjacent land areas that must be reserved in order to discharge the base flood without cumulatively increasing the water surface elevation more than one foot.” Floodways are not mapped for all rivers and streams but are generally mapped in developed areas.

Flood Fringe
The flood fringe refers to the outer portions of the floodplain, beginning at the edge of the floodway and continuing outward. Generally, the flood fringe is defined as "the land area which is outside of the stream floodway but is subject to periodic inundation by regular flooding.” This is the area where development is most likely to occur, and where precautions to protect life and property need to be taken.

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100-Year Flood
The 100-year flood, also called the Base Flood, is the flood having a one percent chance of being equaled or exceeded in magnitude in any given year; it is not a flood occurring once every 100 years. The 100-year floodplain is the area adjoining a river, stream, or watercourse covered by water in the event of a 100-year flood. A 100-year flood is defined by looking at the long-term average period between floods of a certain size, and identifying the size of flood that has a 1 percent chance of occurring during any given year. This base flood has a 26 percent chance of occurring during a 30-year period, the length of most home mortgages. However, a recurrence interval such as “100 years” represents only the long-term average period between floods of a specific magnitude; rare floods can in fact occur at much shorter intervals or even within the same year.

Development
For floodplain ordinance purposes, development is broadly defined as "any man-made change to improved or unimproved real estate, including but not limited to buildings or other structures, mining, dredging, filling, grading, paving, excavation, or drilling operations located within the area of special flood hazard." The definition of development for floodplain purposes is generally broader and includes more activities than the definition of development used in other sections of local land use ordinances.

Base Flood Elevation (BFE)
The term "Base Flood Elevation" refers to the elevation (normally measured in feet above sea level) that the base flood is expected to reach. Base flood elevations can be set at levels other than the 100-year flood. Some communities choose to use higher frequency flood events as their base flood elevation for certain activities, while using lower frequency events for others. For example, for the purpose of storm water management, a 25-year flood event might serve as the base flood elevation, whereas the 500-year flood event may serve as base flood elevation for the tie down of mobile homes. The regulations of the NFIP focus on development in the 100-year floodplain.

Special Flood Hazard Area (SFHA)
Using information gathered in Flood Insurance Studies, FEMA engineers and cartographers delineate Special Flood Hazard Areas on FIRMs. SFHAs are those areas subject to a high risk of inundation by a “base flood” which FEMA sets as a 100-year flood. As discussed further in the next section, SFHAs are regulated zones, requiring the mandatory purchase of flood insurance. They are also subject to special standards and regulations that apply to new construction, and in some cases, existing buildings. Floodplain regulations required by the NFIP apply only to properties located in a SHFA.

However, these are minimum requirements, and local jurisdictions may regulate areas outside of the SHFAs, based on knowledge specific to their area.

Flood Mapping Methods, Products and Programs
Flood Insurance Rate Maps and Flood Insurance Studies
Flood Insurance Rate Maps (FIRMs) and Flood Insurance Studies (FISs) are the basis for implementing floodplain regulations and for delineating flood insurance purchase requirements given that these studies assess the probability of flooding at a given location. FIRMs are developed using water surface elevations combined with topographic data and illustrate areas that would be inundated during a 100-year flood, floodway areas, and elevations marking the 100-year-flood level. In some cases, these maps also include base flood elevations (BFEs) and areas located within the 500-year Local Hazard Mitigation Plan Section 8
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floodplain. Significantly, and of relevance to the urban environment, these maps are prepared only for areas susceptible to coastal and fluvial (riverine) flooding, not pluvial flooding.

FIRMs are the official maps produced by FEMA that delineate Special Flood Hazard Areas (SFHAs) in communities where National Flood Insurance Program (NFIP) regulations apply. FIRMs are also used by insurance agents and mortgage lenders to determine if flood insurance is required and what insurance rates should apply. Communities find it particularly useful to overlay flood hazard areas on tax assessment parcel maps and land use maps. This allows a community to evaluate the flood hazard risk for a specific parcel during review of a development request. FIRM maps can be imported directly into GIS, which allows for GIS analysis of flood hazard areas. Coordination between FEMA and local planning jurisdictions is the key to making a strong connection with GIS technology for the purpose of flood hazard mapping.

In addition to their original purpose of setting insurance rates and regulating flood hazards, FIRMs are used by local and regional planners for other purposes, including land-use planning, emergency preparedness and response, natural resource management, and risk assessment. Given their wide usage, it is important to note that there are many uncertainties inherent in the establishment of FEMA flood zones (Larson, 2009). Furthermore, several recent studies 6F 7 have shown that FIRMs are
inaccurate, which in turn can lead to poor, potentially damaging, planning decisions. Specific limitations of FIRM maps that communities should be aware of are discussed below:
• FIRMs only identify potential flood areas based on the conditions at the time of the Flood Insurance Study (FIS) that led to the development of the maps. As a result, FIRMS do not consider the impacts of changes to the area since the maps were developed, or the impacts that future changes may have on the flood hazard. Conditions that affect the maps and the decisions made on their basis may include changes in corporate boundaries, changes in population, man-made and natural changes to the landscape, removal of vegetation, changes to hydrologic systems, construction of flood control facilities, and climate change. These modifications to the environment may increase or reduce the area susceptible to flooding. Many Flood Insurance Studies that were first completed in the late 1970s and early 1980s have not been updated since, and thus significantly under-estimate the current flood risk in areas where extensive development has occurred since the maps were created.
• The level of detail studied and presented on the maps, as well as the boundaries of the area studied, depend on the type of flood hazard, the funding available, and the risk of flood damage at the time that the FIS was conducted. For instance, areas studied by approximate methods do not provide base flood elevations on the map, and some study areas are limited in extent.

Essentially, not all 100-year and 500-year floodplains have yet been mapped. • The maps typically do not identify all areas susceptible to flooding, such as areas crossed by small drainages, areas of localized ponding during storms, or areas where drainages are restricted by temporary or permanent structures. As mentioned above, FIRMs only cover areas known to be impacted by coastal or fluvial flooding, not areas flooded by standing water or flash

7 https://www.smithsonianmag.com/smart-news/federal-flood-maps-are-outdated-because-of-climate-change-femadirector-says-180980725/ https://www.washingtonpost.com/climate-environment/interactive/2022/fema-flood-risk-maps-failures/ Sanders et al., 2023, Large and inequitable flood risks in Los Angeles, California: Nature Sustainability, https://www.washingtonpost.com/climate-environment/interactive/2022/fema-flood-risk-maps-failures/ Local Hazard Mitigation Plan Section 8
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floods following a rainstorm. This is important in Santa Monica where pluvial flooding may locally impact some areas not identified in the FIRMs.
• The analytical process used to construct these maps relies on many assumptions and limited data. The data used may be too old, incomplete, interpolated, and/or inaccurate. For example, in relatively flat floodplains, small elevation errors in the topography can result in large errors in flood zone boundaries.
• One major drawback is the very short time period for which we have meteorological records.

Research on some parts of southern California has shown that slight climate fluctuations between wet and dry cycles have occurred since the late 1800s (Hereford and Longpre, 2009). The effects of global climate change, which appears to be responsible for significant rainfall events, and large swings between dry and wet periods, are not currently considered and modeled for. • Long-term changes in the watershed or floodplain, primarily as a result of human activities, are also difficult to anticipate. Flood control structures, such as berms and levees, can actually increase the flood risk to other areas. The design of high-density developments often requires taking drainages that used to be spread over a wide area and constricting them into narrow channels, thereby increasing the velocity and erosive power of the flow, and perhaps leading to overtopping. Consequently, there are clearly limitations in using hydrologic calculations based on past, imperfect records to predict the future.
• Larson (2009) also argued that the process of placing a line on a map (flood zone boundaries) incorrectly conveys a sense of certainty about the risk. Since the public and policy makers typically make decisions based on flood zone boundaries, the potential risk to adjacent properties outside the margins defined by those lines is often underestimated. This has recently become apparent in many areas throughout the US that have been impacted by flooding where FEMA mapping showed no hazards 7F
8 . This also means that millions of people that live in areas susceptible to flooding currently have no flood insurance. This became a poignant issue for many homeowners who were impacted by the mudslides and flooding resulting from the atmospheric rivers that hit southern California in 2022-2023, and in early February 2024.

Partly as a result of several of these known issues with FIRMs, FEMA recommends that most property owners, whether residential or commercial, purchase and keep flood insurance even if they are not located in a mapped flood hazard zone. It is important to note that approximately 40 percent of all flood claims occur outside of mapped high flood risk areas, and typical homeowner or business insurance policies do not cover flooding. Temporary situations, such as a storm drain inlet or culvert that becomes blocked by storm debris, can cause localized flooding and property damage. Residents or business owners that rent property can also purchase coverage for the contents of their homes or business inventories. In low to moderate risk areas, property owners should ask their agents if they are eligible for the FEMA Preferred Risk Policy, which provides inexpensive flood insurance protection. Insured property owners can be reimbursed for all covered losses, even if the flood is not officially declared a Federal disaster area.

FEMA also recommends that residents do not forgo purchasing insurance, assuming instead that Federal disaster assistance will pay for flood damage. This is because in order to receive assistance, a community must first be declared a Federal disaster area, and these declarations are issued in less than 50 percent of flood events. Furthermore, Federal assistance is usually in the form of a loan,

8 https://www.washingtonpost.com/climate-environment/interactive/2022/fema-flood-risk-maps-failures Local Hazard Mitigation Plan Section 8
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which must be repaid with interest. If uninsured property owners do receive Federal assistance, they must purchase flood insurance to remain eligible for future disaster relief.

Mapped Flood Areas Outside of the 100-Year Flood Zone
Although many communities rely exclusively on FIRMs to characterize the risk of flooding in their area, there are some flood-prone areas that are not mapped but remain susceptible to flooding.

These areas include locations next to small creeks, local drainage areas, topographically low spots, and areas susceptible to man-induced and pluvial flooding. In order to address this lack of data, jurisdictions can make efforts to develop more localized flood hazard maps. One method that has been employed includes using high-water marks from flood events or aerial photos to better reflect the true flood risk.

Community Rating System
The Community Rating System (CRS) is a voluntary part of the National Flood Insurance Program (NFIP) that seeks to coordinate all flood-related activities, reduce flood losses, facilitate accurate insurance ratings, and promote public awareness of flood insurance by creating incentives for a community to pursue beyond-the-minimum requirements of the NFIP. Any community that is in full compliance with the NFIP’s minimum floodplain management requirements may apply to join the CRS. CRS ratings are on a ten-point scale, from 1 to 10, with 1 being the best rating. Residents of CRS communities who live within FEMA’s Special Flood Hazard Areas (SFHAs) receive a 5 percent reduction in flood insurance rates for every one-point improvement in the community’s CRS rating.

As of October 1, 2023, the City of Santa Monica is not included in the list of CRS-eligible communities8F
9 . For additional information on the Community Rating System, including how to go about becoming eligible, refer to https://www.fema.gov/national-flood-insurance-programcommunity-rating-system.

Risk Mapping, Assessment and Planning (Risk MAP) Program With the Risk MAP Program approved in March 2009, FEMA has moved away from simply portraying flood hazard zones on maps to more accurately communicating and assessing risk to the local community. Building on the digitized maps, FEMA developed a five-year plan to fill in data gaps, increase public awareness, increase their outreach on flood risks, support state and local agencies in risk-based mitigation planning, and provide an enhanced digital platform that improves communication and sharing of risk data.9F
10 Then, in 2011, FEMA started a multi-year project to
improve their guidelines and standards for flood risk analysis and mapping with the goal of bringing overall consistency, clarity, and efficiency to the mapping process. The result of this work was publication, in 2013, of a compendium document covering all standards applicable to the Risk MAP program. This document has been updated since, with the most recent version dating from 2024.10F 11 Today, Risk MAP is meant to be a collaborative effort between FEMA and federal, state, tribal and local partners across the U.S. to identify flood risk, promote informed planning and development practices, and in the process reduce the risk. The process includes several steps that allow for stakeholders (including planners, engineers, real-estate and insurance specialists, and even

9 https://www.fema.gov/floodplain-management/community-rating-system#participating, accessed on 02/08/2024. 10 https://www.fema.gov/pdf/rebuild/map_mod_fact_sheet.pdf 11 https://www.fema.gov/flood-maps/guidance-reports/guidelines-standards/guidance-femas-risk-mappingassessment-and-planning Local Hazard Mitigation Plan Section 8
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homeowners and business owners) to be involved in the discovery and recording of flood-prone areas, create a community outreach plan, and review and comment on preliminary flood maps prepared in response to the discovery phase. The end goal of this program is to inform property owners of their flood risk and most importantly, develop and implement hazard mitigation strategies for flood-prone areas to increase community resilience. Flood risk products that may be created as a result of these efforts include non-regulatory Flood Risk Maps (FRMs), Flood Risk Reports (FRRs), and Flood Risk Databases (FRDs).

FLOOD HAZARD ANALYSIS FOR THE SANTA MONICA AREA
Los Angeles County, including the Santa Monica region, was analyzed by FEMA fairly recently, with the current FIS for the area most recently issued on June 2, 2021. This document can be accessed from https://pw.lacounty.gov/floodzone/moreinfo.cfm with the nine volumes that comprise the study accessible from the section labeled “FEMA Flood Zones.” The FIS includes community descriptions, flooding sources (such as the Los Angeles River), information on historical flooding, existing flood protection measures, hydrologic and hydraulic analyses, and definition of potential flood areas.

The City’s most current FIRM maps became effective on April 21, 2021.11F 12 The 2021 FIRMs include
several revisions for coastal areas such as Santa Monica that reflect updated ocean and topographic measurements at the coastline, physical changes in the watersheds, and in some areas, corrections to an inadvertent “shift” of the flood zone boundaries that occurred in 2008 when the 1980s-era paper maps were converted to digital form.12F
13 The FIRM panels that cover the city of Santa Monica include: 06037C1567G, 06037C1569G, 06037C1588G, 06037C1589G, 06037C1590G and 06037C1751G. The FIRM identifies Special Flood Hazards Areas within Santa Monica that occur along and east of Pacific Coast Highway and extend west into the Santa Monica Bay. The Special Flood Hazards Areas (SFHAs) mapped in Santa Monica are described in Table 8-3 and are shown on Figure 8-3. Note that the section of the Santa Monica coastline that is approximately north of Wilshire Boulevard is classified as both Zone AE and Zone VE, whereas the coastline approximately south of Wilshire Boulevard is only classified as Zone VE.

Table 8-3: Flood Zones Mapped in Santa Monica
Santa Monica
Special Flood
Hazard Areas
Description Area of Santa Monica Map Reference
Zone AE
100-year flood zone along
coastline susceptible to storm
flooding
Coastline north of
Wilshire Blvd.

Figure 8-3,
Bright Pink
Zone VE
100-year flood zone susceptible
to coastal flooding due to storm
surges
Coastline north and
south of Wilshire Blvd.

Figure 8-3,
Light Purple

12 https://www.dpw.lacounty.gov/wmd/NFIP/OPCFEMAMapRevision.aspx 13 https://www.dpw.lacounty.gov/wmd/NFIP/documents/OPCFMR/Open%20Pacific%20Coast%20Study%20FAQ% 20Sheet%20June%202021.pdf
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According to an analysis by City Staff13F
14, 85 beach-front properties along Pacific Coast Highway are located within a SFHA. Owners of these properties are required to purchase flood insurance in order to secure financing if they intend to build or improve their structures. New property owners are required to purchase flood insurance to secure financing to buy the property in the SFHA. Lending institutions that are federally regulated or federally insured must determine if the structure is located in a SFHA and must provide written notice requiring flood insurance. Santa Monica became a participating member of the NFIP in 1983 (City ID No. 060159F), which means that flood insurance is available to any property owner in the city. Being a NFIP member also means that all NFIP-related building code regulations apply. NFIP regulations (44 Code of Federal Regulations (CFR) Chapter 1, Section 60, 3) require that all new construction in floodplains must be elevated at or above base flood level.

The FIRMs that encompass the city of Santa Monica and surrounding jurisdictions also identify areas with a moderate to low risk of flooding. The flood having a 0.2 percent annual chance of occurring (also called the 500-year flood) is usually the basis for these categories, with moderate risk defined as the zone between the limits of the 100-year and 500-year floods, and low risk defined as the area outside of the 500-year flood limits. These zones may also include areas where the base flood is less than one-foot deep, where the drainage basin is small (less than one square mile), or areas that are protected from the base flood by levees. Most of Santa Monica is mapped as having a minimal flood hazard with the majority of the city located outside the 500‐year flood zone (Zone X, unshaded on Figure 8-3). Note, however, that flood-prone areas have been identified just outside the City limits; this could have an impact on motorists attempting to arrive or depart from Santa Monica. For example, sections of Rustic Canyon in Pacific Palisades are designated by FEMA as either a 1% or

0.2% chance of flooding. Likewise, relatively low depressions along Bundy Drive and San Vicente Boulevard near the Brentwood Country Club are considered to have a 0.2% chance of flooding.

Flood insurance is available for properties in these zones, but is not mandated by the NFIP.

14 City of Santa Monica, Staff Report 4422, March 9, 2021 online at https://santamonicacityca.iqm2.com/Citizens/Detail_LegiFile.aspx?Frame=&MeetingID=1262&MediaPosition=&I D=4422&CssClass=
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Figure 8-3: Flood Zones Mapped In and Near Santa Monica Natural Hazards Mitigation Plan Section 8
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The fluvial (riverine) flood hazard in the city is deemed low to nil because there are no floodways extending through Santa Monica today. As mentioned before, prior to 1825, the Los Angeles River flowed through the Ballona gap area, and into the Ballona Lagoon, which historically extended north to what is today the area near the intersection of Main Street and Abbott Kinney Boulevard in Venice (Dark et al., 2011), less than 0.5 miles from Santa Monica. Ballona Creek is now confined to a channel that flows into the Pacific Ocean just south of Marina del Rey. Moreover, geologic maps and older topographic maps of the area suggest that streams draining the foothills of the Santa Monica Mountains in the area east of Brentwood Knoll converged in what today is the east-central part of Santa Monica and then drained west following the low-lying area now mostly occupied by the I-10 freeway.

Pluvial (urban) flooding in Santa Monica is managed by an extensive drainage system that consists of over 2,400 catch basins and 64 storm drain lines, with the Los Angeles County Flood Control District retaining ownership of about 9 to 10 miles of storm drain lines and inlets within the city of Santa Monica. The City’s storm drain pipes range from 16 inches to 11 feet in diameter and are mainly constructed of reinforced concrete; a few structures consist of brick arch drains, concrete open channels, or corrugated metal pipes. Only a small section in the southeast corner of the city drains to Ballona Creek. Historically, the majority of the urban runoff in Santa Monica has drained untreated into Santa Monica Bay via the underground storm drain system described above and five outfalls within City limits. The largest of these outfalls is located at the western terminus of Pico Boulevard and is known as the Pico-Kenter outfall. In addition to runoff from Santa Monica, this outfall discharges runoff from parts of Brentwood and West Los Angeles, and flow conveyed by the 120-inch Santa Monica Freeway storm drain that runs through a portion of the city and drains portions of Los Angeles County as well. Both of these drains come together and empty into the ocean at the western end of Pico Boulevard, along the southern end of the Civic Center. The other four outfalls are located on the beach at Montana Avenue, Wilshire Boulevard, the Santa Monica Pier, and Ashland Avenue. The Montana and Wilshire outfalls typically only discharge runoff to the ocean during heavy rains. The Pico-Kenter, Santa Monica Pier and Ashland Avenue outfalls discharge yearround during both wet and dry periods. In 2022, the City completed construction of the Sustainable Water Infrastructure Project (SWIP) at the Civic Center, which is designed to capture millions of gallons of storm water runoff that once would have flown into the Santa Monica Bay.

The storm drain system in the city of Santa Monica was designed for the 50-year storm (referred to as the “capital flood”) in accordance with the requirements in the Los Angeles County Hydrology Manual (Chapter 4, Section 4.2 Capital Flood Protection14F 15). However, new data indicate that
flooding associated with a 50-year storm (and a 100-year storm) is likely to be more intense than previously thought, which means the values used in the design of the storm drain system may be too small. Specifically, in 2014, the National Oceanic and Atmospheric Administration (NOAA15F 16)
issued supplementary stream gage data that expanded the existing dataset typically used in hydrology analyses. The old stream gage data covered the time period between 1897 and 1970, whereas the new data, published in what is referred to as the NOAA Atlas 14, covers the time period between 1970 and 2010. As a result of the 2014 analyses, NOAA found that precipitation levels in

15 Los Angeles County Department of Public Works, January 2006, Hydrology Manual, available from https://dpw.lacounty.gov/wrd/publication/engineering/2006_Hydrology_Manual/2006%20Hydrology%20Manual -Divided.pdf 16 Perica, S. et al., 2014 (revised), NOAA Atlas 14, Precipitation-Frequency Atlas of the United States, Volume 6, Version 2.3: California: National Oceanic and Atmospheric Administration, 233p.; available from https://www.weather.gov/media/owp/oh/hdsc/docs/Atlas14_Volume6.pdf Natural Hazards Mitigation Plan Section 8
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the south-facing flank of the Santa Monica Mountains are generally about 1 to 2 inches higher for the 100-year 6-hour estimates using the revised Atlas 14 values than the values previously used. In the Los Angeles Basin itself, including Santa Monica proper, the Atlas 14 values are about the same or slightly lower (0 to -0.25 inches) than the previously used values. Combined with the precipitation levels in the mountains, however, the overall runoff input into the storm drain system draining the region is expected to be higher than the previous estimates used in the design of the system.

Moreover, considering that future storms in southern California are expected to be more intense as a result of climate change, with higher maximum 1-day precipitation amounts, as evidenced most recently with the early February 2024 rainfall totals, the storm drain system is anticipated to be under-designed for the unusual but high-intensity storms the region is expected to experience in the future. This in turn means that flood mitigation measures may be limited if the storm drain system is under-designed and does not have adequate capacity to handle the increased flows expected due to climate change.

INUNDATION DUE TO CATASTROPHIC FAILURE OF WATER STORAGE

STRUCTURES
Seismically induced inundation refers to flooding that occurs when water retention structures (such as dams and their associated reservoirs and above-ground water tanks) fail as a result of an earthquake, overtopping, foundation defects, construction errors, or some other reason. Loss of life and damage to structures, roads, and utilities may result from a dam failure. Economic losses can also result from a lowered tax base and lack of utility profits (if the dam is part of a hydroelectric project).

Flooding Due to Dam Failure
Statutes governing dam safety are defined in Division 3 of the California State Water Code (California Department of Water Resources, 1986). These statutes empower the California Division of Dam Safety to monitor the structural safety of dams that are greater than 25 feet in dam height or have more than 50 acre-feet in storage capacity. Dams under Federal ownership and operation, such as those owned by the U.S. Army Corps of Engineers, are not reviewed by, nor are they under the jurisdiction of the California Division of Dam Safety.

There are several reasons why dams can fail. The most common causes include: 1) overtopping (meaning water spills over the top of the dam), 2) foundation defects, 3) cracking caused by natural settling, faulting, landsliding or other deformation of the ground supporting the structure, 4) inadequate maintenance of the structure, and 5) piping (internal erosion of the dam caused by seepage). Combined, overtopping, foundation defects and piping are responsible for approximately 84% of all dam failures that have occurred over the years in the United States.16F 17 Overtopping alone
is responsible for nearly 130 dam failure incidents between 2010 and 2019, based on a database compiled by the Association of State Dam Safety Officials (ASDSO), with the main driver of these incidents being hydrologic events that led to flooding.17F 18 These numbers are sobering considering
that the existing California water storage infrastructure (dams and reservoirs, debris basins, catchment basins, and the causeways and storm drains downstream) are thought to be severely

17 Based on percentages for each cause listed in the Association of State Dam Safety Officials website at https://damsafety.org/dam-failures accessed on October 25, 2023. 18 https://damsafety.org/dam-failures accessed in Oct. – Nov. 2023.

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under-designed for the amount of precipitation and storm frequency expected as a result of climate change, including ARkstorm events such as those experienced in 2023 and 2024.

There have been between 82 and 92 dam failures in California since the 19th century18F 19, with 82 of
those events known to have resulted in uncontrolled release of water from the reservoir. Table 8-4 lists those events known to have occurred in southern California; the list may be incomplete as data on the locations of numerous dams is not available. Three of these dam failure events prompted the enactment of strict dam safety legislation at the State level.19F 20 These dam failures are: 1) St. Francis
dam, 2) Baldwin Hills dam, and 3) Lower San Fernando dam. These dam failures are described further in the following pages.

Table 8-4: Dam Failures in Southern California
Dam Name Location Year Failure Mechanism; Casualties
Sweetwater and
Sweetwater Main
San Diego 1916 Overtopping due to intense rainfall event; 8 deaths.

Lower Otay San Diego 1916 Overtopping during first filling due to inadequate spillway capacity; 30 deaths.

Garfield Avenue South Pasadena 1924 Unknown, but surmised to be related to construction errors
Sheffield Santa Barbara 1925 Earthquake slide or liquefaction Puddingstone Pomona 1926 Overtopping during construction Sweetwater Main San Diego 1927 Inflow flood; hydrologic event Lake Hemet Hemet 1927 Overtopping due to intense rainfall Saint Francis San Francisquito
Canyon
1928 Sudden failure at full capacity through foundation, >450 deaths.

Cogswell Monrovia 1934 Breaching of concrete cover
Baldwin Hills Los Angeles 1963 Leak through embankment due to seepage, internal erosion; 5 deaths, 16,500 evacuated.

Matilija Ventura County 1965 Unknown if it resulted in an uncontrolled release of water from the reservoir; failure mode also unknown.

San Fernando San Fernando
Valley
1971 Seismic shaking and liquefaction; near-failure; 80,000 evacuated.

Wohlford Lake San Diego County 1980 Unknown if it resulted in an uncontrolled release of water from the reservoir; failure mode also unknown.

St. Francis Dam, completed in 1926 in the San Francisquito Canyon near Saugus, was originally 185 feet high and 600 feet long. In response to increased population pressures, the storage capacity of the dam was increased by raising the dam’s height to 205 feet. This was accomplished without increasing the width of the base to compensate for the increased height, a significant issue since this was a curved concrete gravity dam, with its stability derived from a balance between the vertical and horizontal loads. Around midnight on March 12-13, 1928, a massive landslide occurred along the dam’s left abutment; more than 1.5 million tons of schist bedrock slid down and against the dam’s 271 tons of concrete. That, combined with the already unstable dam base resulting from the increased height, led to the catastrophic failure of the dam. A flood wave more than 140 feet high

19 https://npdp.stanford.edu/sites/default/files/reports/npdp_dam_failure_summary_compilation_ v1_2018.pdf; http://npdp.stanford.edu/dam_incidents, both accessed on February 22, 2024. 20 https://water.ca.gov/Programs/All-Programs/Division-of-Safety-of-Dams/History, accessed in October 2023.

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swept down the Santa Clara valley toward the Pacific Ocean. Even 42 miles down-canyon, by Santa Paula, the wall of water was estimated at 25 feet high. Almost everything in its path was destroyed:
livestock, structures, railways, bridges, and orchards. By the time it was over, parts of Ventura County were under 70 feet of mud and debris. The death toll is unknown as there were many undocumented families living in the floodplain below the dam. Some accounts say 420 people were killed; others estimated more than 600.20F
21 Damage estimates topped $20 million (equivalent to more than $366 million in 2024).

The failure of this dam resulted in several positive changes, including the requirement for engineering geologic input in dam projects (it had been absent before then), increased dam safety legislation in California, the review of all federal dams, professional engineering registration, and state-mandated arbitration hearings for all victims of natural disasters (Rogers, 2007). Specifically, and in response to failure of the St. Francis Dam, the State legislature created the Dam Safety Program on August 14, 1929, which required, among other things, the examination and approval of dams built prior to 1929; the review and approval of plans and specifications for new and existing dams; and the supervision during construction and maintenance of all dams under the jurisdiction of the State. Today, this program is managed by the Division of Safety of Dams (DSOD) under the California Department of Water Resources (CDWR).

The Baldwin Hills dam, an earthen dam that created a 20-acre, 70-foot-high reservoir, failed on Saturday, December 14, 1963 at 3:38 in the afternoon (see Photo 8-1). The dam was constructed between 1947 and 1951 for the purpose of storing water for west Los Angeles. The “off-stream” dam did not impound a natural river course, and thus, under the rules at the time, was not a jurisdictional dam under the purview of the DSOD. As a result of this dam failure, in 1965 the California Water Code was modified by the legislature to include off-stream storage reservoirs in the DSOD’s list of jurisdictional dams.

This hilltop reservoir was built in 1951 across the surface trace of several faults and cracks associated with the Newport-Inglewood fault zone and uplift of the Baldwin Hills. A water-injection program to enhance the recovery of petroleum in the adjacent Inglewood oil field was begun in 1954, with a full-scale program instituted in 1957. The injection program caused renewed movement on the faults, including one under the reservoir that experienced normal movement. Movement on the fault caused the reservoir lining to fail; a pencil-thin crack widened to a 75-foot-wide gash, allowing 250 million gallons of treated water to surge out, with the flood sweeping northward in a V-shaped path roughly bounded by La Brea Avenue and Jefferson and La Cienega boulevards. The residences within a square-mile area were inundated with mud and debris, 65 homes were completely destroyed, and more than 200 additional homes and apartments were damaged. Five lives were lost and 27 people were injured; the loss of life would have been much greater had a caretaker not given the alarm approximately three hours beforehand, allowing the police just barely enough time to evacuate the area. Total property damages were estimated at $12 million (approximately $120.7-150.9 million in 2023 dollars), in addition to the loss of the reservoir itself.21F 22 21 https://web.mst.edu/rogersda/st_francis_dam/Impacts%20of%20St.%20Francis%20Dam%20Failurecompressed.pdf accessed at various times in 2024 and March 2025. 22 Hamilton, D.H. and Meehan, R.L., 1971, Ground rupture in the Baldwin Hills: Science, Vol. 172, pp. 333-344; and https://damsafety.org/Incidents
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Photo 8-1: Failure of the Baldwin Hills Dam
Dark spots in the water and upper right-hand quadrant
show the location of the break in the dam.

This is one of the first disaster events documented in a live helicopter broadcast – the live telecast of the collapse from a KTLA-TV helicopter is considered the precursor to airborne news coverage that is now routine everywhere. It took 77 minutes for the impounded reservoir to empty, but it took a generation for the neighborhood below to recover, illustrating the severe, long-term impact of these disasters. Furthermore, failure of this reservoir foreshadowed the end of urban-area earthen dams as a major element of the Department of Water and Power’s water storage system. It also prompted a tightening of Division of Safety of Dams control over reservoirs throughout the State.

The third southern California dam failure (near-failure in this case) that prompted legislative changes occurred as a result of earthquake-induced liquefaction (see Section 6). The Lower and Upper Van Norman (or San Fernando) dams were built in the 1910s to collectively store the water brought into the area by the Los Angeles Aqueduct. Combined, the two dams could store about 80% of the water for the city of Los Angeles. The bulk of these dams were constructed with hydraulic fill; this involves pumping a sludge of sediment and water into place, contained in between embankments or dikes.

The sludge is deposited in layers, with water allowed to drain before a new layer is laid; this causes the finer-grained sediments in the sludge to settle in the center of the dam, creating a clay core. If the water flow is not managed correctly, however, layers or tongues of coarse-grained material may be deposited across the width of the dam, with these layers then allowing for seepage to occur.

Uncompacted hydraulic fill is susceptible to liquefaction when shaken, which is what happened to these dams when the 1971 San Fernando earthquake hit.

Fortunately, at the time, the Upper San Fernando dam was only about one-third full, and the Lower San Fernando dam was about half full. The Lower dam, in particular, had undergone a seismic hazard review in 1966, and by 1967, its operating height had been reduced in response to that analysis. The lower water levels helped avert disaster to areas downstream. Specifically, as a result Natural Hazards Mitigation Plan Section 8
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of the earthquake, the Upper San Fernando dam dropped 3 feet and was laterally displaced about 5 feet. The Lower dam experienced damage to its crest and upstream face, with a large liquefactioninduced shallow landslide that originated from the upstream face failing into the reservoir. To keep the dam from failing catastrophically, the dam operators reduced the water level as quickly as possible. At the same time, the neighborhoods within the potential inundation area were evacuated; an estimated 80,000 people were evacuated from their homes for at least three days. In response to the near-failure of these embankments, the DSOD revisited its seismic evaluation program, and revisited all hydraulic fill dams under their jurisdiction, an effort that resulted in the seismic retrofit of numerous dams in the state. The near-failures also prompted civil engineering departments at various institutions to conduct liquefaction research.22F 2323F24
Flooding Due to Failure of Above-Ground Water Storage Tanks Seismically induced inundation can also occur if strong ground shaking causes structural damage to above-ground water tanks. If a tank is not adequately braced and baffled, sloshing water can lift a water tank off its foundation, splitting the shell, damaging the roof, and bulging the bottom of the tank (elephants foot) (EERI, 1992). Movement can also shear off the pipes leading to the tank, releasing water through the broken pipes. These types of damage occurred during southern California’s 1992 Landers, 1992 Big Bear, 1994 Northridge, and 2019 Ridgecrest earthquakes. The Northridge earthquake alone rendered about 40 steel tanks non-functional (EERI, 1995), including a tank in the Santa Clarita area that failed and inundated several houses below. As a result of lessons learned from those earthquakes, new standards for design of steel water tanks were adopted in 1994 (Lund, 1994). The new tank design includes flexible joints at the inlet/outlet connections to accommodate movement in any direction. However, an analysis of water tanks following the 2003 Paso Robles earthquake found that the design standards should be modified further in seismically active areas to make tanks more earthquake resistant.24F 25 Below-ground water tanks are less likely to be impacted by ground shaking, but are still susceptible to surface ground deformation resulting from fault rupture, liquefaction, or slope failures. Water lost from tanks during an earthquake can significantly reduce the water resources available to suppress earthquake-induced fires. Damaged tanks and water mains can also limit the amount of water available to residents after a severe earthquake. Groundwater wells can be damaged during an earthquake, limiting the water available to the communities dependent on those wells. Therefore, it is of paramount importance that the water storage tanks in the area retain their structural integrity during an earthquake, so water demands after an earthquake can be met. In addition to evaluating and retrofitting to meet current standards, this also requires that the tanks be kept at or near full capacity as much as practical.

23 https://water.ca.gov/Programs/All-Programs/Division-of-Safety-of-Dams/History; https://damsafety.org/incidents/ca00000-124, both accessed on Nov. 6, 2023. 24 Castro et al., 1989, Re-Evaluation of the Lower San Fernando Dam, available from https://apps.dtic.mil/sti/tr/pdf/ADA214589.pdf, accessed on Nov. 6, 2023. 25 Eidinger, J., 2009, Magic R: Seismic Design of Water Tanks: paper accessed from https://www.geengineeringsystems.com/ewExternalFiles/Magic%20R%20%20Seismic%20Design%20of%20Wate r%20Tanks.pdf on Feb. 19, 2023.

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HAZARD ASSESSMENT
Hazard Identification
Hazard identification is the first phase of flood-hazard assessment, and involves estimating the following:
1) the geographic extent of the area at risk from flooding; 2) the intensity of the flooding that can be expected in specific areas of the floodplain; and 3) the probability of occurrence of flood events.

This process usually results in the creation of a floodplain map, with FEMA being the agency that generally creates these products. Floodplain maps often provide detailed information that can assist jurisdictions in making policies and land-use decisions, although communities should recognize that as a result of the limitations in the mapping process and the impacts of climate change, these maps may not capture all flood-prone areas. Maps that show the extent of inundation if a water reservoir fails are also now required for all dams in California under the jurisdiction of the State. We refer to both the FEMA map and the inundation maps available for dams near Santa Monica to address the risk of flooding in the city.

Flood Hazard Mapping and Flooding Potential in the City of Santa Monica The City of Santa Monica has participated in the National Flood Insurance Program since 1974. The potential flood zones mapped by FEMA in the city are shown on the Flood Insurance Rate Maps (FIRMs). The FIRM panels that cover Santa Monica include: 06037C1567G, 06037C1569G, 06037C1588G, 06037C1589G, 06037C1590G and 06037C1751G. The most recent Flood Insurance Study (FIS) for Los Angeles County, which includes Santa Monica, was issued by FEMA on June 2, 2021. Overall, FEMA assigns a Very Low Coastal Flood Risk Index rating to the Los Angeles County region, including Santa Monica25F
26, as discussed further below.

Specifically, the only area in Santa Monica mapped as within the 100-year flood zone is the beach west-southwest of Pacific Coast Highway north of the Pier, and approximately west of the Marvin Braude Bike Trail south of the Pier. Figure 8-3 shows the inundation limits for 100-year flood events as a result of both storm flooding (AE zones in bright pink), and coastal flooding due to storm surges (VE zones in light purple). Zone VE is the coastal area with a 1% or greater chance of flooding with an additional hazard associated with storm waves. The section of the Santa Monica coastline that is approximately north of Wilshire Boulevard is classified as both Zone AE and Zone VE, whereas the coastline approximately south of Wilshire Boulevard is only classified as Zone VE. Based on an analysis by City Staff26F
27 of the 2021 FIRM maps, 85 beach-front properties are located within the 100-year flood zone. The properties adjacent to the coast are also at significant risk of flooding due to tsunamis, as discussed in Section 7.

FIRMs typically also show the estimated limits of areas with moderate to low risk of flooding. The flood having a 0.2% annual chance of occurring (also referred to as the 500-year flood) is usually the basis for these categories, with moderate risk defined as the zone between the limits of the 100- year and 500-year floods, and low risk defined as the area outside of the 500-year flood limits. These

26 https://hazards.fema.gov/nri/coastal-flooding, accessed on 01/31/2024. 27 City of Santa Monica, Staff Report 4422, March 9, 2021 online at https://santamonicacityca.iqm2.com/Citizens/Detail_LegiFile.aspx?Frame=&MeetingID=1262&MediaPosition=&I D=4422&CssClass=
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zones may also include areas where the base flood is less than one-foot deep, where the drainage basin is small (less than one square mile), or areas that are protected from the base flood by levees.

Figure 8-3 shows that most of Santa Monica is mapped as having a minimal flood hazard, with the majority of the city located outside the 500‐year flood zone (Zone X, unshaded). Flood-prone areas are mapped just outside the City limits; this could have an impact on motorists attempting to arrive or depart from Santa Monica. For example, sections of Rustic Canyon in Pacific Palisades are designated by FEMA as either a 1% or 0.2% chance of flooding. Likewise, relatively low depressions along Bundy Drive and San Vicente Boulevard near the Brentwood Country Club are considered to have a 0.2% chance of flooding. Flood insurance is available for properties in these zones, but is not mandated by the NFIP.

Urban Flooding and Existing Flood Protection Measures
in the City of Santa Monica
Santa Monica, like the rest of the extensively developed Los Angeles basin, has a high concentration of impervious surfaces that both collect and concentrate the flow of water. During periods of urban flooding, some streets may concentrate water flow leading to swift currents, and low-lying areas may fill with water. Storm drains, especially if clogged with vegetation and debris, can also result in unintentional localized flooding.

Urban runoff in Santa Monica is captured in a drainage system that consists of over 2,400 catch basins and 64 storm drain lines, and the Los Angeles County Flood Control District also owns about 9 to 10 miles of storm drain lines and inlets within Santa Monica. The City’s storm drain pipes range from 16 inches to 11 feet in diameter and are mostly constructed of reinforced concrete; a few structures consist of brick arch drains, concrete open channels, or corrugated metal pipes.

Historically, urban runoff would be carried by this underground storm drain system and drain untreated into Santa Monica Bay via five outfalls within City limits. A small fraction of the runoff drained into Ballona Creek. The largest of these outfalls is the Pico-Kenter which, combined with the 120-inch Santa Monica Freeway storm drain, empties into the ocean at the western end of Pico Boulevard. Other outfalls are located on the beach at Montana Avenue, Wilshire Boulevard, the Santa Monica Pier, and Ashland Avenue. Since 2022, however, with the construction of the Sustainable Water Infrastructure Project (SWIP) at the Civic Center, millions of gallons of storm water runoff are now captured, treated and used for irrigation and other gray-water purposes, limiting the amount of water that flows into the Santa Monica Bay.

As mentioned before, the FEMA mapping shows that flooding in Santa Monica is generally a concern only along the coastline. However, small and localized flooding may occur in the inland areas of the city, especially if the storm drains are clogged or prove to be inadequate to manage runoff from a large or protracted storm like the one in February 2024. Areas that occasionally flood reportedly include the general vicinity of Wilshire Boulevard to Idaho Avenue, from 4th Street to 7th Street, and along the I-10 freeway, the Pacific Coast Highway, and at the McClure Tunnel. Ponding has also been reported following heavy rain by Michigan Avenue and 16th Street, at Main Street between Hill Street and Ashland Avenue, and by Virginia Avenue Park and Douglas Park.

Proper maintenance of the extensive underground storm drain system is recommended to ensure that urban street flooding remains a relatively low hazard. City staff prepare for storms by closely monitoring storm activity and if there is a forecast of high-volume storms, work to ensure that storm drains are clear, especially in areas prone to ponding. The City may also direct people to stay off the Natural Hazards Mitigation Plan Section 8
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roads during flood/flash flood events. City staff typically address issues of localized ponding with paving or drainage improvement projects.

Inundation Due to Catastrophic Failure of Water Storage Structures Water-Retaining Structures In and Near Santa Monica
The City of Santa Monica owns and operates four drinking water storage reservoirs (Mt. Olivette, Riviera, San Vicente, and Arcadia), two stormwater/urban runoff harvesting tanks, and one treated urban runoff storage tank. These reservoirs and tanks are identified in Table 8-5, and are described further below.

Table 8-5: Water Storage Reservoirs In and Near Santa Monica Owned by Santa Monica Drinking Water Storage Reservoirs and Tanks
Name Address
Storage Capacity
(millions of
gallons)
Type Date
Construction
Mt. Olivette 924 Franklin Street 5 underground 1924
Riviera 1252 Capri Drive,
Pacific Palisades 25 off-stream covered reservoir 1962
San Vicente San Vicente Blvd.
and 26th Street 5 underground 1958
Arcadia 1228 South Bundy Dr.,
Los Angeles 5 partially underground Pre 1927
Reclaimed Water Harvesting and/or Storage Tanks
Clean Beaches Tank 1623 Appian Way 1.6 underground 2018 Sustainable Water
Infrastructure Project (SWIP) –
Stormwater Harvesting Tank
1771 Main Street 5 underground 2022
Marine Park Underground
Storage Tank 1406 Marine Street 0.025 (25,000 gallons) underground 2017 Mt. Olivette Reservoir
The Mt. Olivette Reservoir is located in the northeastern part of the city, along Franklin Street. This subsurface reservoir was built in 1924 and holds up to 5 million gallons (15.3 acre-feet) of water.

This site is not within an Alquist-Priolo Earthquake Fault Zone, but several geologists have mapped a trace of the Santa Monica fault zone along the north side of the Brentwood Knoll, the elevated hill where the reservoir is located, and within 300 feet of the site. At least one published fault map27F 28 shows a trace of the Santa Monica fault through the southeastern corner of the reservoir site. The northeast-sloping face of the knoll, including the northeastern portion of the reservoir site, is also in an area identified by the California Geological Survey (CGS) as susceptible to earthquake-induced slope damage. The slopes in this area could also sustain surficial slope instability following an intense and/or prolonged precipitation event. Due to its location relative to the CGS-defined earthquake fault and landslide zones, the Mt. Olivette Reservoir may be impacted during an earthquake. In August 2023, the Santa Monica City Council approved a contract for engineering upgrades and improvements to the Mt. Olivette Reservoir and all other water storage reservoirs in

28 Yerkes, R.F., and Campbell, R.H., 2005, Preliminary Geologic Map of the Los Angeles 30’ x 60’ Quadrangle, Southern California, Version 1.0: U.S. Geological Survey Open-File Report 2005-1019; available from http://pubs.usgs.gov/of/2005/1019
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the city, with the work expected to be completed over the next two to four years.28F 29 Riviera Reservoir
The Riviera Reservoir, located at 1252 Capri, is located along the northern wall of Santa Monica Canyon in the city of Los Angeles, next to the Riviera Country Club and about two miles north of the city of Santa Monica. Although outside City limits, the dam and reservoir are owned by the City of Santa Monica, and managed by the City’s Department of Public Works. The Riviera Reservoir (California State Dam Number 1043-000, National ID No. CA00876) was constructed in 1962, has a storage capacity of about 25 million gallons (approximately 76 acre-feet), and is an off-stream, covered storage reservoir built with vertical concrete walls with a dam height of 40 feet. Because of its size, this reservoir is considered a dam and is therefore under the purview of the California Division of Dam Safety. Statutes governing dam safety are defined in Division 3 of the California State Water Code.29F
30 These statutes empower the California Division of Dam Safety to monitor the structural safety of dams that are greater than 25 feet in dam height or have more than 50 acre-feet in storage capacity.

According to the Division of Safety of Dams30F
31, the dam is best described as a reinforced concrete
tank. The length of the dam crest is 1,280 feet. As of 2022, this reservoir had been certified, meaning that the dam is considered to safely impound water to the elevation specified in the Certificate of Approval, and is considered satisfactory, meaning that no existing or potential dam safety deficiencies are recognized. This also means that the dam is expected to perform in an acceptable manner under all loading conditions in accordance with the minimum applicable state or federal regulatory criteria.

The Riviera Reservoir is located in an area that the CGS has mapped as susceptible to liquefaction, and adjacent to a slope that could fail as a result of earthquake-induced seismic shaking. If the reservoir failed during an earthquake, then the structures located immediately below the dam at the Riviera Country Club would be impacted. Moreover, flood waters released during the reservoir failure would empty onto the Riviera Country Club golf course, eventually flowing into the Santa Monica Canyon. Thus, inundation from Riviera Reservoir would mainly be confined to Santa Monica Canyon and not cross over into the city. Santa Monica Canyon is a concrete-lined channel that is dry most of the time; the flood control channel will likely contain the increased flow from the dam failure and direct it to the Pacific Ocean. Damage to any homes adjacent to the golf course is considered unlikely, and the travel time of the flood flows to the flood control channel is estimated at 15 minutes. According to the California Division of Safety of Dams, this reservoir has a “high” downstream hazard, defined as expected to result in the death of at least one person should the dam fail when operating with a full reservoir.

San Vicente Reservoir
The San Vicente Reservoir is also located in the northeastern part of the city, in the median of San Vicente Boulevard, near its intersection with 26th Street. The reservoir is entirely underground and

29 https://santamonicacityca.iqm2.com/Citizens/FileOpen.aspx?Type=1&ID=1351&Inline=True, accessed November 8-9, 2023. 30 California Department of Water Resources (DWR), 1986, Statutes and Regulations Pertaining to Supervision of Dams and Reservoirs: Division of Safety of Dams, 46p. 31 https://water.ca.gov/-/media/DWR-Website/Web-Pages/Programs/All-Programs/Division-of-Safety-ofDams/Files/Publications/Dams-Within-Jurisdiction-of-the-State-of-California-Listed-Alphabetically-by-NameSeptember-2022.pdf Natural Hazards Mitigation Plan Section 8
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has a capacity of 5 million gallons of water. This tank is not located in an area mapped as susceptible to fault rupture or slope instability, but it is located near the Santa Monica fault. As a result, this reservoir could experience significant ground shaking if the Santa Monica fault caused an earthquake. In 2018, this reservoir, together with the other three drinking water storage facilities that serve the city, was reviewed by the California State Water Resources Control Board Division of Drinking Water. As a result of that routine survey, several reliability and operational improvements to the reservoir were recommended. These improvements will be conducted under the contract approved by City Council in August 2023, with the work anticipated to be completed by 2028.

Arcadia Reservoir and Water Treatment Facility
The Arcadia Reservoir and Water Treatment Facility is located east of the city of Santa Monica near the intersection of Wilshire Boulevard and Bundy Drive, in the city of Los Angeles. The storage tank has a storage capacity of 5 million gallons and is part of a water treatment facility that removes methyl tertiary butyl ether (MTBE) from the City’s Charnock Well Field and volatile organic compounds (VOCs) from groundwater pumped from the City’s Olympic Well Field. The facility is currently being expanded with the goal of reducing Santa Monica’s reliance on imported water. The multi-barrier treatment facility includes greensand filtration, ultraviolet light advanced oxidation processes, granular activated carbon, and reverse osmosis systems.

Because the Arcadia reservoir is partially buried, failure of the structure may inundate surrounding areas located at lower elevations, with the released water expected to flow to the southwest and possibly south on Bundy Drive. The Arcadia Reservoir is located within the CGS-defined AlquistPriolo Earthquake Fault Zone for the Santa Monica fault, with several different geologists having mapped the fault through this site. Significantly, this fault trace is the same fault previously investigated at both the Veteran’s Administration property in Los Angeles and at University High School, with both studies indicating that this fault has moved in the past 11,700 years and is therefore Holocene-active. The southwestern portion of the Arcadia reservoir is also located in an area mapped as susceptible to liquefaction.

Stormwater and Urban Runoff Harvest Tanks
The City completed construction of two stormwater/urban runoff harvesting tanks aimed to protect water quality in the Santa Monica Bay and increase local water reuse. One of these is the Clean Beaches Tank completed in 2018 that is located in the surface parking lot north of the Santa Monica Pier. Stormwater/urban runoff captured from the Pier drainage area is stored in the 1.6-million-gallon tank and sent to the existing Santa Monica Urban Runoff Recycling Facility (SMURRF) for treatment; the treated water is then used for non-potable and potable reuse. The second is the Sustainable Water Infrastructure Project (SWIP) Stormwater Harvesting Tank located underneath the surface parking lot adjacent to the Santa Monica Courthouse. This tank has the capacity to store 1.5 million gallons of captured stormwater and urban runoff. The stormwater/urban runoff is purified in the SWIP’s Advanced Water Treatment Facility where the final purified water is used for non-potable uses (e.g., irrigation and toilet flushing) and potable reuse (e.g., groundwater recharge via direct injection).

Additional storage tanks like these are reportedly planned throughout the Los Angeles region to alleviate some of the water demands and reduce the need for imported water. One of these tanks was built by the City of Los Angeles at Penmar Park, under the baseball field, just outside Santa Monica city limits. The tank is reportedly not currently operational, but is still there and could thus be used in the future. When the tank was proposed, the water stored there was to be used to irrigate Natural Hazards Mitigation Plan Section 8
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Penmar Recreation Center, Penmar Golf Course, and transferred to a 25,000-gallon storage taken at Marine Park in Santa Monica to irrigate the peak daily needs of the park.31F 32 For these reasons, this
tank is included in Table 8-5 above.

These stormwater harvesting tanks are located in areas of the city considered to be relatively safe from seismic and geologic hazards. This means that these tank locations are not known to be underlain by faults, and are not expected to be impacted by liquefaction or slope instability. The SMURFF is also outside the area expected to be impacted by coastal flooding or a tsunami. Although all three locations (the two storage tanks and the SMURFF) are expected to experience ground shaking as a result of an earthquake on any of the regional active faults, the buried cisterns in particular are expected to perform well. Damage could occur at the piping connections if not fitted with flexible piping or tubing designed to accommodate some movement when the ground is shaking. This is discussed further below.

Water Tank Failure Impacts
The tanks located within Santa Monica are at risk of experiencing very strong ground shaking if the Santa Monica, Palos Verdes, Newport-Inglewood, or Hollywood faults generate an earthquake with its epicenter at or near the city. For this reason, these tanks should be regularly evaluated and their inundation paths should be defined, if at all possible, to determine whether or not habitable structures, and especially essential facilities, such as schools, are located within the inundation zone.

Ideally, in the event of catastrophic breakage, the water stored in these tanks is either contained within the site, or is discharged to a storm drain or channel that limits the impacted area.

It is unclear whether the reservoirs mentioned above are fitted with flexible joints and pipe connections that could withstand shaking associated with an earthquake on a nearby seismic source.

However, if the main water lines leading from the reservoirs to the city are sheared or damaged during an earthquake, Santa Monica could be without water for several days to weeks after the earthquake, depending on the scale of damage. An earthquake on the Santa Monica fault could result in extensive damage to the network of water mains and pipelines that distribute water to all buildings in the region. Rupture of the water mains could also result in the flooding of the areas immediately surrounding, and down-gradient of the failed pipes since several of the reservoirs drain by gravity. These potential ruptures would allow little time to evacuate the areas immediately downgradient from the reservoirs, with the potential for loss of life and damage to property.

Because the entire metropolitan area of Los Angeles, including the city of Santa Monica and neighboring communities, is susceptible to strong seismic ground motion, all above-ground water storage tanks in and upstream from Santa Monica should incorporate earthquake-resistant designs.

These should include flexible pipe joints that can accommodate some movement during seismic events, reducing the potential for breakage of the pipes, leading to accidental releases of water.

The City is currently evaluating design options for the rehabilitation of four water storage reservoirs that serve the community. The evaluation includes failure modes that are caused by seismic events and identify potential mitigation measures to minimize interruption to providing continuous water service to the community.

32 https://yovenice.com/2016/07/09/green-irrigation-for-penmar-park-and-golf-course/ Natural Hazards Mitigation Plan Section 8
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Stone Canyon Reservoir
The Stone Canyon Reservoir, which is located about 4.1 miles northeast of Santa Monica near the residential neighborhood of Bel Air, is owned and operated by the Los Angeles Department of Water and Power. This earthen embankment reservoir (California State Dam Number 6-025; National ID No. CA00083) was constructed in 1924, has a storage capacity of about 3.4 billion gallons (approximately 10,372 acre-feet), is 188 feet tall and 1,150 feet long. Given its size, this dam is also under the jurisdiction of the Division of Dam Safety. The dam is discussed here because if it failed, the flood waters released from the reservoir would inundate portions of the city of Santa Monica, in addition to impacting a significant section of Los Angeles and Culver City. Neighborhoods of the city of Los Angeles that would be impacted include Bel Air, Westwood, West Los Angeles, Mar Vista, Playa del Rey, Westchester, Palms, Venice, and Marina del Rey,.

Roth et al. (200432F
33) evaluated the seismic performance of the Stone Canyon Reservoir with two- and three-dimensional nonlinear effective-stress models using the explicit finite-difference program FLAC. They modeled the reservoir response to shaking levels experienced during the 1994 Mw6.7 Northridge earthquake, and during a hypothetical regional and local Maximum Credible Earthquake (MCE) of moment magnitude (Mw) 8 and Mw 6.5, respectively. Based on these parameters, their analyses suggest that Lower Stone Canyon Dam can be operated safely with the reservoir level at its design elevation of 865 feet.

However, if the reservoir failed during a larger local earthquake, then the eastern portion of the city of Santa Monica, in the area between about Olympic and Pico Boulevards, would be inundated.

Specifically, the flood waters from Stone Canyon Reservoir are expected to have a peak velocity of 1 to 3 feet per second, have a maximum depth of 1 to 10 feet, and arrive in the eastern part of Santa Monica approximately an hour to an hour-and-a-half after the dam failure.33F 34 Significant more
damage is expected to adjacent communities including Bel Air, Los Angeles, Culver City and Venice; refer to Figure 8-4. As of 2022, the California Division of Safety of Dams lists this reservoir as certified, with a satisfactory assessment, and with an “extremely high” downstream hazard. The extremely high downstream hazard is assigned to reservoirs that are “expected to cause considerable loss of human life or would result in an inundation area with a population of 1,000 or more”34F

35. 33 Roth, W.H., Dawson, E.M., Somerville, P., Davis, C.A., and Plumb, C.C., 2004, Evaluating the seismic performance of Stone Canyon Reservoir with 2-D and 3-D analyses: Paper No. 3249 presented at the 13th World Conference on Earthquake Engineering, Vancouver, B.C. Canada, 1-6 August 2004; available from https://www.iitk.ac.in/nicee/wcee/article/13_3249.pdf. 34 GENTERRA Consultants, Inc., 2018, Stone Canyon Dam Sunny Day Dam Failure Scenario, Los Angeles County, California State Dam # 06-025, National Dam ID # CA00083; consulting report prepared for Los Angeles Department of Water & Power, dated June 29, 2018. 35 https://water.ca.gov/-/media/DWR-Website/Web-Pages/Programs/All-Programs/Division-of-Safety-ofDams/Files/Publications/Dams-Within-Jurisdiction-of-the-State-of-California-Listed-Alphabetically-by-NameSeptember-2022.pdf Local Hazards Mitigation Plan Section 8
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Figure 8-4: Dam Inundation Map
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The Los Angeles Department of Water and Power (LADWP) shares its Emergency Action Plan (EAP) for Stone Canyon Dam with Santa Monica emergency response partners regularly. The LADWP conducts regular safety inspections of the dam and monitors water levels in the reservoir to quickly activate response measures if an issue is identified. Its EAP was last revised in February 2024, describing local public safety emergency responsibilities and emergency alerting and communication processes, including regular patrols of the dam as part of its security plan. Areas of Santa Monica that are not directly affected may be indirectly impacted due to large portions of West Los Angeles rapidly evacuating or needing support services, including critical facilities such as schools, hospitals, and college campuses. A major flooding incident would also impact Santa Monica’s mutual aid agencies in Culver City. An imminent failure scenario would result in major long-term recovery impacts for Santa Monica, as the scenario could hinder access to Santa Monica from the east and south for an extended period of time and would result in the relocation of a large portion of the West Los Angeles population, likely impacting the city’s workforce, housing demands, school communities, and business activity.

VULNERABILITY ASSESSMENT – COMMUNITY FLOOD ISSUES
Vulnerability assessment is the second step of flood-hazard assessment. It combines the floodplain boundaries recognized during the hazard identification phase with an inventory of the properties and infrastructure within the floodplain. Understanding the population and property exposed to this hazard can assist in reducing risk and preventing loss from future events.

The vulnerability assessment of the impact flooding would have in Santa Monica considered two separate possible scenarios, as follows: 1) a 100-year coastal flooding event, and 2) inundation of the eastern portion of the city as a result of a “sunny day” failure of Stone Canyon reservoir. The assessment used two separate approaches to calculate the anticipated damage, including cost. The first approach is based on HazUS, a regional multi-hazard loss estimation model and software application developed by FEMA and the National Institute of Building Sciences used to develop multi-hazard losses at a regional scale. Local, state and regional officials can use these loss estimates to evaluate the area’s vulnerability to multi-hazards and prepare for emergency response and recovery.

The coastal floods vulnerability assessment was conducted using a GIS-based version of the most current Flood Insurance Rate Map for Santa Monica (see Figure 8-3), and a Level 1 analysis that relies on the baseline inventories of structures and population counts based on the 2022 National Structure Inventory, and 2020 Census Data and 2022 U.S. Census American Community Survey, respectively. The dam inundation scenario was developed using a user-developed and supplied hazard map that considered only that portion of Santa Monica impacted by the dam inundation event, based on data extracted from the inundation map for a “sunny-day” dam failure event prepared by Genterra Consultants, Inc. (2018; see Figure 8-4). Both analyses considered and provided information on the number and types of buildings within the pathway of the coastal flood events and the inundation zone, that would thus be damaged. Additional information regarding HazUS, including its uses and limitations, is provided in Section 6 – Earthquake Hazards. The HazUS results were deemed to not capture the current value of the properties found to be impacted by the flood and inundation hazards evaluated, so the areas in the City anticipated to be affected by the flood and inundation scenarios were revisited using Crisis Track®, a GIS-based disaster management software that uses tax data to estimate the cost of a disaster. The results of the analyses are presented below and in the following Risk Analysis section.

Local Hazards Mitigation Plan Section 8
City of Santa Monica, California
2025 Flood Hazards Page 8-39
What is Susceptible to Damage During a Flood Event?
The Santa Monica region is approximately 8.4 square miles in area and contains 1,250 census blocks. The region contains over 47,000 households and has a population of about 93,000 people (based on 2020 U.S. Bureau Census data estimates). There are an estimated 27,495 buildings in the region, with a total building replacement value, excluding contents, of $19,682 million (based on 2022 regional population and building value data). The building data used by HazUS estimates that approximately 84.3 percent of the buildings, and 68.7 percent of the building value, are associated with residential housing. For a breakdown of the building exposure by occupancy type and construction type, refer to Section 6.

Critical facilities include police stations, fire stations, hospitals, emergency operation centers, shelters, and other facilities that provide important services to the community. These facilities and their services need to be functional after a flooding event. The database used by HazUS for the Santa Monica region was modified to include two (2) hospitals with a total combined capacity of 531 beds, 27 schools, five (5) fire stations, one (1) police station, and two (2) emergency operation centers (EOCs). The EOC at the Main Library was not included in the analysis. The flood and inundation zones shown on Figures 8-3 and 8-4 were also visually compared to the map of critical and essential facilities in Santa Monica to identify those facilities that could be impacted by these hazards. 100-Year Coastal Flood Event
Only a small portion of this region, and by City staff estimates, 85 buildings, are located within the FEMA-designated floodways (see Figure 8-3). Areas outside of the 100-year flood zone are known to occasionally flood during intense precipitation events, as mentioned previously, but these areas were not considered in the vulnerability assessments as their boundaries are not well-defined.

Critical facilities, and economically important buildings and infrastructure within or very close to the 100-year flood zone include Fire Station 7, the Harbor Patrol, the Santa Monica Pier, the Annenberg Beach House, and the Santa Monica Urban Runoff Recycling Facility (SMURRF). As discussed previously, coastal flooding can impact Pacific Coast Highway (PCH), whereas urban flooding may impact some sections of PCH, the tunnel connecting PCH with the I-10 freeway (McClure Tunnel), and low points along some of the local streets, including Wilshire Boulevard, Michigan Avenue, Idaho Avenue, and 16th Street, to name a few.

Stone Canyon Inundation Event
The portion of the city within the dam inundation failure zone for Stone Canyon reservoir is larger than the area susceptible to coastal flooding, but is still limited. The area considered for the dam inundation analysis is the area of Santa Monica shown on Figure 8-4 shaded in purple. HazUS estimates that there are about 320 structures within the potential zone of inundation. A review of GoogleEarth images of the area suggests, however, that this figure underestimates the total number of buildings in the zone. The HazUS-derived estimates of building exposure by occupancy type to inundation as a result of the catastrophic failure of Stone Canyon dam are summarized on Table 8- 6 below. Note that residential and commercial structures are estimated to comprise nearly 84% of the total number of structures exposed. The potential losses to these structures as a result of a Stone Canyon dam failure event are discussed in the risk analysis section.

Local Hazards Mitigation Plan Section 8
City of Santa Monica, California
2025 Flood Hazards Page 8-40
Table 8-6: Building Exposure by Occupancy Type – Stone Canyon Dam Inundation Event Occupancy Type Stone Canyon Dam Inundation
Exposure (in thousands of $) Percent of Total
Residential 325,263 45.1
Commercial 277,601 38.5
Industrial 103,567 14.3
Agricultural 279 0.0
Religion 5,785 0.8
Government 4,629 0.6
Education 4,649 0.6
Totals 721,773 100.0
The only critical facility located within Santa Monica’s Stone Canyon dam inundation zone is the LA Metro Division 14 facility at the boundary between the cities of Los Angeles and Santa Monica.

The dam inundation maps (GENTERRA Consultants, 2018) also show a private school, New Roads School on Olympic Boulevard, near to or just within the inundation zone. However, there are several critical facilities and infrastructure near the zone that could be impacted as a result of increased traffic during evacuation and potential minor flooding. These include three schools (Edison Language Academy, the Santa Monica College Center for Media and Design, and the Santa Monica College District Offices), the Santa Monica City Yards, the Fire Training Academy, and the I-10 freeway, including several bridges and connector ramps.

RISK ANALYSIS
Risk analysis is the third and most advanced phase of a hazard assessment. It builds upon the hazard identification and vulnerability assessment. A flood risk analysis should include two components: 1) the population and value of the properties that may incur losses from a flood event (defined through the vulnerability assessment; see Table 8-6 above); and 2) the number and type of flood events expected to occur over time.

As mentioned above, the HazUS results presented here for the hazard of coastal flooding are based on an analysis of the FEMA maps available for Santa Monica, combined with results obtained using Crisis Track®, a disaster management software that can help jurisdictions like cities and counties estimate the costs of a disaster (designed for real events). The Crisis Track® software allows calculation of the minimum and maximum cost for residential and commercial properties within the specified hazard zone. Different damage levels based on FEMA’s Preliminary Damage Assessment 4-Point Terminology are applied to generate a range of loss amounts based on the most recent tax assessment values (2023) available at the time of the analysis rather than market valuations. Results provided by both methods are included below.

The dam inundation analysis was also done using HazUS with a user-supplied GIS-based map of the area anticipated to be impacted by failure of the dam, including depth of the flood waters. The HazUS results are supplemented with a Crisis Track® estimate of the cost of the residential and commercial properties located within the inundation zone, based on tax valuations. The results of these analyses are presented in the following sections.

Local Hazards Mitigation Plan Section 8
City of Santa Monica, California
2025 Flood Hazards Page 8-41
General Building Stock Exposure and Potential Building-Related Losses The largest impact on communities from flood events is typically loss of life and property damage.

Nationwide, data from the National Weather Service shows that between 2010 and 2023 included, 101 people on average died each year as a result of flooding.35F 36 Between 2010 and 2022, annual
economic losses in the United States have ranged between about $494 million (2012) and $60.7 billion (2017).36F
37 In California, as a result of the 2022-2023 atmospheric river events, at least 22 people lost their lives and property losses were estimated at about $4.6 billion.37F 38 The type of property damage caused by flood events depends on the depth and velocity of the floodwaters. Faster moving floodwaters can wash buildings off their foundations and sweep cars downstream. Pipelines, bridges, and other infrastructure can be damaged when high waters combine with flood debris. In some regions, extensive damage can be caused by basement flooding. Most flood damage is caused by water saturating materials susceptible to loss (i.e., wood, insulation, wallboard, fabric, furnishings, floor coverings, and appliances). In many cases, flood damage to homes renders them unlivable.
100-Year Coastal Flood Event
Based on data previously obtained by City staff, 85 buildings are known to be located within the FEMA-mapped 100-year flood zone along the Pacific Ocean, west of Pacific Coast Highway. Several of these properties are City-owned and important to the community and the local economy. Coastal flood damage to these structures costs the City both to repair the structures, and as revenue lost if the structures have to be closed to the public while being repaired. For example, the intense rains in early February 2024 caused damage to City-owned property in Santa Monica, which in turn required the expenditure of City funds to pay City staff and subcontractors to clear and clean up debris from flooded areas, and repair damaged fencing and support beams at the Santa Monica Pier.38F
39 The potential cost to private structures within the flood zone as a result of a 100-year coastal flood event is anticipated to cost in the thousands to even millions of dollars.

The HazUS 100-year flood analysis for Santa Monica yielded a potential loss to commercial structures in the coastal flood zone of $4.6 million. The Crisis Track analysis for the area of the city within the 100-year flood zones shown on Plate 8-3 returned the values shown on Table 8-7 below.

Table 8-7: Estimated Economic Losses to Residential and Commercial Buildings in Santa Monica Within the 100-Year Flood Zone Using Crisis Track® Software Damage
Category
Affected
(Green Tag)
Minor
(Green Tag)
Major
(Yellow Tag)
Destroyed
(Red Tag)
Total Cost $4,027,299.20 $10,068,248.00 $24,163,795.20 $40,272,992.00

36 https://www.weather.gov/arx/usflood accessed on March 12, 2024. 37 https://www.statista.com/statistics/237420/economic-damage-caused-by-floods-and-flash-floods-in-the-us/, accessed on March 12, 2024. 38 https://www.ncei.noaa.gov/access/billions/events/US/1980-2024?disasters[]=all-disasters, accessed on March 12, 2024. 39 Based on email correspondence forwarded by Lindsay Call, Chief Resilience Officer, Santa Monica Office of Emergency Management, on February 8, 2024.

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City of Santa Monica, California
2025 Flood Hazards Page 8-42
Thus, the true value of economic losses related to a 100-year coastal flood event in Santa Monica are expected to range between about $4 million and $40 million. Note that these estimates are only for physical damage to the structures, and do not include the cost of damaged utilities or infrastructure. Furthermore, these estimates also do not include damage to the contents of the buildings or loss of inventory, loss of revenue, and the costs of relocation and/or rental while repairs to the damaged structures are done.

Stone Canyon Inundation Event
For the Stone Canyon dam failure scenario, HazUS estimates that about 214 buildings will be at least moderately damaged. This is over 67% of the total number of buildings in the region impacted by the scenario. This figure includes an estimated 23 buildings that will be completely destroyed.

Table 8-8 summarizes the expected damage by occupancy type, and Table 8-9 summarizes the expected building damage by building type. These tables show that residential structures comprise a large number and percentage of the total number of buildings impacted, with all of the “completely destroyed” buildings being residential. The impact to residential and commercial structures is also shown graphically on Figure 8-5.

Table 8-8: Expected Building Damage by Occupancy
as a Result of a Stone Canyon Dam Failure
Damage as a Percentage of the Total Structure
Occupancy
Type
1-10% 11-20% 21-30% 31-40% 41-50% >50%
Counts (%) Counts (%) Counts (%) Counts (%) Counts (%) Counts (%) Agriculture 0 0 0 0 0 0 0 0 0 0 0 0
Commercial 8 53 4 27 1 7 1 7 1 7 0 0
Education 0 0 0 0 0 0 0 0 0 0 0 0
Government 0 0 0 0 0 0 0 0 0 0 0 0
Industrial 8 80 2 20 0 0 0 0 0 0 0 0
Religion 0 0 0 0 0 0 0 0 0 0 0 0
Residential 33 14 79 33 53 22 30 13 20 8 23 10
Totals 49 85 43 31 21 23
Table 8-9: Expected Building Damage by Building Type
Damage as a Percentage of the Total Structure
Occupancy
Type
1-10% 11-20% 21-30% 31-40% 41-50% >50%
Counts (%) Counts (%) Counts (%) Counts (%) Counts (%) Counts (%) Concrete 7 70 3 30 0 0 0 0 0 0 0 0
Manufactured
Housing 0 0 0 0 0 0 0 0 0 0 0 0
Masonry 3 60 2 40 0 0 0 0 0 0 0 0
Steel 2 67 1 33 0 0 0 0 0 0 0 0
Wood 35 15 76 32 50 21 30 13 20 9 23 10
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City of Santa Monica, California
2025 Flood Hazards Page 8-43
Local Hazards Mitigation Plan Section 8
City of Santa Monica, California
2025 Flood Hazards Page 8-44
The HazUS-derived total economic loss in Santa Monica estimated for the Stone Canyon dam failure scenario is $227.17 million, which represents about 31.5% of the total replacement value of the scenario buildings. Building-related losses can be divided into two categories: direct building losses and business interruption losses. Direct building losses are the estimated costs to repair or replace the damage caused to the building and its contents. Business interruption losses are the losses associated with the inability to operate a business because of the damage sustained during the flood.

Business interruption losses also include the temporary living expenses for those people displaced from their homes because of the flood.

HazUS estimates that the total building-related losses amount to $120.27 million. Forty-seven percent (47%) of the estimated losses are related to business interruption. The residential occupancies make up 26.84% of the total loss. Table 8-10 provides a summary of the losses associated with damage to the buildings in the impacted zone.

Table 8-10: Building-Related Economic Loss Estimates Resulting From a Catastrophic Stone Canyon Dam Failure Scenario
Millions of Dollars
Category Area Residential Commercial Industrial Others Total Building Loss
Building 32.43 14.56 4.71 0.69 52.38
Content 18.52 33.47 7.74 4.40 64.13
Inventory 0.00 2.60 1.15 0.02 3.76
Subtotal 50.94 50.63 13.60 5.10 120.27
Business Interruption
Income 0.04 39.28 0.55 1.23 41.10
Relocation 6.42 10.33 0.69 0.66 18.11
Rental Income 3.46 7.78 0.18 0.07 11.49
Wage 0.11 31.20 0.95 3.95 36.20
Subtotal 10.03 88.59 2.37 5.91 106.90
All Total 60.97 139.21 15.97 11.01 227.17
These results are comparable to the estimates calculated using Crisis Track® summarized in Table 8-11 below, given that, as discussed previously, the Crisis Track® software only considers the value of the structures using tax data, and does not include the loss of contents, inventory, or business interruption.

Table 8-11: Estimated Economic Losses to Residential and Commercial Buildings in Santa Monica Within the Stone Canyon Dam Inundation Zone Using Crisis Track® Software Damage
Category
Affected
(Green Tag)
Minor
(Green Tag)
Major
(Yellow Tag)
Destroyed
(Red Tag)
Total Cost $9,306,377.20 $23,265,943.00 $55,838,263.20 $93,063,722.00 Essential Facility Damage
Given that neither of the two hospitals in Santa Monica are located within the flood zone or the potential inundation zone, all of the hospital beds available in the city will be available for use by Local Hazards Mitigation Plan Section 8
City of Santa Monica, California
2025 Flood Hazards Page 8-45
patients after either a 100-year or a dam inundation event. Significantly, however, in the case of the Stone Canyon dam failure, several regional hospitals, including several facilities on the UCLA Medical Center campus, are located within the anticipated inundation zone. It can be expected that the hospitals in Santa Monica would be asked to take on some of the extra patient load resulting from diversion of local patients, in addition to patients injured by the inundation event or the evacuation process itself. All other essential facilities in Santa Monica, including fire stations, emergency operation centers, and schools are expected to not be damaged and be fully operational after a Stone Canyon Dam failure. However, nearby inundation could limit access to the Santa Monica City Yards and the Fire Training Facility. Santa Monica would also be severely impacted by the evacuation and temporary or permanent relocation of several schools, fire stations, and LAPD’s police station in West Los Angeles.

Induced Flood Damage: Debris Generation
HazUS estimates the amount of debris that will be generated by a flood scenario. The model breaks debris into three general categories:
1) Finishes (dry wall, insulation, etc.),
2) Structural (wood, brick, etc.) and
3) Foundations (concrete slab, concrete block, rebar, etc.).

This distinction is made because of the different types of material handling equipment required to handle the debris.

The coastal flooding scenarios do not result in any significant amount of debris as a result of a 100- year flood event.

The HazUS model for the Stone Canyon dam failure estimates that a total of 2,250 tons of debris will be generated. Of this amount, “finishes” comprises 92% of the total, “structures” comprises 4% of the total, and “foundation” comprises the remaining 3%. If the debris tonnage is converted into an estimated number of truckloads, the model estimates that it will require 90 truckloads (@25tons/truck) to remove the debris generated by the inundation event.

Social Impact: Shelter Requirements
HazUS estimates the number of households that are expected to be displaced from their homes due to the flood and the associated potential evacuation. HazUS also estimates those displaced people that will require accommodations in temporary public shelters.

The model estimates that 2 households (or 6 people) will be displaced due to a 100-year flood.

Displacement includes households evacuated from within or very near to the flooded area. Of these, 0 people (out of a total population of 92,984) will seek temporary shelter in public shelters. Similarly, for a 500-year flood event, the model estimates 3 households (or 8 people) will be displaced due to the flood. Of these, 0 people will seek temporary shelter in public shelters. Significantly, the HazUS analysis also estimated that there would not be any injuries or loss of life as a result of the 100-year flood event.

The analysis for a Stone Canyon dam failure scenario estimates that 464 households (or 1,393 people) will be displaced due to the inundation. Of these, 59 people are anticipated to seek temporary shelter in public shelters. Since the flood waters from the hypothetically damaged reservoir are estimated to take at a minimum 60 minutes to reach the city of Santa Monica, the Local Hazards Mitigation Plan Section 8
City of Santa Monica, California
2025 Flood Hazards Page 8-46
model assumed that all people within the potential inundation area would evacuate in advance, and as a result, that there would be no casualties. This is likely to be an incorrect assumption, but realistic estimates of the number of injuries and deaths are not available.

Business and Industry
Storm-flooding events impact businesses by damaging property and by interrupting business. Flood events can cut off customer access to a business as well as close a business for repairs. Roof leaks can impact the contents; in extreme cases, leaks can cause damage to sensitive electrical equipment, with the potential to cause the affected business thousands of dollars in material losses and potential loss of revenue. A quick response to the needs of businesses affected by flood events can help a community maintain economic vitality in the face of flood damage. Responses to business damages can include funding to assist owners in elevating or relocating flood-prone business structures, and loans to make building improvements, such as new roofs. Given that commercial structures are located within the 100-year flood zone, business-related losses associated with damage to the structures and their contents or inventory, and business interruption losses associated with lost wages, loss of income, and relocation and rental income losses would be anticipated.

The HazUS model indicates that commercial properties in Santa Monica within the flood zone will experience minor damage as a result of a 100-year flood event, with anticipated losses amounting to about $4.6 million. This estimate is thought to be low, with most of the losses captured in this estimate related to building and contents damage.

Public Infrastructure
Publicly owned facilities are a key component of daily life for Santa Monica residents. Damage to public water and sewer systems, transportation networks, flood control facilities, emergency facilities, and offices can hinder the ability of the government to deliver services.

A review of past storm damage shows that extensive flooding of streets is not common in Santa Monica. Similarly, as mentioned above, Santa Monica’s City Hall and other essential service buildings are not expected to be impacted by flooding associated with a 100-year flood, or as a result of localized flooding resulting from storm drain obstructions. Nevertheless, based on historical records, the economic losses associated with the cleanup and repair of the flooded areas can be substantial. It is thus important for the City and other responsible agencies to conduct regular inspections of culverts and storm drains to remove debris that may obstruct the flow of water during storms. This should reduce the potential for urban flooding in most areas The City’s water infrastructure at 26th Street and Olympic Boulevard is not anticipated to be inundated if Stone Canyon dam failed. Similarly, the City’s facilities near the intersection of Texas Avenue and Bundy Drive are also not within the dam inundation area.

FLOOD MITIGATION ACTION ITEMS
Based on the information presented in the sections above, flooding issues in Santa Monica are considered minor, with the HazUS coastal scenarios run for the city suggesting limited damage as a result of a 100-flood event, with losses estimated at between $4 million and $40 million. Recent regional storms have shown that minor damage to structures and businesses can cost thousands if not millions of dollars to repair, so these estimates are thought to be realistic. Moreover, in most Local Hazards Mitigation Plan Section 8
City of Santa Monica, California
2025 Flood Hazards Page 8-47
cases, these loss estimates do not include lost revenue due to business interruption. Given that several of the structures in the 100-year flood zone in Santa Monica are public facilities that earn tax revenue for the City, damage to these structures would cost the City two-fold, consisting of both repair and/or replacement costs, and business interruption losses.

A HazUs analysis for Stone Canyon Dam estimates the total economic loss for the flood is $227.17 million, which represents 31.47 % of the total replacement value of the scenario buildings. The building losses for Stone Canyon Dam are broken into two categories: direct building losses and business interruption losses. The direct building losses are the estimated costs to repair or replace the damage caused to the building and its contents. The business interruption losses are the losses associated with the inability to operate a business because of the damage sustained during the flood.

Business interruption losses also include the temporary living expenses for those people displaced from their homes because of the flood. The total building-related losses are estimated at $120.27 million. 47% of the estimated losses were related to the business interruption of the region. The residential occupancies made up 26.84% of the total loss.

Flood mitigation activities listed here include current mitigation programs and activities that are being implemented by the city of Santa Monica, the County of Los Angeles, the U.S. Army Corps of Engineers, and other agencies and organizations.

The City of Santa Monica addresses its localized flooding problems as they arise, typically by deploying pumps to the impacted areas when a storm hits. However, these efforts have at times not been enough to prevent damage.

Main flood control services for the city of Santa Monica are provided by the City’s Public Works Department, with assistance as needed by the Los Angeles County Department of Public Works (LACDPW). Together with the U.S. Army Corps of Engineers, the LACDPW regulates flood levels by adjusting water flows upstream of flood-prone areas.

Because dam failure can have severe consequences, FEMA requires that all dam owners develop Emergency Action Plans (EAP) for warning, evacuation, and post-flood actions. Although there may be coordination with county officials in the development of the EAP, the responsibility for developing potential flood inundation maps and facilitation of emergency response is the responsibility of the dam owner. Dam owners are also required to prepare and submit emergency response plans to the State Office of Emergency Services, the lead State agency for the State dam inundation-mapping program. The flood inundation map prepared by Genterra Consultants, Inc. (2018) for Stone Canyon Dam, which is owned by the Los Angeles Department of Water & Power, was used for the dam inundation scenario described in this report. These documents, including dam inundation maps and emergency response plans are reviewed and updated at least annually to ensure that contact information is up to date. For additional information refer to the section on Inundation Due to Catastrophic Failure of Water Storage Structures.

Cities and counties are also required by State law to have in place emergency procedures for the evacuation and control of populated areas within the limits of dam inundation. In addition, legislation requires real estate disclosure upon sale or transfer of properties in the inundation area (AB 1195 Chapter 65, June 9, 1998; Natural Hazard Disclosure Statement).

Local Hazards Mitigation Plan Section 8
City of Santa Monica, California
2025 Flood Hazards Page 8-48

FLOOD RESOURCE DIRECTORY
The following resource directory lists resources and programs that can assist county communities and organizations. The resource directory provides contact information for county, regional, State and Federal programs that deal with natural hazards.

County Resources
Los Angeles County Public Works Department
900 S. Fremont Avenue, Alhambra, CA 91803
626-458-5100
https://dpw.lacounty.gov/
Sanitation District of Los Angeles County
1955 Workman Mill Road, Whittier, CA 90607
562-699-7411 x2301
https://www.lacsd.org/
State Resources
Governor’s Office of Emergency Services (Cal OES)
3650 Schriever Avenue, Mather, CA 95655
916-845-8911
www.caloes.ca.gov/
California Natural Resources Agency
715 P Street, Sacramento, CA 95814
916-653-5656
http://resources.ca.gov
California Department of Water Resources (DWR)
715 P Street, Sacramento, CA 95814
916-653-5791
https://water.ca.gov/
California Department of Conservation
715 P Street, Sacramento, CA 95814
916-322-1080
www.conservation.ca.gov
Federal Resources and Programs
Federal Emergency Management Agency (FEMA)
FEMA provides maps of flood hazard areas, various publications related to flood mitigation, funding for flood mitigation projects, and technical assistance. FEMA also operates the National Flood Insurance Program. FEMA's mission is to reduce loss of life and property and protect the nation’s critical infrastructure from all types of hazards through a comprehensive, risk-based, emergency management program of mitigation, preparedness, response and recovery.

Local Hazards Mitigation Plan Section 8
City of Santa Monica, California
2025 Flood Hazards Page 8-49
Federal Emergency Management Agency, Region IX
1111 Broadway, Suite 1200, Oakland, CA 94607
510-627-7100
www.fema.gov
Federal Emergency Management Agency (FEMA), Hazard Mitigation Planning California Governor’s Office of Emergency Services
3650 Shriever Avenue, Mather, California 95655
916-799-6499
www.fema.gov/emergency-managers/risk-management/hazard-mitigation-planning http://www.fema.gov/grants/mitigation/state-contacts
FEMA' s Community Status Book Report
This site provides information of California communities participating in the National Flood Program.
https://www.fema.gov/cis/CA.html
National Flood Insurance Program (NFIP)
In Southern California, many cities lie within flood zones as defined in FEMA Flood Maps. The City of Santa Monica is a community within a designated flood zone. As a result, flood insurance is available to citizens in the flood zone that adopt and implement NFIP building standards. The standards are applied to development that occurs within a delineated floodplain, a drainage hazard area, and properties within 250 feet of a floodplain boundary. These areas are depicted on federal Flood Insurance Rate Maps available through Los Angeles County.

P.O. Box 10055, Hyattsville, MD 20782-805
202-646-2500
https://www.fema.gov/flood-insurance
Other National Resources
Floodplain Management Association (FMA)
701 Pleasant Grove, Suite 180, Roseville, CA 95678
916-847-3778
https://floodplain.org/
The Association of State Floodplain Managers
The Association of State Floodplain Managers is an organization of professionals involved in floodplain management, flood hazard mitigation, the National Flood Insurance Program, and flood preparedness, warning, and recovery.
8301 Excelsior Drive, Madison, WI 53717
608-828-3000
http://www.floods.org
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City of Santa Monica, California
2025 Flood Hazards Page 8-50
National Weather Service
The National Weather Service provides flood watches, warnings, and informational statements. 520 North Elevar Street, Oxnard, CA 93030
805-988- 6610
https://www.weather.gov/lox
National Weather Service, Office of Water Prediction
The National Weather Service Office of Water Prediction offers information on floods and other aquatic disasters. This site offers current and historical data including an archive of past flood summaries, information on current hydrologic conditions, water supply outlooks, an Automated Local Flood Warning Systems Handbook, Natural Disaster Survey Reports, and other scientific publications on hydrology and flooding.

National Weather Service, Office of Hydrologic Development 1325 East West Highway, SSMC2
Silver Spring, MD 20910
https://www.weather.gov/owp
National Oceanic and Atmospheric Administration (NOAA)
1401 Constitution Avenue NW, Room 5128, Washington, DC 20230 202-482-6090
http://www.noaa.gov
NOAA's Mission: Science, Service, and Stewardship

1. To Understand and Predict Changes in Climate, Weather, and the Ocean and Coasts Science at NOAA is the systematic study of the structure and behavior of the ocean, atmosphere, and related ecosystems; integration of research and analysis; observations and monitoring; and environmental modeling. NOAA science includes discoveries and ever new understanding of the ocean and atmosphere, and the application of this understanding to such issues as the causes and consequences of climate change, the physical dynamics of high-impact weather events, the dynamics of complex ecosystems and biodiversity and the ability to model and predict the future states of these systems. Science provides the foundation and future promise of the service and stewardship elements of NOAA’s mission.

2. To Share That Knowledge and Information with Others
Service is the communication of NOAA’s research, data, information, and knowledge for use by the Nation’s businesses, communities, and people’s daily lives. NOAA services include climate predictions and projections; weather and water reports, forecasts, and warnings; nautical charts and navigational information; and the continuous delivery of a range of Earth observations and scientific data sets for use by public, private and academic sectors.

3. To Conserve and Manage Coastal and Marine Ecosystems and Resources Stewardship is NOAA’s direct use of its knowledge to protect people and the environment, as the Agency exercises its direct authority to regulate and sustain marine fisheries and their ecosystems, protect endangered marine and anadromous species, protect and restore habitats and ecosystems, conserve marine sanctuaries and other protected places, respond to environmental emergencies and aid in disaster recovery. The foundation of NOAA’s longstanding record of scientific, technical and organizational excellence is its people. NOAA’s diverse functions require an equally diverse set of skills and constantly evolving abilities in its workforce.

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City of Santa Monica, California
2025 Flood Hazards Page 8-51
National Resources Conservation Service (NRCS), US Department of Agriculture NRCS provides a suite of federal programs designed to assist state and local governments and landowners in mitigating the impacts of flood events. The Watershed Surveys and Planning Program and the Small Watershed Program provide technical and financial assistance to help participants solve natural resource and related economic problems on a watershed basis. The Wetlands Reserve Program and the Flood Risk Reduction Program provide financial incentives to landowners to put aside land that is either a wetland resource, or that experiences frequent flooding. The Emergency Watershed Protection Program (EWP) provides technical and financial assistance to clear debris from clogged waterways, restore vegetation, and stabilize riverbanks. The measures taken under EWP must be environmentally and economically sound and generally benefit more than one property. 3550 Harbor Blvd., Suite 2-202, Oxnard, CA 93035
202-720-7246
805-984-2358
http://www.nrcs.usda.gov/
U.S. Geological Services (USGS) Water Resources Division This agency provides current U.S. water news; extensive current (including real-time) and historical water data; numerous fact sheets and other publications; various technical resources; descriptions of ongoing water survey programs; local water information; and connections to other sources of water information.
12201 Sunrise Valley Drive, Reston, VA 20192
888-275-8747
http://ca.water.usgs.gov/
www.usgs.gov/mission-areas/water-resources
Bureau of Reclamation
The mission of the Bureau of Reclamation is to manage, develop, and protect water and related resources in an environmentally and economically sound manner in the interest of the American public. The Bureau provides leadership and technical expertise in water resources development and in the efficient use of water through initiatives including conservation, reuse, and research. It protects the public and the environment through the adequate maintenance and appropriate operation of Reclamation's facilities and manages Reclamation's facilities to fulfill water user contracts and protect and/or enhance conditions for fish, wildlife, land, and cultural resources.