From: Gregory Dagge
To: Meredith Matthews; Stacy Atkins-Salazar; Alex Stillman; Kimberley White; Sarah Schaefer; City Manager"s Office; David Loya; Emily Sinkhorn; Rhea Varley; Engineering Dept; Tabatha Miller; Jennifer Dart Subject: Initiate Adoption of the Comprehensive Update to the City’s Local Coastal Program Date: Tuesday, July 15, 2025 9:42:32 PM
Attachments: image.png
CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you recognize the sender and know the content is safe.
Dear City Council,
Here is information for staff for the City's Local Coastal Program on Sea Level Rise Policy Guidance update to the California Coastal Act, State of California Sea Level Rise Guidance.
Sincerely,
Gregory Daggett
The California Coastal Commission Sea Level Rise Policy Guidance was updated in 2024 to address the three main topics summarized here. Additional information on the changes in the 2024 SLR Policy Guidance Update can be found in the November staff report.
1. New best available science: In June 2024, OPC adopted its most recent update to the State of California Sea Level Rise Guidance, which reflects the previous five years of evolving scientific research on sea level rise. Updates to the Coastal Commission SLR Policy Guidance reflect this best available science, with key changes including an updated discussion of sea level rise science and new tables of anticipated sea level rise amounts.
2. SB 272 (Laird, 2023): This legislation requires local governments to develop sea level rise adaptation plans as part of new or updated LCPs. The updates in the Guidance related to SB 272 are intended to fulfill the legislation’s requirement for the Coastal Commission to establish guidelines for the preparation of those plans. A new section summarizing the language and intent of SB 272 has been added, along with a discussion regarding both the requirements for consistency with SB 272 and guidance and best practices for this type of planning work. A box at the end of Chapter 5 summarizes and highlights the minimum requirements for each of the six sea level rise plan components required by the bill.
3. Environmental Justice: Updates have been made to better integrate environmental justice considerations in the Guidance. These updates build on the Commission’s Environmental Justice Policy (2019) to better inform regulatory and planning processes to address sea level rise through an environmental justice lens. Key changes include the addition of a new environmental justice-focused guiding principle and an expanded discussion on how sea level rise disproportionately impacts environmental justice communities and the importance of meaningful engagement with these communities when conducting sea level rise planning work. Steps related to identifying and meaningfully engaging with environmental justice communities in and around the local jurisdiction or project site were also added to the processes for addressing sea level rise in LCPs and CDPs
CALIFORNIA COASTAL ACT
Chapter 3 - COASTAL RESOURCES PLANNING AND MANAGEMENT POLICIES Article 8 – Sea Level Rise
Section 30270 - Consideration of the effects of sea level rise The commission shall take into account the effects of sea level rise in coastal resources planning and management policies and activities in order to identify, assess, and, to the extent feasible, avoid and mitigate the adverse effects of sea level rise.
Ca. Pub. Res. Code § 30270
Added by Stats 2021 ch 236 (SB 1),s 2, eff. 1/1/2022.
By Pacific Coastal and Marine Science Center
May 5, 2025
Along the Pacific Northwest coast, scientists have long warned of a looming threat: a massive earthquake from the Cascadia Subduction Zone, capable of triggering tsunamis and devastating shaking. Now, new research highlights another, often-overlooked danger—the sudden sinking of the land itself and the longer-term threats posed by coastal flooding.
Media
Floodplain maps and bar graphs depicting the expansion of floodplain after earthquake driven subsidence Sources/Usage: Public Domain. View Media Details
Floodplain maps and bar graphs depicting the expansion of the 1% floodplain after earthquake-driven subsidence today (2023) and in 2100 when the earthquake-driven subsidence is amplified by climate-driven sea-level rise for the (A) Necanicum River, OR; (B) Yaquina Bay, OR; (C) Alsea Bay, OR; and (D) Humboldt Bay, CA. Bar graphs to the Right of each map set show the amount of land area, number of residents, structures, roads, and different land-use types in the 1% floodplain following earthquake-driven subsidence today (2023) and in 2100, when the effects of earthquake-driven subsidence are amplified by climate-driven sea-level rise.
A study authored by a team of scientists including USGS shows that a magnitude 8 or greater earthquake today along the Cascadia megathrust fault—stretching from northern California to Vancouver Island, British Columbia—could cause 0.5 to 2 meters (1.6 to 6.6 feet) of sudden land subsidence, instantly raising relative local sea levels and more than doubling the number of people, buildings, and roads exposed to coastal flooding.
More Than Shaking: The Coastal Sinking Effect
When large subduction zone earthquakes strike, Earth’s crust adjusts to the release of accumulated tectonic stress by deforming: uplifting offshore and subsiding along the coast. Sudden subsidence in coastal regions may persist for decades or more. Geologic evidence of sudden subsidence in Cascadia estuaries has been well documented by paleoseismologists, including quantitative estimates from the last great earthquake in 1700 CE. Using earthquake deformation models based on estimates of coastal subsidence during the 1700 earthquake, the research team simulated the flooding impacts of such a scenario using geospatial analysis. The results show that in many coastal communities along Cascadia, the land would sink enough to cause instant relative sea-level rise. Coastal floodplains would expand, and areas previously considered high and dry could become inundated within minutes.
A Growing Risk by 2100
The threat becomes more severe when combined with projected climate-driven sea-level rise. By the year 2100, the researchers found, a Cascadia earthquake could more than triple current flood exposure, affecting a dramatically larger swath of infrastructure and population.
That’s because the same amount of subsidence that doubles flood risk today would push even more low-lying areas below future sea levels. This compounding effect makes earthquake deformation a critical but underrecognized factor in long-term coastal planning.
Planning for a Dual Threat
This research demonstrates that earthquake and climate impacts are not separate risks—they are interacting forces that must be addressed together. Ignoring the effects of land subsidence in flood models paints an incomplete picture of future risk.
The study notes that coastal planners and emergency managers should incorporate earthquake-induced subsidence into flood hazard assessments, building codes, and evacuation strategies. Many current risk maps and resilience plans account for sealevel rise or tsunami scenarios, but not both in combination with the lasting effects of earthquake subsidence along Cascadia coastlines.
Implications Beyond Cascadia
While the Cascadia subduction zone is a unique and well-studied region, the findings have global implications. Other coastal areas near subduction zones—such as Chile, Indonesia, and Japan—face similar risks from megathrust earthquakes that could alter land elevation and magnify flooding threats in a warming world.
Read the study: Increased flood exposure in the Pacific Northwest following earthquakedriven subsidence and sea-level rise.
Monster earthquake could sink swath of California,
dramatically heightening flood risk, study says
Waves on a beach and small fire in the distance.
A beach campfire provides warmth at sunset on the Samoa Peninsula in Eureka on Sept. 2, 2022. (Myung J. Chun / Los Angeles Times)
By Rong-Gong Lin II
Staff Writer
April 30, 2025 4 AM PT
During an earthquake scenario with the highest level of subsidence, or land sink, the area at risk of flooding would expand by 116 square miles.
Among the areas at risk after a megaquake along the Cascadia subduction zone are the Humboldt Bay region, including the area around Eureka and Arcata.
A long-feared monster earthquake off California, Oregon and Washington could cause some coastal areas to sink by more than 6 feet, dramatically heightening the risk of flooding and radically reshaping the region with little to no warning.
Those are the findings of a new study that examined the repercussions of a massive earthquake on the Cascadia subduction zone, which stretches from Northern California up to Canada’s Vancouver Island.
The study, published Monday in the journal Proceedings of the National Academy of Sciences, concluded that in an earthquake scenario with the highest level of subsidence, or land sink, the area at risk of flooding would expand by 116 square miles, a swath that’s 2½ times the size of San Francisco.
Such a scenario would more than double “the flooding exposure of residents, structures and roads,” and officials would need to contend with a future of “compromised roadways and bridges,” as well as lifelines and infrastructure that are either more frequently flooded or permanently inundated, the study’s authors wrote.
CALIFORNIA
These California coastal cities face heightened flood danger from tsunami, data show
March 18, 2025
In other words, a powerful earthquake in this area would risk “drastically altering shorelines and causing profound, lasting impacts to coastal populations, infrastructure, and ecosystems,” the study said. Unlike relative sea-level rise that’s driven more gradually by climate change, a rise resulting from a major earthquake “will happen within minutes, leaving no time for adaptation or mitigation.” The last megaquake on the Cascadia subduction zone, registering a magnitude 9, occurred in 1700. Based on archaeological evidence, villages sank and had to be abandoned, according to the U.S. Geological Survey.
From California’s North Coast to Washington state, scientists say, the next great earthquake — magnitude 8 or higher — could cause land to sink by 1.6 to 6.6 feet, the same range seen during the 1700 earthquake.
Currently, more than 8,000 people live in flood plain areas along estuaries in the Cascadia coastal region. But in the event of a high level of subsidence after an earthquake, that figure would nearly triple to more than 22,000, the study calculates.
Nearly 36,000 structures would be threatened by the resulting quake-influenced flood plain, a 168% increase from the current figure of about 13,000.
CALIFORNIA
The California tsunami danger is real. The 7.0 earthquake is wake-up call to prepare
Dec. 6, 2024
And an additional 777 miles of roadway would be in the new flood plain, nearly tripling the at-risk total to 1,212 miles of road.
The flood plain is defined as areas that have at least a 1% chance of flooding each year, which is considered to be “high risk,” according to the Federal Emergency Management Agency. These are areas that have at least a 1-in-4 chance of flooding during a 30-year mortgage. Flood insurance must be purchased by home and business owners in high-risk areas with mortgages from federally regulated or insured lenders.
Radiocarbon dating suggests there have been more than 11 great earthquakes off the shore of California’s North Coast, Oregon and Washington state over the last 6,000 to 7,000 years — recurring every 200 to 800 years, the study said.
“Gradual climate-driven sea-level rise is not the only inundation threat,” the study said. “Coastal subsidence from the next great [Cascadia subduction zone] earthquake may produce” more than 3 feet of sudden relative sea-level rise sooner than otherwise expected.
CALIFORNIA
A potent threat of major earthquake off California’s northern coast March 12, 2014
When discussing a future megaquake on the Cascadia subduction zone, “We often hear about the tsunami and the shaking. But there’s the subsidence that’s going to persist — for decades to centuries —after the earthquake, and just totally alter the flood plains,” Tina Dura, assistant professor of geosciences at Virginia Tech and the study’s lead author, said in an interview.
One estimate published by FEMA is that a magnitude 9 earthquake along the full length of the 800-mile fault zone would leave 5,800 dead from the earthquake alone. An additional 8,000 would die from the resulting tsunami that could rise as high as 80 feet and offer coastal areas as few as 10 minutes of warning. Total economic losses could hit $134 billion.
In the 1700 Cascadia megaquake, oral histories describe tsunamis more than 50 feet high wiping out coastal villages. In Anacla, a village on what is now called Vancouver Island, only 1 out of more than 600 people survived, according to the U.S. Geological Survey.
The tsunami was so strong that it uprooted trees. When it finally receded, treetops were found strewn with trash and victims’ limbs. “Ghost forests” of rot-resistant trees found in tidal marshes and estuaries in the region are evidence that land sank during the quake and drowned the trees, the USGS said.
The results of the latest study should be a wake-up call to residents and government officials considering a postquake response, scientists say. There already are parts of U.S. Route 101 that routinely flood during exceptionally high king tides, Dura said, “and places like that could be kind of a hot spot for seeing flooding right away after the subsidence” following a megaearthquake.
Another factor officials should consider is whether crucial infrastructure, such as airports, would fall within the quake-expanded flood plain, scientists say.
CALIFORNIA
These are some of the major earthquake risks facing California June 11, 2021
Authorities may also want to consider avoiding building infrastructure such as schools, fire stations and wastewater treatment plants in areas “that we’ve shown are likely to become in the flood plain,” Dura said.
Notable areas at risk of land sinking after a megaquake along the Cascadia subduction zone, according to the study, include:
California’s Humboldt Bay region, including the area around Eureka and Arcata.
In Oregon, Waldport and Bayshore along Alsea Bay; Newport and South Beach along Yaquina Bay; and Gearhart and Seaside along the Necanicum River.
In Washington, the Willapa Bay area, including the communities of Long Beach, Ocean Park, Tokeland and Raymond; and the Grays Harbor area, including Ocean Shores, Westport and Aberdeen.
Traditionally, scientists and government officials have focused on climate-change-driven sea-level rise to calculate the projected increased risk of coastal flooding. But the study suggests that neglecting the role of major earthquakes would be shortsighted.
“Earthquake-driven coastal subsidence following recent historical earthquakes has had severe consequences for communities, leading to permanent land loss, infrastructure damage and forced relocation,” the study said.
CALIFORNIA
String of earthquakes rattles L.A.: Are they telling us something bigger? June 10, 2024
One example was the magnitude 9.1 earthquake off Japan’s east coast in 2011, which caused some land to sink by up to 3 feet. In one area of the town of Ishinomaki, the sinking land forced people to contend with regular flooding, according to news reports.
Another magnitude 9.1 earthquake that struck near Sumatra, Indonesia, in 2004 caused land subsidence of up to 3 feet. Areas used for aquaculture have since suffered chronic tidal flooding, leading to oversalinization, and land has been lost, the study said.
The magnitude 9.2 earthquake in Alaska in1964 caused land to sink by more than 6 feet along the coast, “rendering roads, docks and waterfront areas uninhabitable, in some cases necessitating relocation of communities to higher ground or raising waterfront facilities and airstrips above high tide,” the study said. In some places, the subsidence was even more severe.
A broken street.
Before the magnitude 9.2 earthquake of 1964 in Alaska, the sidewalk in front of stores on the right-hand side of this photo from Anchorage were level with those on the left. (U.S. Geological Survey )
And a magnitude 9.5 earthquake in Chile in 1960 caused up to 8 feet of coastal subsidence, “permanently submerging coastal pine forests and farms and converting them to intertidal marshes, and flooding coastal towns and forcing residents to abandon homes,” the study said.
Besides Dura, there are 19 other co-authors to the study, with affiliations including Singhofen Halff Associates of Orlando, Fla.; the University of Oregon; Rowan University; the University of North Carolina Wilmington; Durham University; the USGS; Cal Poly Humboldt; the Oregon Department of Geology and Mineral Industries; the University of Hong Kong; and Nanyang Technological University in Singapore.
Lori Dengler | How a great Cascadia
earthquake might remake the Pacific
Northwest coastline
A “ghost forest” of Sitka spruce trees near Neskowin in Tillamook County, Oregon, was submerged over 2,000 years ago in a past Cascadia earthquake. (Photo by Rob Degraff/Contributed).
This week marks a decade since I retired from teaching at is what is now called Cal Poly Humboldt. My final 106 Earthquake Country lecture was on May 7, 2015. That lecture included a summary of the earthquakes that had occurred during the term. The largest and deadliest that semester was a M7.8 in Nepal and a wrap on earthquake hazards.
I organized hazards in categories according to primary causes: shaking due to the passage of seismic waves, fault rupture displacement land on either side of the fault, and land elevation changes producing permanent raising or lowering of the ground. While each was the result of fault slip, the underlying physics in each case is different — seismic waves are triggered by the “kick” the fault rupture gives to the ground, fault displacement can move the ground on opposite sides of the fault many tens of feet for great quakes, and elastic rebound of stressed ground can produce noticeable changes in elevation.
My 106 students knew about ground shaking as almost all had felt a quake before entering my classroom. Most had some idea of fault slip although often with erroneous ideas of faults swallowing people or dropping California into the ocean. Almost no one was aware of how land levels could change both during earthquakes and in the long intervals between them. In the past two decades, as public awareness of a great earthquake on the Cascadia subduction zone has grown, there has been much outreach on shaking hazards and tsunami potential but little focus on how the land might change and its implications.
Two weeks ago, a paper was published in the Proceedings of the National Academy of Sciences about the likely scale of Cascadia land level changes and how earthquake-produced subsidence needs to be included in sea-level rise discussions and coastal planning. It was a large team effort led by Tina Dura of Virginia Tech, who I worked with when she was a postdoc at Humboldt. The paper quantifies how the areas vulnerable to coastal flooding are likely to expand in the Cascadia region from North Coast California to Washington and exacerbate climatechange-induced rising seas. It has garnered much media attention but also some confusion about the implications.
Abrupt earthquake-triggered land changes are not a new idea. The earliest written accounts of great earthquakes causing the land to rise or fall are from China and date back to nearly 4,000 years ago. Oral histories of people around the globe contain earthquake stories and some describe how they cause the land to change. Some of these are from the land you might be currently living on.
Deborah Carver, good friend and wife of geologist Gary Carver, spent decades researching the Wiyot, Yurok, and Tolowa earthquake/tsunami stories. She found a Humboldt Times article published in 1855 stating, “The Bay Indians have a tradition that this bay was produced by an earthquake which swallowed up the land, destroyed a large and powerful Indian tribe — only a few escaping.” She found published and unpublished stories from UC Berkeley anthropologists Goddard, Waterman, and Kroeber collected in the early 20th century (stories in link below).
The Yurok stories of Earthquake and Thunder, who ran up and down the coast stomping the ground and allowing the ocean to come in, were first thought to be of mythic and cultural relevance, but not descriptions of actual events. But physical evidence of repeated downward jerks of Humboldt Bay and a better understanding of the earthquake cycle changed that assessment. The land can drop suddenly in earthquakes and in shallow places like Humboldt Bay, a few feet of subsidence will greatly expand its area.
To understand why the ground drops or rises in great subduction zone earthquakes like Cascadia, think elasticity. Stretch a rubber band or other elastic material and let go. It returns to its original shape. All that deformation was just stored in the material. Crustal rocks are likewise elastic to some degree. If you squeeze or stretch them over decades or centuries, they store that energy just like a rubber band. An earthquake suddenly releases the pull or push, and the ground can quickly snap back to its previous shape.
In subduction zones like Cascadia, the squeezing lasts for a very long time. Gravity is constantly pulling the subducting slab downwards. This pull is resisted by friction that is locking the subducting slab to the overlying plate. In most of Cascadia, it’s the Juan de Fuca plate being pulled down and to the east, and Oregon and Washington that are stuck on top. That pull can be measured — slowly shifting the surface upwards and in an east-northeast direction. It takes very sensitive instruments to measure the distortion, the amplitude is on the order of an inch or two per century. But the area being squeezed is very large and when the locked zone is suddenly broken in a great earthquake, that squeezed zone suddenly relaxes, causing the land to drop.
The classic example of this subsidence is the 1964 M9.2 Alaska earthquake. George Plafker spent months mapping coastal changes in detail after the earthquake and documented for the first time the regional elevation changes a great earthquake can produce (see Not My Fault 5/24/2017 for more detail on Plafker’s work). An area roughly 500 miles long and 300 miles wide had changed in height. Areas more than 80 miles from the continental shelf and edge of the subduction zone had dropped and areas closer to the rupture had risen.
Many people have worked on this pattern of uplift and subsidence in great quakes since 1964. I saw firsthand the effects in 2005 when I studied the 2004 Andamans-Sumatra earthquake and again in 2010 and 2011, working in Chile and Japan. Each earthquake is unique, and the areas of uplift and subsidence depend on the size of the earthquake, the orientation of the fault rupture, and the distance from the coast. The coastal areas I visited had dropped between 1 and 6 feet, causing additional damage to infrastructure and exacerbating tsunami impacts.
Dura’s group focuses mainly on post-quake earthquake subsidence and how it contributes to sea level rise. This makes sense as all of the Oregon and Washington coasts are far enough away from the continental shelf and the rupture tip to be in the down-dropped areas. Her team examined paleoseismic data from coastal wetlands and the majority of data points suggest roughly 6 feet of coseismic subsidence. There are a few sites in Oregon with values as low as
1.5 feet and one near the mouth of the Columbia River that is nearly 9.
The picture becomes more complicated in our part of Cascadia, south of the Oregon border, because we are closer to the continental shelf and where the Cascadia megathrust fault breaks the seafloor surface. Plafker found that the zone closer to the tip of the fault generally uplifts during great quakes. He found this not only in Alaska, but in his re-examination of the even larger M9.5 Chile quake in 1960. But the pattern isn’t smooth because it is influenced by secondary faults which may also rupture in a great earthquake.
Such is likely the case in Humboldt County, where splay faults off of the subduction zone are exposed on land. Faults like the Little Salmon that run along the base of Humboldt Hill, the Fickle Hill fault in Arcata, and the Mad River fault zone from McKinleyville to Trinidad might all influence what goes up and what does down in our area. The general picture in the Humboldt Bay area from both coring and the Yurok and Wiyot oral history is subsidence on the order of one to three feet. The Cape Mendocino area, where the subduction zone breaks the sea floor very close to the coast, shows repeated evidence of uplift. But uplift is generally not wellpreserved in the geologic record.
I agree with Tima and colleagues that coseismic land level changes deserve higher planning priorities, not only for sea level rise planning, but also in short-term earthquake response planning. County and city planners need to take earthquake land level changes into consideration in the siting of future development and zoning decisions. In Humboldt County, we can’t predict exactly what will go up or down or by how much, but I am sure the result will be disruption of roads, our port, and other infrastructure that will likely take months to years to repair.
Note: the paper “Increased flood exposure in the Pacific Northwest following earthquake-driven subsidence and sea-level rise” can be read at
https://www.pnas.org/doi/10.1073/pnas.2424659122. You can access Deborah Carver’s compilation of North Coast native earthquake stories at https://npshistory.com/publications/redw/native-stories-earthquakes-tsunamis.pdf.