From: Gregory Dagget
To: Meredith Matthews; Stacy Atkins-Salazar; Alex Stillman; Kimberley White; Sarah Schaefer; City Manager"s Office; David Loya; Peter Lehman; Scott Davies; Abigail Strickland; Ashton Hamm; Dan Tangney; Matthew Simmons; Emily Sinkhorn Subject: Old Business, Adoption of the Comprehensive Update to the City’s Local Coastal Program Date: Sunday, October 12, 2025 8:13:39 PM
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Dear City Council, Planning Commission and City leadership, The LA Times‑reported story April 30th,2025 by Rong-Gong Lin, warns that a Cascadia “monster” earthquake could cause rapid land subsidence of up to several feet, greatly expanding coastal flood exposure and making earthquake-triggered inundation a near‑instant, long‑lasting hazard.Arcata’s LCP update process and related grant-funded vulnerability work address sea level rise and coastal hazards but emphasize gradual SLR scenarios and hydraulic modeling improvements tied to tides, waves, and stormwater; existing materials focus more on progressive inundation than sudden, seismic subsidence events.
* Explicit planning for earthquake‑triggered permanent subsidence and its compounding with SLR — The LCP update and SLR work discuss gradual sea level rise and hydraulic pathways but do not clearly treat abrupt, permanent land drop from a Cascadia event as a separate scenario with multi‑decadal consequences.
*Integrated hazard scenarios that combine earthquake subsidence + storm surge + future SLR — Current modeling improvements emphasize local hydraulics, waves, and rainfall but appear not to present combined worst‑case compound scenarios that the LA Times study highlights (e.g., immediate several‑foot subsidence plus long‑term higher baseline). *Critical infrastructure re‑location and lifeline redundancy tied to sudden subsidence — The City’s documents note wastewater facility planning and levee work, but there is no clear binding policy or trigger tied to seismic subsidence thresholds for retreat, mandatory relocation, or hard lifeline relocations (roads, wastewater, power, emergency access).
*Post‑quake permanent land‑use redesign guidance and zoning tools — LCP updates and public processes discuss coastal land use and hazard avoidance but lack prescriptive, post‑subsidence land‑use frameworks (e.g., immediate land‑classification changes, building moratoria, managed retreat corridors) that would be needed if broad areas become permanently lower and regularly inundated. *Rapid‑response mapping, public warning and evacuation protocols for immediate inundation after a major quake — Studies stress that earthquake subsidence happens within minutes, leaving little adaptation time; the LCP materials don’t clearly integrate near‑term emergency mapping, signage, or evacuation changes specifically for subsidence‑driven flooding. *Long‑term financing and legal strategy for large‑scale retreat/compensation — Grant funding supports assessments and modeling, but there is no detailed financing plan or legal strategy (eminent domain, buyouts, easements, infrastructure funding) articulated in the LCP materials for the much larger exposure the LA Times study indicates.
Short action checklist the Council can approve immediately • Insert a seismic‑subsidence scenario requirement into the LCP resolution and scope of work. • Direct staff to produce combined subsidence+SLR hazard maps for adoption into the LCP within the next modeling deliverable.
• Request a joint county/state coordination memo to identify funding options for large‑scale adaptation/retreat tied to a subsidence scenario.
Arcata’s LCP update process and related grant-funded vulnerability work address sea level rise and coastal hazards but emphasize gradual SLR scenarios and hydraulic modeling improvements tied to tides, waves, and stormwater; existing materials focus more on progressive inundation than sudden, seismic subsidence events.
Sincerely,
Gregory Daggett
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.
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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.
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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.
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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.
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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-changedriven 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.
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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 Humboldt and Arcata Bay during a 2021 king tide. | File photo by Tim Hanan.
USGS; Cal Poly Humboldt; the Oregon Department of Geology and Mineral Industries; the University of Hong Kong; and Nanyang Technological University in Singapore.
A Major Cascadia Zone
Earthquake Could Cause Much of
Coastal Humboldt to Rapidly Drop
Into the Sea, New Study Finds
ESPAÑOL
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Those of us who live in earthquake country tend to have two primary anxieties about the unsettling events: the shaking itself (collapsed roofs, crumbling buildings, etc.) and a tsunami (giant wall of water, ‘nuf said). However, a newly published
study invites us to adopt an altogether new earthquake-related fear: rapidly sinking land.
The study, published Monday in the Proceedings of the National Academy of Sciences (PNAS), found that a major earthquake along the Cascadia subduction zone will likely cause between 1.6 and 6.5 feet of sudden subsidence long the Washington, Oregon and northern California coasts.
This abrupt land sinkage would dramatically raise the relative sea level of coastal communities in a matter of seconds. In the worst-case scenario, the subsidence would more than double the flooding exposure of coastal residents, structures and roads. And those risks would remain long after the shaking settles. “This lesser talked about hazard is going to persist for decades or centuries after the earthquake,” the study’s lead author, Dr. Tina Dura, told NBC News. “The tsunami will come in and wash away and it’s going to have big impacts, don’t get me wrong, but the lasting change of the frequency of flooding … that’s going to have to be dealt with.”
Such damaging events have already happened elsewhere on the globe. Earthquake-driven coastal subsidence followed recent earthquakes in Chile, Alaska, Southeast Asia and Japan, resulting in such severe consequences as permanent land loss, widespread infrastructure damage and forced relocation.
These risks are exacerbated by climate-driven 21st century sealevel rise. In some communities along the coastal Washington, Oregon and northern California, the effects of sea-level rise are somewhat moderated by gradual coastal uplift. In fact, coastal uplift rates exceed the current rate of sea-level rise in such places as Astoria, Ore., Port Orford, Ore., and our neighbor to the north, Crescent City.
Floodplain map and bar graph depicting the expansion of the 1% floodplain (an area with a 1-in-100 chance of flooding each year) after earthquakedriven subsidence today (2023) and in 2100 when the earthquake-driven subsidence is amplified by climate-driven sea-level rise for Humboldt Bay. The bar graph shows 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. | Creative Commons license CC-BY-NC-ND 4.0.
Humboldt Bay residents aren’t so lucky. Here, the study’s authors note, “complex regional tectonics are causing gradual subsidence, resulting in the highest recorded Pacific-coast RSLR [relative sea-level rise] rate of 4.7 [millimeters per year].” In other words, the ground beneath our feet is slowly dropping as the water rises.
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The study defines a “major earthquake” as a magnitude 7.7 to
9.2, and while that may sound unlikely (knock wood), scientists were able to discover how often they occur by radiocarbondating plant fragments preserved within pre-earthquake peat or overlying mud. The results? There have been 11 such “great earthquakes” along Cascadia’s coasts over the past 6,000-7,000 years. They hit roughly every 200 to 800 years.
The most recent “great earthquake” along the Cascadia
subduction zone struck in 1700 — more than 300 years ago. The CSZ is a major offshore fault line running from Northern California to British Columbia.
The authors of the new study warns that coastal flood hazard analysis often overlooks the risk of sudden earthquake-triggered land subsidence.
“This study underscores the need to consider combined
earthquake and climate impacts in planning for coastal
resilience at the Cascadia subduction zone and globally,” it says.
The combined effects of earthquake subsidence and climatedriven sea-level rise could more than triple the flood exposure of residents, structures and roads in the next 75 years, the study found. The coastal areas at risk are home to airports, major highways, wastewater treatment facilities, businesses and homes.
County leaders recently completed a Sea-Level Rise Adaptation Plan for Humboldt Bay/Eureka Slough Area. It advocates “an incremental approach” to planning, using various “shorter-term actions” to reduce immediate risk and buy time for more ambitious “longer-term actions” to address future conditions. It does not appear to contemplate the possibility of long-term projections becoming a reality all at once.
Tsunamis along the Cascadia Subduction Zone can be triggered in a mega earthquake when the Juan de Fuca plate is rapidly pushed underneath the North American plate, displacing
massive amounts of water. That same movement can cause
coastal land to suddenly drop, according to Dura, an assistant professor in geosciences at Virginia Tech.
If that happens, she recently told the San Francisco
Chronicle, “All the sea-level rise you expected by 2300 is going to happen in minutes.”
You can download the study and read it for yourself via the link below. The study was funded by the Cascadia Region Earthquake Science Center, part of the National Science Foundation. ###
DOCUMENT: Increased flood exposure in the Pacific Northwest following earthquake-driven subsidence and