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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 sea-level 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 earthquake-driven subsidence and sea-level rise.