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Increased Flood Risks in the Pacific Northwest Due to Earthquake-Driven Subsidence and Rising Sea Levels

The Pacific Northwest faces significant threats from potential seismic activity and climate change. Recent research from Virginia Tech highlights the dual hazard of powerful earthquakes coupled with rising sea levels, which could lead to augmented flood risks that would affect thousands in northern California, Oregon, and Washington.

A study published in the Proceedings of the National Academy of Sciences reveals that a major earthquake could cause coastal land to subsist by as much as 6.5 feet. This phenomenon could expand the federally designated 1% coastal floodplain—areas with a 1-in-100 chance of flooding in any given year—by between 35 to 116 square miles.

Lead researcher Tina Dura, an assistant professor of geosciences, emphasized that this is the first quantitative assessment of how a Cascadia subduction zone earthquake might influence land use and complicate recovery timelines in affected areas.

The study indicates that southern Washington, northern Oregon, and northern California—regionally populated zones—would experience the most significant repercussions. Utilizing thousands of earthquake simulations, Dura’s team evaluated the anticipated land subsidence from an impending large-scale earthquake in the Cascadia subduction zone.

By applying geospatial analytics, the researchers assessed how the floodplain would increase across 24 estuaries and communities in the region, considering scenarios where an earthquake could occur today or by the year 2100, when climate change could exacerbate sea-level rise.

Following an earthquake today, an estimated additional 14,350 residents, 22,500 structures, and 777 miles of roads would fall within the newly expanded floodplain, effectively more than doubling flood exposure.

Critical infrastructure such as five airports, 18 essential facilities—including schools and hospitals—eight wastewater treatment plants, and various potential contaminant sources would also be at risk.

Looking ahead to 2100, projections from the Intergovernmental Panel on Climate Change (IPCC) suggest that sea levels could rise by up to 3 feet along the Cascadia subduction zone. This rise would significantly magnify the flood risks associated with earthquake-driven ground subsidence, potentially tripling the exposure for residents and infrastructure.

“The immediate aftermath of an earthquake, coupled with rising sea levels, could severely delay recovery efforts due to compromised infrastructure,” Dura noted. “Long-term consequences might render many coastal communities uninhabitable.”

Low-lying areas currently utilized for agriculture, such as grazing and farming, could incur substantial economic damage from increased inundation. This would likely lead to soil salinization, making agricultural land unusable. Additionally, the erosion of vital natural ecosystems like coastal estuaries could worsen, diminishing their ability to act as protective buffers against storm surges and wave energy.

Dura warns that the loss of these ecosystems may be irreversible, especially with physical barriers like topography and human development limiting migration inland.

The research team at the Cascadia Region Earthquake Science Center (CRESCENT) is cataloging geological evidence of past earthquakes to better understand subsidence risks. Their findings indicate that approximately every 200 to 800 years, significant earthquakes have occurred in the Pacific Northwest, the last of which resulted in a noticeable coastal land drop between 1.5 to 6.5 feet.

“Cascadia presents a unique situation,” Dura remarked. “While it’s not heavily populated overall, the communities located in estuaries are directly within the subsidence zone, which could have dramatic implications during and after future seismic events.”

Global Context of Subduction Zones

Subduction zones are prevalent around the world, including regions off the coasts of Alaska, Japan, and South America, where tectonic plates interact. These geological activities create significant pressure that, when released, can result in powerful earthquakes and tsunamis. Following an earthquake, flooding can occur almost immediately due to the sudden subsidence of the land.

The cascading impacts of these earthquakes extend globally, as exemplified by notable historical earthquakes that have reshaped landscapes and affected communities. Such events underscore the necessity for robust hazard assessments and effective mitigation strategies that can be beneficial in tectonically active regions worldwide.

More Information: Dura, Tina, “Increased flood exposure in the Pacific Northwest following earthquake-driven subsidence and sea-level rise,” Proceedings of the National Academy of Sciences (2025). DOI: 10.1073/pnas.2424659122.

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phys.org