Earthquake simulations reveal hidden flood risks along Pacific Northwest coast

Virginia Tech researchers have identified a scenario in which tectonic activity and environmental changes converge to elevate flood threats across the Pacific Northwest. Their modeling points to a heightened danger if a major earthquake hits the Cascadia subduction zone while sea levels continue their upward trend, coastal areas in northern California, Oregon, and Washington could experience a significant increase in flood exposure.

A major earthquake along the Cascadia subduction zone could cause sudden land subsidence of up to 2 m (6.5 feet), allowing seawater to reach areas previously above flood thresholds. As a result, currently unaffected locations—including residential zones and transportation corridors—may become subject to regular or severe flooding.

The analysis focused on coastal regions with significant population and infrastructure density, including southern Washington, northern Oregon, and northern California. These areas, located near the Cascadia subduction zone, are particularly vulnerable to earthquake-induced subsidence and subsequent flooding.

To get a clearer picture, researchers conducted thousands of earthquake simulations. They studied how much the land could drop during a major seismic event and combined that with mapping tools to trace how flood zones might spread.

Their analysis covered 24 estuaries and coastal towns. Since no one knows when the next major quake will hit, they explored two timelines. In the first one, the event happens now. The second exhibit is set in the year 2100, when higher sea levels would make the flooding much more destructive.

Scientists also estimated that if a major quake happened now, more than 14 000 people and over 22 000 buildings would suddenly be inside areas at risk of flooding. Around 1 250 km (777 miles) of roads could also be affected, more than twice the current flood exposure. The impact wouldn’t stop there. Floodwaters could reach five airports, dozens of essential sites like hospitals, schools, police and fire stations, and key parts of the region’s infrastructure. Wastewater treatment plants, an electric substation, and over 50 sites that store or handle hazardous materials could also be at risk.

By the end of the century, sea levels along the Cascadia coastline are expected to rise by a considerable margin. According to climate projections, it can be somewhere around 90 cm (3 feet) above current levels. If a major earthquake were to strike by then, the combined effect of sinking land and higher seas could drastically increase flood risk. This will place far more homes, roads, and residents in danger than today.

To make matters worse, essential natural areas like wetlands, dunes, and beaches would face significant damage. These areas act as speed-breakers during storms, reducing the strength of waves and preventing erosion. Without them, homes and communities are left more exposed. Once these ecosystems are lost, recovery is unlikely, especially with limited space to move inland due to natural barriers and existing development.

Experts from the Fralin Life Sciences Institute pointed out that losing intertidal wetlands would hamper important natural functions, including water filtering, wildlife habitat, and carbon storage. They explained that these wetlands play a major role in trapping carbon, but once they’re worn away or turned into tidal flats, that ability drops sharply.

The Cascadia subduction zone, part of the Pacific “Ring of Fire,” is known for producing powerful earthquakes and volcanic activity. Despite its potential, it hasn’t seen a massive quake since January 1700, which is why studying the land’s past shifts is more important than ever right now to understanding future risks.

Studies from the Cascadia Region Earthquake Science Center, based at the University of Oregon, show physical signs of past land sinking caused by large earthquakes. Their work, supported by the National Science Foundation (NSF), brings together experts across disciplines to better understand the seismic risks along the Cascadia subduction zone.

By examining coastal sediments going back 6 000 to 7 000 years, the team found evidence of at least 11 major earthquakes, occurring every 200 to 800 years. The most recent event caused the coastline to suddenly drop by as much as 2 m (6.5 feet).

They pointed out that while the region isn’t highly urbanized, many estuaries have small communities located directly in areas most vulnerable to land subsidence. In their view, future quakes in Cascadia could cause more severe ground sinking than similar events elsewhere.

Researchers pointed to past quakes as a clear reminder of what can happen when the ground moves in places like Cascadia. In Chile, a massive earthquake turned farmland and pine forests into marshes. In Alaska, towns and airstrips had to be moved to higher ground. Indonesia lost parts of its coastline, and Japan’s disaster damaged ports and led to a nuclear emergency. These happenings show a pattern. For regions built near subduction zones, the risks are very real and very long-lasting. What’s happened in other parts of the world could just as easily happen here.

References:

¹ Increased flood exposure in the Pacific Northwest following earthquake-driven subsidence and sea-level rise – T. Dura,W. Chilton, et al. – Proceedings of the National Academy of Sciences – April 28, 2025 – DOI https://doi.org/10.1073/pnas.2424659122