How Earthquakes and Rising Seas Could Expand Flood Zones in the Pacific Northwest

What is it about?

A large (>Magnitude 8) earthquake on the Cascadia Subduction Zone could suddenly lower coastal land by up to 9 feet. This would cause floodplains to reach farther inland, putting more communities, infrastructure, and ecosystems at risk. We analyzed 24 estuaries from Washington to northern California and found that earthquake-driven land subsidence could expand flood zones by up to 115 square miles. Sea-level rise from climate change alone could add another 40 square miles. If both happen together, flood exposure could triple in some places. We also show that land may stay low for decades to centuries after the earthquake, meaning the increased flood risk could be long-lasting. Our findings suggest that current flood and tsunami maps may underestimate future risks. Planning for this combined hazard is critical for protecting people, services, and coastal ecosystems.

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Photo by Daniel Halseth on Unsplash

Photo by Daniel Halseth on Unsplash

Why is it important?

Critical services—like hospitals, roads, power stations, and wastewater treatment plants—could be permanently exposed to flooding. Farmlands could be damaged by saltwater, and wetlands that protect coasts from floods may disappear. Current flood and tsunami maps don’t fully account for these combined effects. To keep communities safe, we need to update hazard maps, strengthen infrastructure, and plan for a future where the coastline has changed.