Cascadia subduction zone earthquake could be even worse than feared
When an earthquake rips along the Cascadia Subduction Zone fault, much of the U.S. West Coast could shake violently for five minutes, and tsunami waves as tall as 100 feet could barrel toward shore. But that’s just the start of the expected horrors.
Even if coastal towns in Northern California, Oregon and Washington withstand that seismic onslaught, new research suggests, floodwaters could seep into many of these vulnerable communities for good. That’s because entire coastal shorelines are expected to drop by as much as 6½ feet when the earthquake strikes, according to new research published Monday in the Proceedings of the National Academy of Sciences.
Researchers analyzed seismic and flood modeling to produce some of the most detailed estimates of how the Cascadia earthquake would drop — or subside — coastal shorelines and found that it could affect more than double the number of people, structures and roads currently at risk. The effect will also worsen over time, as intensifying climate change raises sea levels further.
“This lesser talked about hazard is going to persist for decades or centuries after the earthquake,” said Tina Dura, the study’s lead author. “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.”
Dura said geologic fossil evidence shows that previous Cascadia earthquakes immediately dropped the land level and turned dry ground into tidal mudflats in estuaries along the Pacific Northwest.“That’s going to happen again and we’ve built up a lot of those areas,” said Dura, who is an assistant professor of geosciences at Virginia Tech. “That’s how we have ports there ... and that’s where we built towns, and all that area is gonna drop down maybe over a meter, up to two meters.”
The Cascadia Subduction Zone fault, which runs offshore along North America’s West Coast from Northern California to northern Vancouver Island, represents a looming threat. The fault has the capability to produce a magnitude-9.0 earthquake, and a large temblor is expected there at least once every 450-500 years, on average. The last of those major quakes occurred in 1700.
When the fault rips, experts have said, it will precipitate the worst natural disaster in the nation’s history. A 2022 state and federal planning exercise for a Cascadia earthquake predicted about 14,000 fatalities, more than 100,000 injuries and the collapse of about 620,000 buildings in the Pacific Northwest, including 100 hospitals and 2,000 schools.
The new research suggests that coastal planners must seriously reckon not only for the threat of intense shaking and tsunami waves, but also for the long-term reshaping and rapid sinking of the coastline itself.
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