New study details ‘startling’ effect large tsunami would have on Western Washington

A volcanic eruption halfway across the globe had the West Coast of the United States under a tsunami warning on Jan. 15. What local officials are more worried about, though, is the potential for a tsunami caused by an earthquake right here in the Puget Sound region.

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The Washington State Department of Natural Resources (DNR) recently published a study detailing the possible damage from a tsunami following a 9.0 Cascadia Subduction Zone earthquake, known more colloquially in seismology circles as “the Big One.” As DNR Chief Hazards Geologist Corinna Allen describes, the results of that study were “startling.”

“Following a large subduction zone earthquake, we expect that the first wave will arrive on the outer coast in the La Push area, about 10 minutes after the start of the earthquake,” Allen told Seattle’s Morning News on KIRO Newsradio. “The earthquake shaking could last anywhere from three to six minutes for one of these large subduction zone events, so that leaves very little time for evacuation in many coastal communities.”

Allen estimates that the earthquake could produce waves measuring up to 45 feet above tide levels. Once tsunamis reach land, water levels across beaches on the outer Olympic Peninsula could be as high as 100 feet, as well as 20 to 50 feet high in “many communities.”

“It’s an incredible amount of water in a very short amount of time,” she described.

Study shows impact of Cascadia tsunami on Puget Sound communities

In the event of such a tsunami, Allen recommends evacuating to higher ground if possible. If that’s not an option, “vertical evacuation is essential” for coastal communities.

That has many areas preparing vertical evacuation structures, operating as towers that can be built on top of existing structures, like parking garages, hotels, or schools. The Washington Emergency Management Division believes the state needs at least 50 such structures — and as many as 85 — in order to provide vertical evacuation options to residents in at-risk communities.

That said, Allen also points out that this represents “our worst case scenario.”

“It is startling, and it is scary,” she said. “But it is more likely that we are going to experience a smaller tsunami from a distant source earthquake event, like the 1964 Alaska earthquake.”

“However, we do have a number of earthquake sources in Washington that could cause significant tsunamis, so trying to understand those hazards is critical for being able to plan for them,” she added.

You can learn more about the DNR’s research on tsunami hazards at this link.

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