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Study shows impact of Cascadia tsunami on Puget Sound communities

Seattle's Fenix Underground storefront after the 2001 Nisqually earthquake. (FEMA, Washington State Governor's Office)

Thanks to a new study from the Washington State Department of Natural Resources (DNR), we now have an idea of the potential impact of a tsunami on Puget Sound communities if there were a 9.0 magnitude earthquake along the Cascadia subduction zone.

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The study, prepared by geologists within the Washington Geological Survey division of DNR, was conducted to develop preparation and response plans for those in heavily populated areas that would be impacted by a Cascadia earthquake and tsunami.

“Millions of people live along the Salish Sea, and knowing what to expect is critical to creating response plans so that we can be ready when — not if — an inevitable Cascadia mega-quake hits,” said Hilary Franz, commissioner of public lands, in a written release. “It’s our duty to put the training and knowledge of our highly-trained geologists to work to make sure our communities can be prepared and safe.”

The study tracks the projected impacts from Blaine to Olympia of a 9.0 earthquake on the Cascadia subduction zone. The last Cascadia rupture was 321 years ago, DNR says, and experts estimate a 10-17% chance Washington experiences another rupture in the next 50 years.

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The first tsunami waves, according to the study, would reach Whidbey Island within 90 minutes of the earthquake, and more inland locations 2-4 hours after.

“Wave activity may inundate shorelines for some 14 hours, with strong currents remaining in inland waters for more than 24 hours,” DNR writes.

The inundation ranges from a high of 13 feet at the Vashon Island Ferry Terminal to six inches at the Port of Olympia. Seattle’s Harbor Island could see 3.7 feet of inundation, the Snohomish River Delta 5.7 feet, and the Port of Tacoma 3.5 feet.

“The tsunami would first arrive as a trough, with sea level gradually receding in all inner coastal waterway locations,” DNR reports.

The model does not account for tide stages or local tsunamis caused by earthquake-induced landslides, and the DNR says local crustal faults could be a larger hazard for some communities.

Find the new study and maps, as well as more tsunami information online here.

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