SEATTLE, WA — About nine years ago, WSDOT released an unnerving video showing what would happen to the Alaskan Way Viaduct if a magnitude 7.0 earthquake hit the region. This week, geologists at the state Department of Natural Resources created a similarly scary video, although without all the death and destruction of the viaduct one.
The new state DNR simulations show how a tsunami would hit Washington in the event of a magnitude 9.0 earthquake along the Cascadia Subduction Zone. The worst of the tsunami would hit the Pacific Coast and San Juan Islands area — but the interior of Puget Sound between Edmonds, Tacoma, and Hoodsport could see waves 10 feet or higher, the simulation shows.
The good news is that residents in the Seattle-Tacoma area would have a long time to get out of the way. The simulation estimates that it would take about two hours for a wave to travel across the Salish Sea and down into central Puget Sound.
In the simulation, areas that get hit hardest in Puget Sound are highlighted in red. Locally, Tacoma's Commencement Bay and the southern ends of Vashon and Maury islands get hit with the worst of the waves, the simulation shows.
But the simulation isn't necessarily meant to be an exact gauge of how bad the tsunami will be. Corina Forson, DNR's chief hazards geologist, says the video is meant to give Washington residents a general idea of how one type of Cascadia Subduction Zone scenario would play out.
"These simulations really drive home how tsunami will impact all of Washington's coastlines," she said.
The waves will also slosh around inside Puget Sound for a long time. After about 3-1/2 hours, Commencement Bay will empty as water rushes to inundate Stanwood on Camano Island. The parts of the video in blue where water will fall below normal levels. During those times, boats, commercial ships, and ferries could be in danger of getting stranded.
The state also released individual videos for specific areas, including Bellingham and the San Juan Islands. Two hours after the quake, Bellingham Bay will empty, but 30 minutes later will get hit by a wall of water about 10 feet high, the video shows.
Tsunamis happen when a large amount of water gets disrupted by an event like an earthquake or landslide. The big waves can travel as fast as 500 mph, but typically slow to around 30 mph in shallow areas near land. Tsunamis are also unpredictable, and the first wave isn't always the most destructive.
The simulations released this week only show what happens up to 5-1/2 hours after the initial earthquake. Tsunami activity can continue for up to 24 hours after a quak, Corson said.
"A small nondestructive tsunami in one place may be very large and violent a few miles away. This is because coastal areas have different beach slopes and different offshore and coastal geographical features, such as reefs, bays, and river mouths," the state DNR describes on its tsunami hazards webpage.
The state has mapped some tsunami areas in Puget Sound. Luckily, many of the areas that will be hit hardest are not very residential, like Sodo in Seattle. You can check this state map to see if your neighborhood is at risk during a tsunami.
And one more thing to remember: the SR 99 tunnel that replaced the Alaskan Way Viaduct has been built to withstand a magnitude 9.0 earthquake, according to WSDOT.
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