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Simulation parameters for the scenario that generated the most shaking for the Seattle area involved an off-shore hypocenter. (Flickr Image via UW News)

“None of the pictures is rosy,” says a line in a new report from the University of Washington detailing a research project that simulated 50 different ways that a magnitude-9.0 earthquake on the Cascadia subduction zone could unfold.

It’s the understatement of the year if you live in fear of the “Really Big One” and what kind of destruction such a major earthquake could cause in the Pacific Northwest. But there are scenarios in which such an event — which last happened in 1700 — could be labeled best-case, depending on where you live and where the epicenter is located.

Of course, worst-case is also a possibility and the story in UW News only needs to use such phrases as “sediment-filled basins like downtown Seattle” to really drive that point home.

The project, to be presented Tuesday in Seattle at the annual meeting of the Geological Society of America, was led by Erin Wirth, a UW postdoctoral researcher in Earth and space sciences. The science is ultimately aimed at helping people prepare and providing engineers with data to assess how tall buildings might respond to a predicted pattern of shaking.

“There had been just a handful of detailed simulations of a magnitude-9 Cascadia earthquake, and it was hard to know if they were showing the full range,” Wirth said. “With just a few simulations you didn’t know if you were seeing a best-case, a worst-case or an average scenario. This project has really allowed us to be more confident in saying that we’re seeing the full range of possibilities.”

Watch this Facebook video to see seismic simulations for strong shaking in Seattle and a “better-case” scenario:

How will a 9.0 Cascadia earthquake affect Seattle?

We know the "really big one" is coming. But what exactly is going to happen in cities along the coast? One University of Washington scientist created 50 simulations to show how strong the shaking will be. Here are two scenarios for Seattle — a "strong shaking" scenario and a "better case" scenario. Read more about the research here: http://www.washington.edu/news/2017/10/23/50-simulations-of-the-really-big-one-show-how-a-9-0-cascadia-earthquake-could-play-out/

Posted by University of Washington News on Monday, October 23, 2017

The offshore zone, with the potential to bring deadly shaking to coastal Washington, Oregon, British Columbia and northern California, is scary because the Juan de Fuca oceanic plate is slowly moving under the North American plate. Scientists say a major jolt is unleashed roughly once every 500 years, and that timeframe means it could happen again any day.

According to UW News, Wirth ran simulations using different combinations for three key factors: the epicenter of the earthquake; how far inland the earthquake will rupture; and which sections of the fault will generate the strongest shaking.

An epicenter fairly close to beneath Seattle can actually present less intense shaking because seismic waves will radiate away from the city.

“But when the epicenter is located pretty far offshore, the rupture travels inland and all of that strong ground shaking piles up on its way to Seattle, to make the shaking in Seattle much stronger,” Wirth said.

Considering what Seattle is built on, and how long shaking from a 9.0 could last, the scenario is not pretty.

“We are finding large amplification of ground shaking by the Seattle basin,” said collaborator Art Frankel, a U.S. Geological Survey seismologist and affiliate faculty member at the UW. “The average duration of strong shaking in Seattle is about 100 seconds, about four times as long as from the 2001 Nisqually earthquake.”

Wirth’s presentation will take place Tuesday from 9:10 to 9:25 a.m. in the Washington State Convention Center’s room Chelan 4.

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