9.0 Cascadia Quake Destruction Scenarios Vary, According To New UW Research

SEATTLE, WA - For most locals, the reality of a magnitude 9 Cascadia megaquake is a constant concern. But a new University of Washington study of possible scenarios shows that the megaquake could be easier on the Puget Sound region depending on the location of the epicenter.

University of Washington researchers looked at 50 different Cascadia scenarios to determine what it would mean for urban areas like Seattle and Portland. The Cascadia subduction zone runs from British Columbia to Northern California. It's where the North American Plate and the Juan de Fuca Plate meet. The Juan de Fuca Plate is sliding under the North American Plate, and someday the two plates will spring apart - that's when the big one will hit.

There's no way to predict where the quake's epicenter will be, which is why UW looked at so many different possible scenarios. One of the best scenarios for Puget Sound would be a rupture up by Neah Bay in the far northwest corner of the state. In that scenario, according to the research, the worst shaking would radiate out into the Pacific Ocean.

“Surprisingly, Seattle experiences less severe shaking if the epicenter is located just beneath the tip of northwest Washington,” UW postdoctoral researcher in Earth and space sciences Erin Wirth said. “The reason is because the rupture is propagating away from Seattle, so it’s most affecting sites offshore. 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.”

If the quake strikes farther out in the Pacific, the shaking would likely radiate into Puget Sound, causing stronger shaking, according to the research.

But even in that best-case scenario, the quake will be very destructive. Downtown Seattle will likely experience strong shaking because of the sedimentary soil in that area. The shaking could last up to 100 seconds in the Puget Sound area.

Wirth's research will be presented Tuesday at the Geological Society of America meeting in Seattle.

Image courtesy University of Washington