UW Professor: 'No Cause For Alarm' During Seismic 'Slow Slip' Event

SEATTLE, WA - The periodic "slow-slip" seismic event began deep underneath Washington this week, and with it comes a slightly higher risk of a Cascadia subduction zone megathrust earthquake.

But University of Washington earth sciences professor Ken Creager, who studies slow-slip events, advises Puget Sound residents not to lose sleep worrying about it.

"You should always be prepared because [a Cascadia subduction earthquake] is going to happen," he told Patch.com. "There is some reason to think the probability is higher during [slow-slip], but it's still highly unlikely. I don't think it's cause for alarm."

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Slow-slip events have preceded some recent large earthquakes, which is why scientists believe the chance for a Cascadia quake is slightly higher right now. The March 2011 subduction quake in Japan was preceded by a slow-slip event, as was a 2014 magnitude 8.1 quake near Chile.

Slow-slip events are characterized by a series of small tremors deep underground. During past events, the tremors have traveled north toward Vancouver, and then turned around and moved south, sometimes as far as the Columbia River near the Oregon border.

However, if the tremors move continue moving west toward areas where the North America and Juan de Fuca plates are locked together, that might be cause for alarm.

"That might be cause for further discussion," Creager said.

The Cascadia subduction zone runs from northern California up to Vancouver Island. Deep underground, the Juan de Fuca plate is sliding underneath - subducting - the North America plate. The plates are locked together in some spots. At some point, the pressure will cause the two plates to unlock and spring apart. When that happens, the Pacific Northwest will be rocked by up to a magnitude 9.0 earthquake and a tsunami.

The slow-slip event can last anywhere between two and six weeks - and, as Creager says, it's never a bad time to be prepared for a large earthquake. Scientists estimate there's a 37 percent chance of a Cascadia quake occurring within 50 years.

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