Cascadia Megaquake Risk Rises As 'Slow Slip' Event Begins

SEATTLE, WA - The chance of a Cascadia subduction zone megaquake is slightly higher right now - that's because the yearly seismic "slow-slip" seismic event has started, putting pressure on the tectonic plates along the Pacific Coast.

Slow-slip - also called an episodic tremor and slip, or ETS - officially started May 8, according to the Pacific Northwest Seismic Network (PNSN). During ETS, after 14 months of moving eastward, the Juan de Fuca tectonic plate stalls and moves west, putting stress on the Cascadia subduction zone.

"It's loading up the edge of the lock zone of the Cascadia subduction zone more rapidly than normal tectonic processes would do," Bill Steele, director of communications at the PNSN, said during last year's ETS. "You're getting seven months of strain accumulation applied to the back edge of the fault over a week."

This year's ETS started under the Kitsap Peninsula and then moved north (see photo above). As it usually does, the slow tremor has turned around, heading south toward Oregon.

Steve Malone, a PNSN professor emeritus, is tracking the 2018 ETS. The event looks pretty standard so far, Malone wrote in a blog post about the event.

While it's true there is risk of a Cascadia rupture right now, it's pretty slight. If anything, ETS is a good time to go over your earthquake safety plan.

"You should always be prepared because [a Cascadia subduction earthquake] is going to happen," University of Washington earth sciences professor Ken Creager said during the 2017 ETS. "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."

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 will inundate coastal areas.

Image courtesy Pacific Northwest Seismic Network