Puget Sound Seismic Tremor Event Has Begun, Seismologists Say

SEATTLE, WA — You likely won't feel anything, but a major seismic event has begun underneath Puget Sound that will bring dozens of tiny tremors to the region — and perhaps raise the chance of a Cascadia subduction zone earthquake.

On Tuesday, seismologists with the Pacific Northwest Seismic Network said that the 2019 slow-slip event — also called an episodic tremor and slip, or ETS — has begun. ETS happens about every 14 months when small tremors migrate through the region.

According to the PNSN, the 2019 ETS began around Aug. 11 near San Juan Island, and the tremors were heading west-south-west.

"Over this week the tremor has on average gotten stronger and more frequent but is not yet as near continuous as during some previous ETS events. Thus far I have not been convinced that the GPS data is yet showing displacements consistent with a full-blown ETS, but then it is early in the sequence, and I am just a seismologist," PNSN professor emeritus Steve Malone said in a blog post. "If this activity continues as I expect it likely will then it likely will spread a bit farther south but it should mostly head to the northwest. Of course the timing for this is anyone's guess."

To understand why this is happening — and why it increases the risk of a subduction zone quake — you have to take a dive into the geology of the region.

The Cascadia subduction zone runs from northern California up to Vancouver Island deep under the Pacific Ocean. The zone is where the Juan de Fuca plate is sliding eastward underneath the North America plate. That's called subduction.

The plates are locked together in some spots. At some point, that locked pressure will be too much and the two plates will spring apart. When that happens, the Pacific Northwest will be rocked by a Cascadia subduction zone earthquake as big as a magnitude 9.0.

During slow-slip, the Juan de Fuca tectonic plate stalls and moves westward, which could release pressure on those locked portions of the fault zone, allowing them to spring apart — and voila, Cascadia subduction zone earthquake. The tiny tremors under Puget Sound signal that the plate reversal has begun.

That's the theory, at least. Malone told Patch back in May that the ETS-Cascadia theory hasn't been tested thoroughly. It also happens in other areas. The Oregon ETS, which happens about every 22 months, got underway about three months ago.

Slow-slip or not, a Cascadia quake will happen. The U.S. Geological Survey said in February there's a 14 percent chance that a Cascadia megaquake will hit in the next 50 years.

ETS is a good time to think about earthquake preparedness, experts say. There's actually an emergency preparedness fair coming up in Sammamish on Sept. 7, and the Federal Emergency Management Agency has tips on how to get ready for a quake.

On a positive note, the PNSN just this week got a $10.4 million federal grant to beef up the ShakeAlert early-warning system. ShakeAlert will eventually be able to provide Puget Sound residents between a few seconds and a few minutes of warning before strong shaking begins. That's enough time to shut off gas and water lines, and for people to seek shelter.