The earth under Western Washington has been working overtime in May. The Pacific Northwest Seismic Network (PNSN) reports that an Episodic Tremor and Slip Event started May 8 — and is still underway.
PNSN Professor Emeritus Steve Malone confirmed the consistent tremors started "only slightly north of where many in the past have started."
The activity, where movement of plates is coupled with tremors, is centered just west of Puget Sound near the Olympic Mountain range and is so deep that it people can't feel the shaking.
The map below is a snapshot of the first 10 days of activity, color-coded chronologically. The red dots show the most recent activity.
In March, PNSN posted a prediction of these tremors for Washington and British Columbia. The event was expected to start in April or May, 'based on the roughly 14-month repeat period observed since around 2000.'
Malone notes this episodic tremor event is similar to a pattern set from 2007-2011. Now he expects the tremors to continue into early June, spreading slightly more south and even further up north. He writes, "Thus far I (a seismologist) don't see any evidence of geodetic slip in the GPS data provided by PANGA. You can look for yourself at the Central Washington University PANGA web page."