Northwest tremors no cause for concern, seismologists say

Seismologists say the seismic activity along the West Coast from British Columbia to Washington state this week is not hundreds of small earthquakes, as early reports shared on social media speculated, and doesn’t indicate that the “big one” is imminent.

What was detected this week are known as tremor episodes, routine low-level tectonic activity that occurs when low-lying plates slide over each other as part of what seismologists call a "slow slip.” Over the course of the week the impact area of the tremor episodes migrated from Vancouver Island to central Washington.

“This is thoroughly normal behavior,” Joan Gomberg, research seismologist at the United States Geological Survey (USGS), said Wednesday.

A slow slip can take two weeks to a month to complete. It occurs in a much deeper zone than the one that produces strong earthquakes, and can be hard to detect without instruments.

Any relationship between the slow slips and strong earthquakes is speculative, Gomberg cautioned.

“These episodes happen about once a year in our region," said John Vidale, a seismologist at the University of Washington in Seattle.

That fact hasn’t stopped many Washington residents from fearing the worst.

“Somebody put up a map with the tremors and labeled it in a way that people would think there were 400 little earthquakes right under Seattle, and people weren’t sure what to make of it,” Vidale said.

“Earthquakes happen really fast, in a matter of minutes or seconds. Slow, deeper slippage happens over weeks … and doesn’t send out strong waves," Gomberg said. "It’s the waves that are sent out in a really fast slip that knock things down and do damage.”

It’s been about 300 years since the last major earthquake in the Pacific Northwest, and on average they occur roughly every 500 years, Gomberg continued.

Many Americans live on the North American plate, which rides on top of three smaller plates including the Juan De Fuca plate in the Puget Sound region near Seattle. It is in the shallowest part of the interface between the North American and Juan De Fuca plates that the risk of a strong earthquake is strongest.

Gomberg said that “there’s certainly no cause for alarm” from the latest tremor episodes.

“But the important thing is to always be prepared," she said, "because someday we will have a major earthquake and it's good to be aware.”