Pacific Northwest records thousands of tiny tremors during seismic shuffle

VANCOUVER (NEWS 1130) — New data shows western Washington State and Vancouver Island might be ending their near-annual shuffle away from the mainland.

Seismologist John Cassidy with Natural Resources Canada notes western Washington State has moved between 4 to 6-millimeters away from the mainland over the past week or so, with thousands of tiny tremors detected.

“Because of that slow movement of the ocean plate that’s being pushed beneath us, Vancouver Island for example is normally moving to the northeast, it’s normally moving towards Vancouver at about a centimetre a year,” he explains.

“But about every 14 months, Vancouver Island changes direction — as does Washington and Oregon and the Northern Part of California — for about two weeks and instead of moving towards the northeast moves toward the ocean”

This change in direction is known as an episodic tremor and shift (ETS) event.

“What we see are these thousands and thousands of tiny tremors that we can record, there’s ground-shaking associated with these, a very, very minute amount of shaking that nobody feels,” Cassidy says.

“We find this very narrow band of tremors that happens at the same time that we’re changing direction and moving back toward the ocean. It’s quite a remarkable process and the movements are very slight. It’s a big area that’s moving towards the ocean.”

These events are predictable, and not a sign of an impending earthquake.

“It doesn’t tell us that a ‘big one’ is going to happen, but it’s really a good reminder that this is an earthquake zone.”

Cassidy notes the type of earthquake people are referring to when they refer to ‘The Bing One’ is a magnitude 9 subduction quake, and we are in the geological “window” for one of those.

“It could happen tomorrow afternoon, it could happen in 500 years. It’s not something that can be determined on an exact basis,” he notes.

There have been 19 such quakes in the last 10,000 years. The last one was 320 years ago. Cassidy says the shortest time between two events was 250 years, while the longest was closer to 1,000.