‘Out of the ordinary’ tremor continues near Whatcom with 3,000-plus events in past week
Nearly a week since the Pacific Northwest Seismic Network first tweeted that it was monitoring an “out of the ordinary” series of 3,000 small seismic events just northwest of Whatcom County under Vancouver Island, another 3,000-plus events have occurred.
“If you’ve checked the Tremor Map in the last couple days, you may have noticed an uptick in activity,” the network tweeted Tuesday, Feb. 2. “Tremors in between ETS (episodic tremor and slip) events are normal, but starting on about Jan. 26th, tremor under Vancouver Island started getting stronger and longer over the next few days.”
Episodic tremor and slip is observed in subduction zones when non-earthquake rumbling, or tremor, and slow slip occur along the zone where one tectonic plate is pushed beneath another. It usually occurs beneath a locked zone of the fault that can generate great quakes, according to the Incorporated Research Institutions of Seismology. Tremor can release enough energy to equal a 7-magnitude quake, though its rumblings are rarely felt.
Since Jan. 26, the Pacific Northwest Seismic Network’s Tremor Map has logged 6,217 seismic events — most under southern and central portions of the island in British Columbia, but a handful were recorded in the San Juan Islands on the U.S. side of the Haro Straight an under the northwest tip of the Olympic Peninsula. Two were even recorded in western Whatcom County waters in the Straight of Georgia — a pair of 1.8-magnitude events that occurred 12:45 p.m. and 1:25 p.m. Friday, Feb. 5.
Almost all of the tremors had magnitudes smaller than 2.0, though a handful of 2.0s and 2.1s were seen in the list.
The Pacific Northwest Seismic Network tweet said the high amount of activity is “out of the ordinary,” but said it wasn’t believed to be indicating any hazards or tectonics along the Cascadia Subduction Zone.
“It’s important to keep in mind that ‘ordinary’ still isn’t a very well understood term when it comes to tremor,” the network said. “We’ve been studying tremor for less than two decades, so what may seem irregular right now might be normal over the 300-plus years since the last CSZ (Cascadia Subduction Zone) earthquake.”
Action has remained steady since the Feb. 2 tweet, with 3,308 events recorded since that date, according to the Tremor Map, including 404 events on Sunday, Feb. 7. The daily high was 686 recorded Feb. 3, with a low of 54 recorded the first day on Jan. 26 and with the average since then being 444.1 recorded events per day.
In a Feb. 1 blog post, Pacific Northwest Seismic Network Professor Emeritus Steve Malone said the recent burst seen beneath Vancouver Island doesn’t “look to be a typical inter-ETS tremor patch that is usually down-dip (east-northeast) of the main ETS patches and lasts no more than a few days.”
“Our general conclusion is that over periods of years most (all?) sections of the Cascadia Subduction Zone see slow slip (ETS) events in the subduction fault zone down-dip of the locked zone,” Malone wrote.
The Pacific Northwest Seismic Network is operated cooperatively by the University of Washington and the University of Oregon to monitor earthquake and volcanic activity across the region, utilizing more than 300 seismograph stations across the region.
