Scientists watching ‘out of the ordinary’ string of tremors just northwest of Whatcom
The Pacific Northwest Seismic Network is monitoring an “out of the ordinary” series of nearly 3,000 small seismic events over the past week that have rattled just northwest of Whatcom County under Vancouver Island.
“If you’ve checked the Tremor Map in the last couple days, you may have noticed an uptick in activity,” the network tweeted Tuesday morning. “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.
As of early Tuesday afternoon, the network’s tremor map shows 2,909 epicenters, most under southern and central portions of the island in British Columbia, but a handful were even recorded in the San Juan Islands on the U.S. side of the Haro Straight.
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.”
Incidentally, Jan. 26 marked the 321st anniversary of the last great Cascadia Subduction Zone earthquake that shook the Pacific Northwest on Jan. 26, 1700.
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.”
“This latest batch seems to be a reactivation of part of the standard ETS region,” Malone wrote. “It seems to have started in the region on Vancouver Island that had some of the earliest tremors during the Oct. 4-Nov. 16 ETS event, a burst going from Oct. 15-17.
“I had expected that this latest burst might migrate north-west to fill in an area that has not had significant tremor since a short burst in January-February of 2020. However, up until today, it seems to be slowly moving south-east back into an area that had significant tremor a few months ago.
“But this afternoon tremor started lighting up the northern part of this patch and so maybe ultimately it will have some that continues on to the northwest.”
Malone reiterated that he didn’t think the recent tremor signals any hazard or significant changes in the tectonics of the Cascadia Subduction Zone.
“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.
