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‘Swarm’ of earthquakes off Vancouver Island a precursor?

This map from U.S. Geological Survey shows the location  of a trio of large earthquakes that struck within an hour of each other late Sunday night, Oct. 21, followed by three smaller quakes, off Vancouver Island/
This map from U.S. Geological Survey shows the location of a trio of large earthquakes that struck within an hour of each other late Sunday night, Oct. 21, followed by three smaller quakes, off Vancouver Island/ Courtesy to The Bellingham Herald

A swarm of large and moderate earthquakes that struck Sunday night and Monday morning off Vancouver Island hasn’t changed stresses on the nearby Cascadia fault, which scientists say is overdue for a massive seismic event.

“It’s the same as it was yesterday,” said Doug Gibbons, a research engineer at the Pacific Northwest Seismic Network.

“Until you get up to magnitude 7 or greater, you’re not really changing the stress level to a large degree.”

Three earthquakes — all magnitude 6.5 or greater — struck within an hour of each other Sunday night, followed by four smaller quakes Monday off Vancouver Island, about 275 miles west of Bellingham.

Gibbons said that the temblors likely aren’t the precursor of a larger quake.

“We don’t see any indications of a foreshock,” he said.

No damage was reported from the largest quakes, according to the U.S. Geological Survey. A quake of 6.1 to 6.9 can cause serious damage in populated areas.

There also was no immediate tsunami threat, according to a post at the National Tsunami Warning Centers.

FEMA released a video on tips on what people should do in the event of an earthquake.

Sunday night’s swarm of quakes occurred just one week after the Great ShakeOut, a worldwide series of earthquake drills.

“It certainly heightens awareness,” Gibbons said, noting the tremors’ timing.

USGS geophysicist Amy Vaughan told The Associated Press that the quakes were lightly felt onshore.

In a Twitter post, British seismologist Stephen Hicks at the University of Southampton said the shaking occurred on the Sovanco transform fault, which is near the San Juan de Fuca plate and the Cascadia subduction zone.

“Looks like we have an earthquake triplet, folks!” Hicks tweeted about midnight West Coast time.

“A question that continues to puzzle earthquake scientists: Why did the 3 fault segments, which were all presumably close to failure anyway, rupture in three separate earthquakes over an hour period, versus a single M7.0 rupture???”

Seven earthquakes were recorded within miles of each other after the first one, a magnitude 6.6 temblor that struck at 10:40 p.m. PDT Sunday. All seven quakes were between 6 miles and 11 miles deep, according the PNSN website.

Canadian Broadcasting Corp. seismologist and meteorologist Johanna Wagstaffe reported that the quakes were relatively shallow in depth and showed a horizontal “strike-slip” movement, which lessened the tsunami threat.

“If any one of these quakes had hit closer to land, there would have been devastating consequences,” Wagstaffe reported on CBC.

By 2:30 p.m. PDT Monday, a total of 213 responses were received at the USGS “Did you feel it” citizen science web portal.

Scientists think that region is overdue for a massive earthquake, a megaquake on the scale of 9.0 or greater.

Scientists and emergency management officials have been aware of the Cascadia fault’s potential for devastation, even before a 2015 New Yorker article warned of that possibility.

Gibbons said Northwest residents can expect further shaking in the region for the next several days. “Because that was a crustal event, there’s probably going to be small- to medium-size aftershocks for up to a week or so,” he said.

Robert Mittendorf: 360-756-2805, @BhamMitty
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