What does a 3.1-magnitude earthquake feel like?

Earthquake seismologist John Cassidy initially thought the 10 seconds of rumblings he felt in his Cordova Bay home Saturday night could have been caused by an earthquake, but he wasn’t certain.

“It wasn’t a distinct jolt, not waves, but more rumbling like a train,” said Cassidy, head of the earthquake seismology section for Natural Resources Canada.

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“I heard as much as I felt it,” said Cassidy. “And I did wonder about the Growlers, or a helicopter can cause vibrations in our house. I thought it was an earthquake but it wasn’t a real distinctive one.”

Residents throughout the capital region reported rumblings when a magnitude-3.1 earthquake, as reported by the United States Geological Survey, struck Saturday at 10:57 p.m. in Washington state’s San Juan Islands. (It was first reported as a 3.2.) 

The quake hit about 20 kilometres east of Sidney.

“It was mostly just a reminder that we live in an active earthquake zone,” said Cassidy.

Southwestern B.C. is one of the most seismically active regions in Canada, with about 400 earthquakes each year. About a dozen are felt.

“Small earthquakes are happening every day that people aren’t feeling,” said Cassidy. “Then we have damaging ones that happen decades apart and then we have some of the world’s largest earthquakes centuries apart.”

The quake Saturday was a miniature North American crustal quake — one of three types of quakes that occur in subduction zones in southwestern B.C. and also in southern Alaska, Japan, Chile and Mexico, said Cassidy.

The North American crustal quakes have the potential to be the most damaging because they are closest to the Earth’s surface and closest to populated centres, said Cassidy.

Juan de Fuca Plate earthquakes stretching from the middle of the Strait of Georgia to southern Puget Sound have been the most frequent in the region. A 6.8 magnitude quake shook Seattle in 2001, causing major damage, and was felt in Greater Victoria, causing minor damage, he said.

Plate Boundary earthquakes that occur along the boundary between the North American Plate and the subducting Juan de Fuca Plate are called megathrust quakes because they can be up to magnitude 9 and cause large tsunamis. They occur every 200 to 800 years, with the last in 1700.

Seismologists can see that Vancouver Island is buckling under the pressure of this type of earthquake building. “We can measure that everyday,” said Cassidy.

There’s no correlation, however, between the quake Saturday and the so-called Big One. That quake is predicted to have a 10 to 15 per cent chance of occurring in the next 50 years. Overall, there’s a one-in-three chance of a damaging earthquake on southern Vancouver Island in the next five decades.

The European Mediterranean Seismological Centre solicits testimonials about quakes around the world. There, a Sidney resident wrote of the Saturday event: “First it felt like something hit the house and then the window rattled.”

Another Sidney resident described it as “a mini explosion” while a third thought a plane had crashed at the airport. Their dog was upset.

In Victoria, 25 km from the epicentre, a resident reported their headboard rumbling and walls shaking. Another said it was the “most alarming shaking I’ve felt in the 22 years since I’ve lived here.”

Others slept through it or didn’t detect it because, as one Colwood man said, his house is on a bed of rock.


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