News / Vancouver

A 6.5-magnitude earthquake struck off the B.C. coast… and nobody felt it

Seismologists say a slow earthquake struck Vancouver Island on Dec. 21 and is still being detected in northern Washington.

A map showing slow earthquakes occurring between Dec. 21, 2015, and Jan. 12, 2016, along the Cascadia subduction zone.

Courtesy Pacific Northwest Seismic Network

A map showing slow earthquakes occurring between Dec. 21, 2015, and Jan. 12, 2016, along the Cascadia subduction zone.

A huge earthquake measuring at least 6.5 on the Richter scale recently shook Vancouver Island— and nobody felt it.

That’s because the tremor happened over about eight days.

Called a “slow earthquake,” the most recent event started Dec. 21 below the surface of Vancouver Island and kept shaking until Dec. 30 before migrating south to northern Washington, where continued seismic activity was recorded as recently as Tuesday.

“I like to describe them as the banana slug of earthquakes,” said University of Washington graduate student Shelley Chestler, who is writing her thesis on slow earthquakes. “While they work the same way as normal earthquakes as far as two plates slipping past each other, that slip happens much more slowly in the order of a month rather than 15 seconds.”

While the 4.9-magnitude earthquake that struck off the coast of Saanich on Dec. 29 did coincide with the ongoing slow earthquake beneath Vancouver Island, Chestler said it’s unclear if the two were connected. 

Slow earthquakes, also known as slow slip events, are clusters of tremors that shake the ground like normal earthquakes, but the ground motions are so slight that only sensitive instruments called seismometers can detect them. Along the Cascadia subduction zone beneath southern Vancouver Island and northern Washington, slow earthquakes are recorded every 12 to 15 months.

Chestler said slow earthquakes can release the same amount of energy as a magnitude-6.5 to 6.8 normal earthquake, but because they release this energy more slowly, they can't be felt.

Simon Fraser University earth sciences professor John Clague describes the phenomenon as a band of lights that turn on and off steadily.

“Along this band, all hell breaks loose and they start blinking as all these earthquakes start occurring,” he said. “The sources of these earthquakes are typically quite far below the earth’s surface. Probably about 30 or 40 kilometres.”

While slow earthquakes are being actively researched, Chestler said scientists are still puzzled about what causes them and why they don’t happen all the time.

But she emphasized that slow earthquakes are nothing to be alarmed about.

“They’re not making it more likely for the Big One to occur at any time and they don’t generate dangerous shaking,” she said. “They’re just there and they’re pretty interesting and cool.”