5.4 quake offshore west of Port Alice, followed by aftershocks

A magnitude 5.4 earthquake struck offshore, 181 kilometres west of Port Alice Sunday morning.

The 8:20 a.m. earthquake was followed by two aftershocks and “dozens of smaller, really tiny aftershocks,” said Natural Resources Canada seismologist John Cassidy.

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The shallow quake struck at 8:20 a.m. at a depth of 10 kilometres. It was followed by a magnitude 4.8 aftershock at 9:48 a.m. and a magnitude 4.2 at 10:14 a.m. in the same area.

There are no reports of damage and none would be expected.

The earthquake was in the same region as a sequence of larger earthquakes on Oct. 21 and 22. Two of the quakes were magnitude 6.5, the third was magnitude 6.8.

“These are likely linked to those previous earthquakes, either aftershocks or readjustments,” said Cassidy. “We’ve been seeing ongoing aftershocks in that area since the end of October.”

These earthquakes don’t tell us that we’re any closer - or any further way - from the big earthquake, said the seismologist. They’re not releasing stress from the other types of earthquakes in the region, he said.

“It’s a complicated area,” Cassidy explained. “It’s not one of the earthquakes where the ocean plate is pushed beneath the continent.”

Sunday’s earthquake occurred in the northern Explorer Plate. This is an area just west of Vancouver Island where two different tectonic plates are being pushed towards Vancouver Island.

“It’s a region where we have plates moving towards us, but these plates are also sliding past one another. So there are a lot of different faults in that offshore region. We’ve had well over 700 earthquakes in that region in the past year,” said the seismologist.

The earthquakes are not related to the big subduction faults, said Cassidy. The ocean plate is really quite thin in this area.

“As you go down, it gets hot pretty fast and the rocks can’t support earthquakes. For earthquakes you need the rocks to be relatively cold and brittle so they can actually break,” said Cassidy.

However, these earthquakes are scientifically important. Data from recordings of these earthquakes give scientists a better idea of the faults, how waves propagate and how shaking on land varies from one site to another, said Cassidy.

They also serve as a reminder that we live in an active tectonic region and have earthquakes in this area, he said.

People should be prepared and know what to do during an earthquake, said Cassidy. He recommends people look at Shakeout B.C. at shakeoutbc.ca.

Cassidy also recommends the Capital Regional District Prepare Yourself website at crd.bc.ca/prepare-yourself

ldickson@timescolonist.com

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