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Investigating BC's 'Big One'


Urban areas inland would feel the tremors from a major earthquake
Urban areas inland would feel the tremors from a major earthquake

Andrea Stockton, staff writer

August 12, 2011 — The potential for a megathrust earthquake is possible along BC's coast. New studies show tremors could reach 100 km inland.

The areas in red are most at risk for seismic activity
The areas in red are most at risk for seismic activity

Nature Geoscience recently published a new study, which says the fault line in the Pacific Northwest is about seven kilometres deeper than previously thought.

Andrew Calvert is an earth scientist at Simon Fraser University and is the lead author of the study. He says the greater depth affects the ground shaking along a lot of the Pacific Northwest coast and buildings and urban areas about 100 km inland could feel the tremors.

“If there were an earthquake on that fault, which we call a megathrust earthquake, it would actually break the fault over quite a long distance, so 500 to a 1000 km or more along the coast line. And so, that break would happen over two or three minutes and it would mean that the length of the ground shaking would actually be quite long compared to some of the local earthquakes we have.”

Calvert adds that a typical local earthquake usually affects the area for just 10 to 20 seconds.

How would a megathrust earthquake compare?

“A modern equivalent to what would happen here was basically what we saw in Japan in March, where there was essentially a magnitude 9 earthquake off the east coast of northern Japan,” warns Calvert. “And that strong ground shaking and the tsunami that's generated would be very similar to what would occur here in the event of a megathrust earthquake.”

Geologists say people would probably have about 30 minutes after the earthquake to get to higher ground before the wave would inundate the coast. When the earthquake strikes, the flexure of the tectonic plates is what displaces the water soon after.

Tsunami of 1964. Courtesy: Alberni Valley Museum
Tsunami of 1964. Courtesy: Alberni Valley Museum

The seismic hazard along the coast is significant and is something that could happen at any time.

While the province is preparing for the “Big One”, Calvert says it's worth bearing in mind that there are other earthquakes that can occur that are not on the plate boundary.

“There can be magnitude 7 earthquakes in the overriding North American plate, which are close to the surface and there can also be magnitude 6.5 to 7 earthquakes in the subducting plate. And both of these types of earthquake are likely to be closer to the urban areas in the Pacific Northwest and so they do represent a significant seismic hazard as well, and they occur more frequently.”

Canadians are encouraged to be alert and ready for natural disasters. In preparation for a possibly damaging event, BC takes part in a massive earthquake drill every year.

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