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Trio of quakes off B.C. coast remarkable, but not alarming says scientist

SIDNEY, B.C. - In earthquake-prone British Columbia, which experiences about 1,500 quakes a year, a preoccupation with The Big One is never far from the surface.

SIDNEY, B.C. - In earthquake-prone British Columbia, which experiences about 1,500 quakes a year, a preoccupation with The Big One is never far from the surface.

It breaks through almost every time a quake does more than rattle the kitchen glassware. So it wasn't surprising when the airwaves buzzed with Big One talk after three sizable temblors rocked the ocean floor off the West Coast.

A magnitude 6.5 quake was recorded just after 3 a.m. local time last Saturday under the Pacific Ocean south of the Queen Charlotte Islands.

A second, similar-sized quake occurred about 45 minutes later in roughly the same location.

Wednesday morning, a third quake, measured at magnitude 6.1, was traced to the same spot.

Enough to send ripples through quake-sensitive British Columbians who wondered whether this cluster of large shakers might not be a precursor for The Big One.

The answer from a Canadian scientist is no.

Well, probably not.

"These earthquakes don't mean we're any closer to the big earthquake we expect, or that the big earthquake is any further away," says Dr. John Cassidy, part of a team of leading-edge researchers at the Pacific Geoscience Centre near Victoria.

But they do serve as a reminder that coastal B.C. is a very active region where earthquakes occur every day, he says. And once every couple of centuries it gets slammed by a "really big one" - the last an undersea quake in the year 1700.

"It was a magnitude 9 and ruptured the fault from northern Vancouver Island to northern California," says Cassidy.

The biggest quake ever recorded on land in Canada, a magnitude 7.3, struck central Vancouver Island in 1946 at a place called Forbidden Plateau near Campbell River.

The quake, felt as far away as Portland, Ore., and Prince Rupert on B.C.'s North Coast, crumbled chimneys even in Vancouver and sent frightened residents running into the streets, according to Natural Resources Canada account.

Two people died - one drowning when a small boat was capsized by the quake-generated tsunami and another in Seattle from a heart attack.

So the prospect of a very big and very destructive quake is very real, too.

The root of the threat on the West Coast is an area scientists refer to as the Cascadia Subduction Zone, stretching from a fault off the Queen Charlottes off the northern coast to California's infamous San Andreas fault.

To keep the image simple, Cassidy describes two continental plates under the Pacific Ocean moving eastward, being pushed beneath North America's continental plate at a spot in the ocean several hundred kilometres west of Vancouver Island.

Where the plates meet, pressure and friction builds. When they crack or slip, energy is released in the form of earthquakes.

Generally, the larger the magnitude number, the more damage is likely.

But the magnitude scale is logarithmic, not linear, and Cassidy acknowledges this often has people confused, particularly in this case where all three recent quakes were over the magnitude 6 mark.

Going from magnitude 6 to 7, the level of ground shaking goes up by 10 times while the energy release goes up by 32 times. The ground shaking at magnitude 8 is 100 times stronger, and the energy release is 1,000 times stronger, he says.

The trio of plus-6 quakes occurred at the northern tip of the zone, where lower magnitude quakes, while frequent, are more the norm.

Unfortunately, despite sweeping advances in the field, seismologists are still unable to predict where the next devastating quake will occur.

But they are getting a much better handle on the process.

"What we're doing is improving our understanding of where earthquakes occur, how often and, ultimately, how will the ground shake during future earthquakes," says Cassidy.

He sees that as the key, giving engineers and designers the data they need to build bridges, pipelines and buildings to withstand that level of shaking.

"Then it doesn't matter if the earthquake happens tomorrow, or in a year or in 10 years, because your infrastructure will survive," he adds.

"So, it's really the building codes that provide our defence against earthquakes."

Cassidy is realistic enough to admit that predicting earthquakes may never be possible.

But he says scientists are making great advances in their understanding though studies relying on ultra-accurate global-positioning satellite mapping to identify areas where energy is being stored.

"So this is all-new research and is being folded into the national building code of Canada and providing us with, really, a much better defence against earthquakes," he said.

 
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