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British Columbia On Alert For Major Earthquake


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B.C. put on alert for huge quake

JANE ARMSTRONG

From Saturday's Globe and Mail | February 03, 2007

VANCOUVER — Scientists have alerted British Columbia's emergency-planning department to the possibility of a catastrophic earthquake striking the province's southwest coast next week.

While the probability of a quake is still low, rapid strides in earthquake detection have given federal scientists with the Pacific Geoscience Centre on Vancouver Island greater confidence in their ability to predict when and where one will occur. Garry Rogers, a seismologist at the centre, compared the current earthquake odds to the dangers of driving a car.

“Everyone drives their car every day, and the probability of getting in a car accident is small,” Dr. Rogers said. But during rush hour, the probability of getting into an accident is much higher. “Well, Vancouver Island is now driving in rush hour.”

What prompted the alert was a series of imperceptible tremors emanating from deep beneath the ocean, which scientists now recognize as ominous warnings that the earth is on the move again off Vancouver Island.

They now estimate the long-awaited giant quake will hit closer to the island's western shoreline than previously thought.

The tremors occurred on what is known as the Cascadia subduction zone, which lies beneath the Pacific Ocean off the West Coast and runs from Vancouver Island to Northern California. The rumblings began last week near Puget Sound near Seattle and made their way north to Vancouver Island in recent days.

The tremors — known in earthquake-speak as an episodic tremor and slip — monitor the ongoing strain between the solid earth on the West Coast and the offshore Juan de Fuca Plate.

The two plates are rubbing against one another, with the offshore plate continually pushing against and under the North American Plate.

The recent tremors mean that even more stress is building between the two, which scientists believe will one day rupture into a major earthquake the size of the one off the coast of Indonesia on Dec. 26, 2004, which killed thousands.

Dr. Rogers said the chances of a major earthquake striking southwestern B.C. spike during the tremor events. The current tremor session is expected to last for another week.

A tremor event is similar to an earthquake, but it occurs at a deep level on a fault where the rocks are hot and elastic.

Instead of the offshore tectonic plates slipping steadily under North America, the scientists now say there are periodic jumps that pass stress up to the more shallow locked section of the fault, where earthquakes occur.

“It's piling [stress] on the upper portion and eventually it will fail,” causing a giant earthquake, Dr. Rogers said.

He offered another analogy to describe the effects of the stress buildup caused by the tremors.

“It's like adding straws to the proverbial camel's back,” he said. “Every time we add a straw, we get closer to failing the camel's back. One of those straws will break the camel's back.”

Scientists at the geoscience centre first discovered the tremor events in 1999. Data showed that seven GPS sites, which were strung along from Vancouver Island to Seattle, were moving out to sea.

It's long been known that Vancouver Island is slowly moving eastward toward the mainland. But suddenly, the data showed the movement was reversed.

Eventually, scientists linked this reversal of movement to the tremor events. Right now, Dr. Rogers noted, Vancouver Island is again moving west.

The scientists' findings were presented four years ago at the annual meeting of the American Geophysical Union in San Francisco.

Since then, dozens of scientists from around the globe have begun tracking tremor events.

One day, one of these tremor events will cause the fault line to rupture, he said, resulting in an earthquake magnitude measuring as high as 9.

It would not be the first great megathrust quake to devastate the West Coast of North America. The geological record on the West Coast has shown that giant earthquakes occur once every 500 years.

The last one struck on Jan. 26, 1700, causing widespread destruction, flooding and a giant tsunami. Indian villages along the coast were wiped out and the tsunami reverberated as far as Japan.

Because B.C.'s West Coast is earthquake territory, the province is more prepared than many jurisdictions. Every B.C. municipality is required by law to have an emergency plan.

Dr. Rogers said a giant quake would cause widespread damage. The rupture line will be about 1,000 kilometres long and the tremors would continue for minutes — not seconds.

The offshore rupture would cause the continental shelf to spring upward, causing a tsunami that would slam the west coast of Vancouver Island, causing property damage and loss of life on coastal communities.

The good news is that West Coast scientists say advances in detecting tremor events have translated into improved methods of pinpointing where the inevitable rupture will occur.

Previously, they could provide a rough location — up to 100 kilometres off Vancouver Island — of the next quake. New data show the quake will occur 25 kilometres closer to shore, meaning the damage will be greater.

As a result of this find, scientists will urge some affected municipalities to toughen their building codes, Dr. Rogers said.

The Geological Survey of Canada added for an average week, the chances of a mega thrust earthquake occurring is 0.0005 per cent (or one in 200,000); during a week of Episodic Tremor and Slip this increases to 0.026 per cent (or one in 4,000) -- a 50-fold increase. We are at a time when a mega thrust quake could occur.

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B.C. tremor scare toned down

CanWest News Service; Victoria Times Colonist

Published: Sunday, February 04, 2007

VICTORIA - Vancouver Islanders, stocking up on emergency supplies after warnings of increased chances of a major earthquake, can relax - a little.

The latest episode of subterranean tremors has ended earlier than expected, seismologists at the Pacific Geoscience Centre in Sidney said Saturday.

"Things have quietened down very quickly. We don't know why," said Natural Resources Canada seismologist John Cassidy.

The tremors, measured by sensitive instruments, are caused by the slipping of the bottom plate in the Cascadia fault, which runs off the west coast of Vancouver Island, and beneath the Island.

Scientists at the Pacific Geoscience Centre discovered the episodes occur about every 14 months and add stress to the shallow, locked part of the plate, bringing it closer to giving way, which would result in a major earthquake. When the plate fails, it is expected the earthquake will measure about nine on the Richter scale.

The next expected episode of tremor and slip will be about April 2008.

Seismographs show the tremors have stopped, but data from other instruments does not yet show whether Vancouver Island is still being pulled towards Japan or if it is again moving in its usual direction towards Ottawa, Cassidy said.

Victoria Times Colonist

© CanWest News Service 2007

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