By JANE ARMSTRONG
Toronto Globe and Mail
February 05, 2007
While the probability of a quake is still low, rapid strides in earthquake detection have given federal scientists with the Pacific Geoscience Center on Vancouver Island greater confidence in their ability to predict when and where one will occur. Garry Rogers, a seismologist at the center, 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," 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.
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, Rogers said.
Scientists at the geoscience center 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.
Scripps Howard News Service, http://www.scrippsnews.com
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