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By MARK HUME
Wednesday, September 14, 2005 Updated at 6:15 AM EDT
Vancouver A silent tectonic event, so powerful it has shifted southern Vancouver Island out to sea, but so subtle nobody has felt a thing, is slowly unfolding on the West Coast.
Scientists who are tracking the event with sensitive seismographs and earth orbiting satellites warn it could be a trigger for a massive earthquake -- some time, maybe soon.
But they are quick to add that the imperceptible tremors emanating from deep beneath the surface are sending signals scientists are not yet able to comprehend fully and "the Big One" might yet be 200 years off.
What they do know is that the earth is moving this week on the West Coast as two massive tectonic plates slip past each other.
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"Southern Vancouver Island is sort of sliding towards the west right now. We're moving towards Japan," said John Cassidy, a seismologist with Natural Resource Canada at the Pacific Geoscience Centre near Sidney, B.C. "It's a very small amount. We've moved about three millimetres to the west over the past couple of days."
The event, known as an episodic tremor and slip, is a predictable, cyclical phenomenon that is adding pressure to a zone where the Juan de Fuca Plate and the North American Plate are locked, just off Vancouver Island. While the two plates are slipping in some areas, in another they remained locked. That locked zone is where the next megathrust earthquake is expected to come from when it suddenly releases.
Mr. Cassidy said a slip event occurs every 14 months, and when it does, scientists believe the chance of an earthquake the size that triggered the Asia tsunamis increases.
One researcher has likened the event to going up a step on a staircase, at the top of which sits a megathrust earthquake. But nobody knows where the top is or where we are on the staircase.
The geological record on the West Coast has shown that megathrust earthquakes occur roughly once every 500 years. The last one struck on Jan. 26, 1700, which leaves a window of possibility 200 years wide.
"We know there will be another megathrust earthquake, but we don't know when," Mr. Cassidy said.
But he said the slip is important because it is the only predictable event related to earthquakes, and it may hold clues as to when and where a megathrust will occur.
He said experts are so convinced the event is a potential trigger that they have advised emergency preparedness officials to be alert.
"We're in a time window of higher hazard," Mr. Cassidy said. "It's likely that one of these slip events will [one day] trigger a megathrust earthquake."
Mr. Cassidy said a slip event is not an earthquake but involves the release of tectonic pressure in tremors that seismographs are picking up.
"It's really a very subtle shaking. It's different from an earthquake. It has a different frequency content. It's more of a continuous signal, rather than an earthquake which would start with a bang. . . . There's no jolt to this so people don't feel it."
Satellites have tracked the shift of southern Vancouver Island to the west using global positioning technology that can detect minuscule movement.
Tectonic forces usually push the island east, but during a slip event it slides west for about two weeks.
"The normal movement to the east can be thought of as earthquake hazard. That's energy being stored for the next big megathrust earthquake and on top of that regular motion we have this cycle that adds a little more stress every 14 months. So that's why we say it becomes a trigger [event]," Mr. Cassidy said.
Herb Dragert, a seismologist with the Geological Survey, first detected the phenomenon of the slip event in 1999, and since then, it has been confirmed by scientists in the U.S. and Japan.
Mr. Dragert said slip events have led to new understandings of how the earth is moving 25 to 45 kilometres beneath the surface, where heat is so intense that rock is like soft wax.
"Before our investigations began, about three or four years ago, we thought once we get to those depths temperatures were hot enough to kind of make this a sloppy, plastic area and no stress could accumulate over a long term," he said.
"Now because of this phenomenon . . . we know that's not the case. . . . A small amount of stress accumulates and then for whatever reason, at 14 months . . . it slips. And that's one of our slip events. . . . Not only does it slip, but while doing so it produces these distinct seismic tremors."
