Slipping fault may increase quake risk

The ground isn't shaking, but a geologic fault in the Pacific Northwest has been silently slipping for the past 11 days — and may be raising the risk of a major earthquake.

Scientists at the Geological Society of Canada (GSC) say an earthquake on the Cascadia subduction zone off the coast of Washington, Oregon and Vancouver Island in British Columbia may be up to 30 times more likely over the next two weeks while the unusual seismic event, called "episodic tremor and slip," is under way.

Vancouver Island has moved about 0.14 inch westward since seismic instruments first detected the event Sept. 3 on the Olympic Peninsula.

"The plate is already moving, and this could be the seed that starts a bigger slip," said GSC seismologist Taimi Mulder.

But the risk estimate is based on a computer model that makes many assumptions about a poorly understood phenomenon. And even if it's true, the overall chance of an earthquake remains slight.

Discovered only a few years ago, the slip events occur every 14 to 15 months.

They seem to originate deep inside the Earth, where the offshore Juan de Fuca plate is diving under the continental plate, said University of Washington seismologist Steve Malone.

Unlike an earthquake, where the plates slip violently, these events cause no detectable ground motion but seem to be increasing the strain on the fault.

The Cascadia subduction zone last ruptured in 1700, generating a massive earthquake and tsunami on a par with last year's Sumatra disaster. Scientists have searched the seismic record to see if a silent slip event preceded that earthquake, but there weren't enough instruments in the area to tell.

Once they confirmed the current slip event was under way, Canadian scientists alerted the British Columbia emergency-management agency and issued a public notice.

The University of Washington seismology program put a notice on its Web site but didn't contact emergency managers.

"I don't think we understand this well enough to have people get too excited," Malone said.

Sandi Doughton: 206-464-2491 or sdoughton@seattletimes.com