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Deep beneath Seattle, a 6.7 'silent quake' nobody felt

Global Positioning System 'blip' leads scientists to discovery

Friday, April 20, 2001


SAN FRANCISCO -- In August 1999, a 6.7-magnitude "silent earthquake" started deep beneath Seattle and moved north into Vancouver Island over the next 35 days.

"It was equivalent in energy to the Nisqually quake," said Herb Dragert, a scientist with the Geological Survey of Canada in Sidney, B.C.

But unlike the magnitude-6.8 temblor that hit the Puget Sound area on Feb. 28, this one didn't shake us. Not yet anyway.

The "slip event" happened so slowly that at first, nobody noticed its quiet move from the subduction zone under Seattle along the tectonic plate boundary to Canada.

Dragert and his colleagues describe their discovery of this "slow, silent quake" using Global Positioning System (GPS) data in today's Science magazine. The report has been a big hit here at an annual meeting of the Seismological Society of America.

Scientists have long recognized the existence of these slow quakes -- which aren't really earthquakes at all since the Earth doesn't shake, rattle or roll -- and their potential contribution to the geological stress that can ultimately lead to a real quake.

The massive killer quake of magnitude-9.5 in Chile in 1960, for example, happened within days of one of these slow, silent types. Nobody knows precisely what the connection might be, but everyone is certain it's worth close attention.

"This is the kind of data that gives us a clue as to what's going on down there in these subduction zones," said Tom Heaton, a leading quake expert at the California Institute of Technology.

The Cascadian Subduction Zone, which encompasses Western Washington and Oregon, is located where two of the tectonic plates that make up the Earth's crust collide. The offshore Juan de Fuca Plate runs into and dives under, or "subducts," the North American plate. The plates move in a slow, subterranean collision at an average speed of four centimeters per year.

One might think the slow release of seismic energy along the Cascadian Subduction Zone would be a good thing. The alternative -- an instant release of built-up stress -- can produce the planet's largest quakes, of magnitude-9 or greater.

But Dragert doesn't think the slow quake helps at all. It might even hurt.

"You're still building stress on the locked zone," where friction between the tectonic plates is highest, he said. Instead of accumulating stress continuously over time, as many scientists had assumed, the zone is "getting yanked" with a slow quake, he added.

The last great stress reliever on the Cascadian Subduction Zone was a massive quake that paleoseismic studies have determined hit the Pacific Northwest in January 1700.

If the subduction zone has built up sufficient stress over time, Dragert said, one of these slow silent slips between the plates could trigger the big one.

The discovery of this latest slip took Dragert by surprise. And it took place slowly.

As one of a number of scientific teams in the United States and Canada using GPS satellites to study ground deformation due to tectonic movement over time, his team noticed a "blip" in the data from one of the GPS stations.

"We figured something went wrong at one of the stations," Dragert said. But when he started looking at the data from other stations, they all showed the same blip -- a reversal in the direction of ground movement over a period of a week or two.

"We didn't know what it meant at first," he said.

After extensive analysis of the data last year, Dragert and his colleagues Kelin Wang and Thomas James concluded the land surface in Puget Sound and southern Vancouver Island had been in a sort of westerly rebound from the gradual release of the locked-up area between the two tectonic plates.

For Dragert, this demonstrates the value of using GPS technology for the study of earthquakes and seismic hazards.

"This kind of thing won't ever show up on a seismometer," he said. Ground deformation tracked by GPS will tell us a lot more about the routine behavior of moving rock below us, Dragert said, and may even help forecast unusual build-up in the strains and stresses that can lead to a quake.

"We've known that GPS is a useful tool for monitoring ground deformation," said Tony Qamar, a University of Washington seismologist. "But up until now, we didn't know if it would have any use for earthquake hazard assessment."

The state of knowledge still isn't close to allowing scientists to predict when a specific quake will happen, Qamar cautioned. But gaining a better understanding of the stresses and strains beneath us, he said, will help in accurately estimating general quake risks for specific locations.

1998-2001 Seattle Post-Intelligencer

-- Martin Thompson (, April 20, 2001


bUT Will GPS ever know who had an orgasm where?

-- John littmann (LITTMANNJOHNTL@AOL.COM), April 20, 2001.

Page URL: f=/stories/20010420/538768.html

April 20, 2001

Silent slips could cause giant quake on West Coast Shifting plates off B.C. coast building pressure, scientists warn

Margaret Munro National Post VANCOUVER - Gradual, previously unnoticed earthquakes have been discovered off Canada's West Coast by federal scientists who suspect the geological slips could trigger potentially catastrophic quakes.

Writing in the journal Science today, Dr. Herb Dragert and his colleagues at Natural Resources Canada say a silent slip event occurred in southwestern B.C. over 42 days in the summer of 1999, moving Vancouver Island a few millimetres to the southwest.

The slip would have caused a 6.7- magnitude earthquake if it had occurred instantly. But it occurred so slowly, it was discernible only to scientists using military satellites to monitor the geological forces building along the West Coast.

Scientists have long recognized the existence of these slow quakes -- which are not really earthquakes at all since the Earth does not shake, rattle or roll -- and their potential contribution to the geological stress that can ultimately lead to a real quake.

The tectonic plate under the Pacific Ocean is trying to slide under the plate supporting the North American continent, but the two plates are locked deep underground and storing pressure, the geoscientists say.

The geoscientists, who can measure the coastline and mountains buckling and bulging under the strain, predict the pent-up energy will eventually be released in a giant earthquake that could rip down the coast from Vancouver Island to northern California. Coastal communities and cities including Vancouver, Victoria and Seattle could be hit hard.

Dr. Dragert and his colleagues at the federal Pacific Geoscience Centre use the U.S. military's Global Positioning System of satellites to measure subtle shifts in the landscape as the geological forces build.

They have discovered that mountains on Vancouver Island and northwestern Washington State are moving northeastward at about 1.5 centimetres a year. But the scientists say the movement reversed in August, 1999, and a silent slip event is the most likely explanation.

The slippage occurred gradually, working its way from Puget Sound to Vancouver Island during August and September. Dr. Thomas James, one of the report's co-authors, says there is evidence silent slips also occurred in 1994 and in December, 2000.

The slippage is believed to have occured about 40 kilometres underground, where temperatures are about 500 degrees Celsius and the crustal plate can slide more smoothly, without the friction that sets off earthquakes.

Dr. James says the silent slips suggest the geological stress generated by the colliding tectonic plates does not accumulate steadily, as scientists had thought. Rather, it could build in discrete pulses -- the slip events -- that lead to giant quakes, which occur off the Pacific northwest about every 500 years. The last great earthquake on the west coast of North America occurred Jan. 26, 1700, which scientists documented based on sand layers, buried trees and reports of tsunami in Japan.

The scientists say each silent slip event could bring the locked tectonic plates closer to failure. It is also conceivable a slip event could trigger a larger earthquake.

Silent slip events are considered responsible for the deadly 9.5- magnitude quake in Chile in 1960 and 8.2-megathrust quakes in Japan in 1944 and 1946.

Dr. James stressed in an interview that the discovery of silent slip events off the B.C. coast does not make the earthquake threat any more ominous.

''We don't want to alarm people,'' he said. "It does not change the odds of how often a giant quake will occur or enable scientists to predict when one will strike."

Dr. James and his colleagues would like to expand their GPS network to get a better read on the slips. The Canadian scientists have 12 GPS antennae in their West Coast network. The Japanese have about 1,200 antennae in their earthquake-monitoring network.

-- Martin Thompson (, April 24, 2001.

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