'Silent' Quake Gently Rolling Through Puget Sound Area Right Now

SEATTLE - You can't feel it, but at this moment, parts of Washington and British Columbia are having an earthquake. It is a slow-moving trembler that can't be felt and won't cause any injuries or damage. Still, by the end of the event, which already has lasted more than two weeks, it is likely to have released about as much energy as equivalent to a 6.7 quake – similar to that the Nisqually earthquake did in February 2001.

The movement is occurring deep beneath the Strait of Juan de Fuca and parts of Vancouver Island in the area where the Juan de Fuca tectonic plate slides beneath, or subducts, the North American plate. The event began on Feb. 26 and could continue for another two or three weeks.

And scientists are now learning that these quakes may not be all that rare.

For years, a Canadian scientist fretted about tiny tremors that registered on seismographs - when they registered at all - only as background noise.

But about six months ago, Garry Rogers decided that the little high-frequency tremors signaled the slow, so-called "silent earthquakes" deep below the surface of the Earth - quakes that over a period of weeks can release as much energy as damaging surface temblors.

"People have been ignoring them for years," Rogers said of the tremors in a telephone interview Friday from the Pacific Geoscience Center in Victoria, British Columbia.

He and colleagues plan to outline the connection between the tremors and the silent quakes next month at the Seismological Society of America's annual meeting in Puerto Rico.

Now "we know that as soon as we see these tremors, that it's started," he said.

Rogers "has been worrying about this for years now," said a colleague, geophysicist Tony Qamar at the University of Washington's Seismology Lab in Seattle. "In the early days, some people tried to tell him it's not anything to worry about."

Rogers said the tremors "look very much like wind noise," but when he looked outside while they were registering, trees were still and there clearly was no wind.

After studying the data - and realizing that the tremors coincided with Global Positioning System readings that indicated silent quakes or "slip events" - he waited. Researchers at Central Washington University in Ellensburg had determined that the deep quakes occur every 12 to 14 months, so action was expected this spring.

As soon as the tiny tremors started Feb. 26, Rogers got UW seismologist Steve Malone on the phone and said, "This is what they look like - there they are!"

Of course, researchers still need to determine what it all means.

The new information aids scientists' growing understanding as they study data pouring in from seismographs posted around the region to detect surface motion and from the GPS readings, which rely on satellite data and have been tapped for Northwest seismic research since 1995.

Officials at the UW, Central Washington and the Geological Survey of Canada have recorded at least eight "slips" since 1992, each releasing energy about equal to a magnitude 6.7 quake. The last previous one was in February 2002.

While GPS may be best known for helping drivers or high-tech hikers figure out where they are, it has also enabled scientists to determine the movement of the vast tectonic plates that underlie the Earth's surface.

In the Northwest, researchers have learned that steady northwest pressure from the offshore Juan de Fuca plate is shoving it underneath the North American plate at a rate of about 1.5 inches a year, actually moving land at the surface.

A few years back, seismologist Herb Dragert and colleagues at the Geological Survey of Canada concluded that the silent quakes - or "slips" - represented a westerly recoil from that slow, steady shoving. The silent quakes are so deep that their movement is imperceptible to humans.

The place where the two plates overlap, about 20 miles below the Earth's surface, is called the Pacific Subduction Zone, where depth and heat make the rocks relatively "hot and mushy," Rogers said.

The question is whether these remarkably regular adjustments - the deep quakes - contribute to surface quakes by stepping up the pressure, or reduce the frequency of surface quakes by releasing built-up pressure.

Rogers is inclined to support the theory that the silent quakes increase the pressure until one becomes "the straw that broke the camel's back" and the Earth snaps back with a major subduction zone earthquake, with disastrous results on the surface.

But there are arguments and reputable researchers on both sides, Qamar noted - all with extensive data to bolster their viewpoints.

A major subduction zone quake can exceed a magnitude 8 and cause massive damage, land-elevation changes and tsunamis in the Pacific Ocean. Washington experienced its last major subduction zone quake 300 years ago, in 1700.

Scientists believe such quakes occur every 500 years or so - a tough range for emergency planners to work with.

Now that scientists know they can follow the action from the surface, "maybe we can learn something about this area where the mushy rocks are and really understand the physics of what's going on," Rogers said.

Japan, with a similar geological situation and an extensive network of seismographs - some as far as 600 feet below the surface and impervious to urban impacts - has been making strides in the same general sphere of knowledge.

For More Information:

University of Washington Seismic Lab -- www.ess.Washington.edu
Pacific Geoscience Center in Victoria, British Columbia -- www.pgc.nrcan.gc.ca.