Deep tremors suggest mega-quake could hit closer to Seattle, earlier then thought

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Using sophisticated seismometers and global positioning systems, scientists have been able to track minute movements along two massive tectonic plates colliding 25 miles or so underneath the Puget Sound basin.

Their early findings suggest a mega-earthquake could strike closer to Tacoma and Seattle than earlier thought.

The deep tremors, which can't be felt by humans, routinely occur every 15 months or so and can continue for more than two weeks before dying back to undetectable levels.

The instruments are detecting an inch or two of movement - known as "episodic tremor and slip" - as the Juan de Fuca plate grinds and sinks beneath the North American Plate. Closer to the surface, the two plates are locked together. When they snap, scientists say, the result could be a 9.0 or greater earthquake and a massive tsunami.

But where once they predicted a mega-earthquake would be centered just off the Northwest coast, the scientists, using data from the tremors research, now say it could be 30 miles or more inland, under the Olympic Peninsula.

"The closer you are to the source, the stronger the shaking," said Steve Malone, a research professor emeritus at the University of Washington.

Exactly how much stronger and how much more damage such a quake would cause in the Puget Sound area has not been calculated, Malone said.

While there is still plenty of debate about the findings within the scientific community, and while they may not be consistent with the models geologists have developed, state officials are aware of the latest studies.

"People are aware of the possibility," said John Vidale, a professor of geophysics at the University of Washington and the state's seismologist. "We haven't exactly calculated the impact, but bringing the fault closer (to metropolitan areas) could increase the shaking."

Earlier calculations creating a virtual earthquake using a supercomputer indicated that such a mega-earthquake in the Northwest could result in ground motion of 1.5 feet per second in Seattle, nearly six inches per second in Tacoma, Olympia and Vancouver, and three inches in Portland, Ore. That would be more than enough to cause major damage.

But the virtual earthquake was modeled on the fault being offshore, not one nearly within sight of Puget Sound's metropolitan areas.

"If we are right, it could be a lot stronger," said Malone.

Scientists have spent years studying what is known as the Cascadia subduction zone, an area where the two tectonic plants collide, stretching roughly 600 miles off the Pacific coast of northern California to southern British Columbia.

As the Juan de Fuca plate slides under the North American plate, they can become locked. When plates become locked, pressure builds. The pressure is released in what scientists call a megathrust earthquake, which can easily be magnitude 9.0. The Sumatra-Andaman Islands earthquake the day after Christmas 2004 was a 9.2 megathrust earthquake that produced a devastating Indian Ocean tsunami.

The last megathrust earthquake along the Cascadia subduction zone, estimated at 9.2, was in January 1700. It produced a tsunami that reached Japan. Cascadia subduction zone megathrust earthquakes happen on average every 400 to 500 years, but they can happen as little as 300 years apart or as much as 800 years.

A megathrust earthquake would be different than those that shake the Northwest occasionally. Those come along seismic faults that aren't believed to be tied directly to plate tectonics.

Since the deep tremors were first detected 15 years ago, scientists have been trying to determine what was causing them along the Cascadia subduction zone. Eventually, they concluded the tremors reflected the slippage of the Juan de Fuca plate under the North American plate.

"It's a burst of noise that can go on for up to 24 hours over a period of several weeks," said Herb Dragert, a geophysicist with the Geological Survey of Canada in Victoria, B.C., who was among those who first developed the theory.

What is unique about the deep tremors, which occur in an area stretching from roughly Olympia to Vancouver Island, is that they reappear roughly every 15 months. While tremors have been detected elsewhere along the Cascadia subduction zone, none are as regular or prolonged as those in the Puget Sound basin, Dragert said.

According to the timetable, episodic tremor and slip should be going on just about now. But instead it came this spring, catching scientists by surprise. Malone said some tremors were detected southwest of Olympia last week, but it was too soon to determine if they were part of a new episode or just isolated ones.

If all the energy associated with tremors over two weeks released in just 10 seconds, Vidale said, it would equal a 7.0 earthquake.

Seismographs and other equipment detecting tremors, earthquakes and other ground movements are located at more than 300 stations around the region.

The slippage the tremors are associated with is also adding pressure to the locked zone closer to the surface of the earth, Dragert said.

"Every 15 months it's like tightening the guitar string a little more," Dragert said. "You don't know whether it will take it beyond the break zone."

The deep tremors are being tracked at other subduction zones around the globe, including in Japan, Mexico, Chile and Alaska.

In the Northwest, the tremor and slip zone represents the "deepest and most landward limit" of where a mega-rupture along the subduction zone could occur, Dragert said.

"If the tremors mark the edge of the lock zone," Malone said, "it would be under the Olympics."

Dragert, Malone and Vidale all say they have come a long way over the past decade in understanding the deep tremors.

"We need to know a lot more," Dragert said.
[link to earthweb.ess.washington.edu]