WASHINGTON Using sophisticated seismometers and GPS devices, scientists have been able to track minute movements along two massive tectonic plates colliding 25 miles or so underneath Washington states Puget Sound basin. Their early findings suggest that a mega-earthquake could strike closer to the Seattle-Tacoma area, home to some 3.6 million people, than was thought earlier.

The deep tremors, which humans cant feel, occur routinely every 15 months or so and can continue for more than two weeks before becoming undetectable.

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, it could produce a massive 9.0 or greater earthquake and a tsunami.

The largest earthquake ever recorded was 9.5 on the Richter scale, in Chile in 1960. The largest in North America was the 9.2 Great Alaska Earthquake in 1964, which killed nine people and spawned a tsunami that struck the Northwest coast. The 1906 San Francisco earthquake, which killed 750 to 2,500 people, was estimated to be an 8.2.

Under pressure

Whereas the scientists once predicted a mega-earthquake would be centered off the Northwest coast, now using data from the tremors research they say that it could be 30 miles or more inland, under the Olympic Peninsula, which lies to the west of Seattle and Tacoma.

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, hasnt been calculated, Malone said.

Scientists have spent years studying whats known as the Cascadia subduction zone, an area where the two tectonic plants collide that stretches roughly 600 miles off the coast of Northern California to southern British Columbia, Canada.

As the Juan de Fuca plate slides under the North American plate, the two can become locked. When plates become locked, pressure builds. The pressure is released in what scientists call a mega-thrust earthquake, which easily can be magnitude 9.0.

The Sumatra-Andaman Islands earthquake the day after Christmas in 2004 was a 9.2 mega-thrust quake that produced a devastating Indian Ocean tsunami that killed an estimated 233,000 people in 11 countries.

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

Different type of quake

A mega-thrust earthquake would be different from those that shake the Northwest occasionally. A mega-thrust quake occurs right on the boundary of two tectonic plates, while other earthquakes occur along cracks in the plate. John Vidale, a professor of geophysics at the University of Washington and the states seismologist, likened whats going on beneath the Earths crust to a bunch of blocks jostling around. Where the smaller blocks collide, you can have more standard-type quakes. Where the biggest blocks the tectonic plates collide, you have a mega-thrust earthquake.

Whats unique about the deep tremors, which occur in an area stretching roughly from Olympia, Wash., to Canadas Vancouver Island, is that they reappear about every 15 months. While tremors have been detected elsewhere along the Cascadia subduction zone, none is as regular or as prolonged as those in the Puget Sound basin, according to Herb Dragert, a geophysicist with the Geological Survey of Canada in Victoria.

Every 15 months, its like tightening the guitar string a little more, Dragert said. You dont know whether it will take it beyond the break zone.

On the Web

Pacific Northwest Seismic Network:

www.ess.washington .edu/SEIS/PNSN

U.S. Geological Survey: www.usgs.gov

Geological Survey of Canada: http://tinyurl.com/5n97nf