EarthquakesMega-quake could strike near Seattle

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 state’s 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 can not feel, occur routinely every 15 months or so and can continue for more than two weeks before they die 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, it could produce a massive 9.0 or greater earthquake and a tsunami.

By comparison, 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.

McClatchy Newspapers’ Les Blumenthal reports that whereas the scientists once predicted that a mega-earthquake would be centered just 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 across Puget Sound. “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, hasn’t been calculated, Malone said.

While there’s still plenty of debate about the findings within the scientific community, and while they may not be consistent with the models that 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. Scientists have spent years studying what’s known as the Cascadia subduction zone, an area where the two tectonic plants collide that stretches roughly