Mega-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’t 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, in Chile in 1960.
The largest in North America was the 9.2 Great Alaska earthquake in 1964, which 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 8.3.
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.
Seattle Earthquake Risk
Seattle’s tallest buildings are at risk of collapse during a rupture of the Cascadia fault zone in the Pacific Northwest, say U.S. seismic experts. The Cascadia subduction zone is likely to produce the strongest shaking experienced from earthquakes in the lower 48 states, said seismic experts from the California Institute of Technology in Pasadena.
While the Pacific Northwest has experienced little seismic activity in 200 years, there is growing consensus the Cascadia subduction zone ruptures in giant earthquakes exceeding 9 on the Richter scale, Caltech scientists Thomas Heaton and Jin Yang said Thursday at the Seismological Society of America’s annual meeting in Monterey, Calif. The Cascadia subduction zone last ruptured in 1700, Heaton and Yang said.
Simulations at Caltech show such earthquakes, which last for more than 4 minutes and are dominated by low-frequency motions, would be exacerbated by the geography of the Seattle basin and cause severe damage among modern high-rises, said Heaton and Yang.
via Source
