Mercury News (Link) (August 15, 2009)
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.