3.7 quake under Whidbey Island rattles windows

The 3.7 magnitude earthquake under Whidbey Island came about an hour before most alarm clocks were set to go off, but University of Washington scientists say it should serve as a wake-up call that a much bigger quake is likely in the future.

The deep quake came from the same zone that produced the destructive 6.8 Nisqually quake in 2001, which cracked the state Capitol dome and rained bricks down from historic buildings in Seattle's Pioneer Square neighborhood.

"Every 20 to 30 years, we have one at 6.5 or greater," said UW seismology lab coordinator Bill Steele. "It is the most frequent source of damaging earthquakes in the region and it will produce big ones in the future."

Wednesday's quake was 36 miles deep, in the Juan de Fuca tectonic plate, the same source as the 7.1 Olympia quake in 1949.

The "big one" could come from that plate slipping under the North America plate. That interface has produced quakes of magnitude 8 or 9 about every 500 years. The last one was about 300 years ago.

Each whole number increase on the magnitude scale describes ground motion that is 10 times greater than the previous whole number.

Western Washington experiences frequent quakes of about magnitude 3.0 - about 10 a year - Steele said. Most of these are not felt by many people because the shaking is less because the quakes occur so deep under the Puget Sound.

The Nisqually quake, under the Nisqually River delta south of Seattle between Tacoma and Olympia, was the largest quake to shake Western Washington in more than a half-century.

But that quake, which disrupted operations at Seattle-Tacoma International Airport and damaged the Alaska Way Viaduct through downtown Seattle, was not a "megathrust" quake that would cause catastrophic damage on the West Coast.

A megathrust quake would involve the breaking of a tectonic plate - a piece of the earth's outer shell - and would likely have a magnitude of about 9. The last "megathrust" quake - commonly referred to as "the big one" - happened in the Northwest in 1700.

Washington's previous two destructive earthquakes occurred in 1949 and 1965.

The "big one" could also bring a tsunami as high as 80 to 100 feet, similar to the one that sstruckSumatra in 2004.

Research by Brian Atwater of the U.S. Geological Survey has linked a "ghost forest" in a Washington state tidal marsh, believed to be related to an earthquake, to a tsunami recorded in a Japanese historical document on Jan. 27, 1700.

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On the Net:

http://www.pnsn.org/recenteqs/Quakes/uw07011209.htm