Hi All. It seems that by being slow the most recent quake in Washington may have been the deciding factor that prevented it from being a 9.0Md instead of a 6.8Md. Take Care…Don in creepy town.

Thursday, March 1, 2001

Damage limited by deep, slow earthquake

SEATTLE (AP) -- Scientists analyzing preliminary data from the powerful earthquake that rumbled deep beneath the Pacific Northwest have sobering news: The region got off easy.

Seattle and surrounding communities ducked a geological punch in the magnitude 6.8 quake. Scientists said the earthquake's considerable depth -- about 33 miles underground -- blunted its enormous power.

"It's like throwing a pebble into a pond and watching the waves dampen out as they extend," said Tony Crone, associate chief scientist of the U.S. Geological Survey's hazards team. "The earthquake's energy had to travel 30 miles in every direction from its point of origin before it hit the surface."

The Northwest also benefited from a very dry winter. Combining an earthquake with typically rainy Seattle weather could have set off mudslides capable of dragging steeply sloped neighborhoods into Puget Sound.

The epicenter of Wednesday's quake was about 35 miles southwest of Seattle and 11 miles northeast of the state capital, Olympia. Tremors were felt as far away as Portland, British Columbia and Salt Lake City.

The quake was very near the location of a magnitude 7.1 earthquake that occurred in 1949 and a magnitude 6.5 that hit in 1965. In terms of energy released, this earthquake was only about one-third as strong as the 1949 quake.

The Pacific Northwest is earthquake prone, although not necessarily the kind of shallow shakers that beset much of California. Many of those "slip-strike" quakes occur when a fault line in a single geologic plate suddenly moves like a boxer's sharp jab.

Wednesday's quake was more like sumo wrestling. It occurred deep within the Earth's crust where two plates bump and grind in a region known as a subduction zone.

A smaller plate known as the Juan de Fuca extends from British Columbia and the North Pacific. It moves eastward and dives beneath the North American plate, which is moving northwest, at a rate of about 2 inches per year, said USGS seismologist Robert Norris.

The plates carry on this epic underground struggle continuously. Rocks on the edge of the Juan de Fuca plate are chewed and crushed by the pressure, then swallowed deeper into the hot Earth and recycled, eventually to become new rock material and molten lava.

As the lower plate moves deeper, pressure builds up and is released in earthquakes like this one, Norris said.

An earthquake that occurs in the deep zone generally produces less damage but is felt over a larger area, said Gerick Bergsma, a research assistant at the University of Washington Seismology Lab. Aftershocks are rarely felt above ground.

Scientists say the deep geological features that generated Wednesday's quake could have unleashed something much worse, including a magnitude 9.0 earthquake.

"If there were a magnitude 9.0 earthquake in the Pacific Northwest, it could have a global impact," Crone said. "The evidence hints the entire zone could fail in one great earthquake."

That kind of seismic energy could unleash a tsunami, or giant wave, across the Pacific capable of swamping coastal cities in Japan or Korea thousands of miles east.

The region has seen earthquakes that powerful, including one in 1700 that sparked a tsunami detected in Japan.

"We know we can have much larger earthquakes on that interface," said David Wald, a USGS seismologist. "This was not the largest earthquake we can have, not by a long shot."

For more information, see:

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

USGS National Earthquake Information Center at neic.usgs.gov

Berkeley Seismology Lab at www.seismo.berkeley.edu/seismo

Incorporated Research Institutions for Seismology at www.iris.washington.edu