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THE NORTHWEST QUAKE

Temblor's 30-Mile Depth Helped Keep Deaths Down

March 01, 2001|USHA LEE McFARLING and KENNETH REICH | TIMES STAFF WRITERS

The Pacific Northwest was spared far greater damage and a potentially high death toll in Wednesday's 6.8-magnitude earthquake mainly because the temblor was centered 30 miles beneath the Earth's surface--too deep to wreak the destruction typically associated with that large a quake, scientists said.

"Shallow, big earthquakes are bad for people. Large earthquakes that are deeper tend to be felt for a large area but not be as damaging," said Rob McCaffrey, a geophysicist at Rensselaer Polytechnic Institute in Troy, N.Y., and an expert on the area where the quake occurred.

California generally experiences the shallower, more damaging type of earthquakes. For example, the 1994 Northridge quake that killed 57 people and caused $40 billion in damage was on a sloping thrust fault three to 12 miles beneath the surface. A similar quake in Seattle could have resulted in a massive loss of life and terrific toll on the area's infrastructure.

"Certainly, if you put this size earthquake directly beneath the city--shallow like in Northridge--you would have expected far greater damage," said Thomas Heaton, an earthquake engineering professor at Caltech.

The 10:54 a.m. earthquake may have shocked many Seattle residents, but it was not a surprise to seismologists--who for decades have known that the Pacific Northwest, like California, is riddled with faults and earthquake hazards.

"This is a tragic wake-up call," said Thomas Brocher, a geologist with the U.S. Geological Survey in Menlo Park, Calif.

Brocher is involved in a massive project to use ultrasound-like scans and seismic waves generated by underground explosions to map the geological structures in the Seattle area that are associated with earthquakes.

"It's a reminder that the Pacific Northwest is earthquake country, and you have to be prepared."

Wednesday's quake occurred about 35 miles southwest of Seattle, on the edge of Puget Sound.

Washington state seismologist Tony Qamar said that it was a type of quake known as a "within-slab," or "within-plate" because it occurred within one of the Earth's plates.

The temblor was centered deep below the surface--in a region where the relatively small Juan de Fuca Plate under the Pacific Ocean is sliding beneath the larger North American Plate that underlies British Columbia, Washington, Oregon and Northern California.

This seismic hot spot is known as the Cascadia subduction zone. This subduction causes major earthquakes and eventually results in volcanic eruptions in the Cascade Range.

"This is a pretty common type of earthquake to occur in that area," said Kate Miller, who heads the department of geology at the University of Texas at El Paso and has studied the subduction zone for a decade.

Similar quakes have hit the area before: There was a 6.5 temblor in 1965, and a 7.1 quake in 1949 killed eight people and caused landslides in the area.

The earthquakes are the result of the incredible tensions in the Earth's depths.

There is stress from the twisting, wedging and bulging of the slab, pressure from the weight of rocks overhead, tension as the slab is pulled downward by gravity and still-unexplained forces that might be caused by minerals metamorphosing into new compounds in the heat and pressure.

While such slab earthquakes are expected, the processes involved are far from being fully understood. A slab can generate several types of earthquakes as it migrates downward during the subduction process.

"The processes that are generating earthquakes, and why the area under Seattle is so active, is still a big question," said Anne Trehu, a professor of geophysics whose building at Oregon State University in Corvallis shook noticeably during the quake.

A leading theory, according to Qamar, is that stretching and pulling of the plate creates most of the tension that triggers the earthquakes.

Wednesday's temblor was one of three types of quakes that afflict the area--and the one least likely to cause significant damage. "It's not a great disaster," said Robert Crosson, a geophysicist at the University of Washington in Seattle. "This definitely fits a pattern we have observed before."

The earthquake with the potential to do the most damage and generate the "Big One" would be a "mega-thrust" quake occurring where the North American Plate meets the Juan de Fuca Plate 50 miles off the Washington and Oregon coasts.

"A quake there might reach magnitude 9 and send a tsunami into Puget Sound. This would be the most devastating," said Seth Stein, a geophysicist at Chicago's Northwestern University.

Recent geological detective work has shown that a major earthquake did occur along the 600-mile-long plate boundary in 1700, causing tsunamis that devastated the Pacific Northwest and sloshed onto the shores of Japan.

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