UW Today

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December 2, 1998

Subduction zone quake could shake Puget Sound area harder than expected

News and Information

SAN FRANCISCO – Recent satellite measurements by University of Washington seismologists indicate the “locked zone” between the Juan de Fuca and North America plates is wider in the Seattle area than previously believed. That means the Puget Sound lowlands are likely to experience significantly greater motion during a subduction-zone earthquake than scientists earlier thought.

A poster to be presented Sunday (Dec. 6) at the fall meeting of the American Geophysical Union in San Francisco displays measurements showing the Seattle area is moving to the east-northeast at 3 to 5 millimeters a year relative to eastern Washington. While that is less than the 10 millimeters a year measured on the Washington coast, it is greater than existing earthquake models predicted, said Anthony Qamar, a UW research associate geophysics professor.

The measurements indicate that in western Washington, the area where the Juan de Fuca and the North America plates are locked together is wider than previously thought. Therefore, an earthquake in the Cascadia subduction zone would bring stronger shaking than expected to the Puget Sound area.

“It is the sudden unlocking of the two plates and the accompanying rebound of the stressed plates that creates the giant subduction earthquakes that visit the Pacific Northwest every 500 years on average,” Qamar said.

He is to present the poster with Giorgi Khazaradze, a UW geophysics graduate student, and Herb Dragert, a research scientist with the Geological Survey of Canada.

The Washington coastline is being compressed at an even greater rate than the Puget Sound region because it is much nearer to the area just offshore where the Juan de Fuca plate dives beneath the North America plate, they said. But, unlike the relatively steep dive that occurs off the coast of southern Washington and Oregon, the Juan de Fuca plate takes a shallower angle in the Puget Sound region. That means it takes longer for the brittle rocks to reach a depth at which they become plastic and easily slide past the North America plate. The locked zone is the shallow region where the brittle rocks of the two plates come together and do not slide past each other except in great earthquakes.

Since the Puget Sound region is farther from the line where the two plates initially meet, the shaking during an earthquake would be less intense in Seattle than on the coast. However, that is offset by indications the locked zone is wider than previously believed. That means the fault on which the earthquake waves originate is wider and therefore closer to Seattle, which would add to the level of shaking in the Puget Sound area.

The measurements also show that besides being squeezed from west to east by the Juan de Fuca plate, the region also is being squeezed from south to north because California is being dragged northward against Oregon and Washington. The east-west movement is currently faster, but the resulting stresses are relieved every few hundred years by great earthquakes in the Cascadia subduction zone. The pressure from the south actually is stronger but it accumulates more slowly. That pressure appears to be the cause of periodic shallow earthquakes on fissures such as the Seattle Fault, Qamar said.

The measurements came from six global positioning system (GPS) stations in western Washington installed in 1995 and 1996. Radio signals broadcast by GPS satellites are used to measure the changing distance of the stations relative to a station at Penticton, B.C.

The original stations, paid for by the U.S. Geological Survey, were the precursor to the Pacific Northwest Geodetic Array, a system of GPS stations operated by the UW, Central Washington University, Renssalear Polytechnic Institute, Oregon State University, the U.S. Coast Guard and the Geological Survey of Canada. New stations are being paid for with grants from the USGS and the National Science Foundation.

It will take two or three years of additional measurements from the entire array in British Columbia, Washington and Oregon, which now includes 10 measuring sites, to confirm the theory of the wider locked zone, Qamar said. Eventually, he said, the data will lead to better ability to predict earthquake hazards.

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For more information, contact Qamar at (206) 685-7563; by e-mail at tony@geophys.washington.edu. During the meeting Dec. 6-10, you may leave a message for Qamar at the San Francisco Hilton, (415) 771-1400, or contact the AGU press room at (415) 905-1007.