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Northwest squeeze: quake in the making.

Northwest squeeze: Quake in the making

Plate tectonic forces are warping the Pacific Northwest in a manner that will eventually lead to a large, potentially giant earthquake there, according to scientists who have studied geodetic measurements in the region. No one knows when the future shock might hit, but its size could rival the largest the world has seen this century.

The Pacific Northwest lies in the middle of a collission zone where the small Juan de Fuca plate crashes into and dives under the larger North American plate. In other parts of the world this "subduction" process has generated huge earthquakes, and geoscientists have long wondered whether the Northwest is also prone to such massive temblors.

Some researchers discount the possibility, suggesting that the Juan de Fuca plate -- moving about 4 centimeters per year -- is sliding smootly beneath the North American continent. But many geoscientists think the small plate has become stuck as it scrapes underneath North America. Such locking action would build energy until the Juan de Fuca plate suddenly bolts forward, generating a tremendous jolt possibly larger than magnitude 9.

To find out what gives deep beneath the Northwest, James C. Savage and Michael Lisowski of the U.S. Geological Survey in Menlo Park, Calif., examined measurements made by bouncing laser light off reflectors in the Olympic Mountains of Washington state to detect subtle warping in the region. They also looked for signs of movement in tide gauge records along the Washington-Oregon coast. Both sets of data indicate the region is storing strain from a locked subduction zone beneath, the researchers report in the April 5 SCIENCE.

The work supports geologic findings along the coastline suggesting the region has experienced extremely large jolts in the past (SN: 2/17/90, p.104). From the evidence, investigators have deduced that the most recent quake probably occurred about 300 years ago. While the interval between quakes runs on average about 600 years, some intervals appear much shorter.
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Title Annotation:plate tectonic forces in the Pacific Northwest will lead to earthquakes there
Publication:Science News
Date:Apr 20, 1991
Words:328
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