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A summer spate of Pacific quakes.

Something is stirring off the coast of northern California and southern Oregon, and it's not a sea monster. Four sizable quakes rocked this region during July and August--a testament to the plate tectonic activity along the Pacific Northwest coastline.

The summer shakes began on July 12, when a magnitude 6.9 tremor occurred under the seafloor about 100 kilometers off the coast near Crescent City, Calif. On Aug. 16, a region just south of the July epicenter generated a magnitude 6.2 quake, followed a day later by a magnitude 7.1 quake. Splitting that pair on Aug. 17, an onshore, magnitude 6.0 quake rattled the Cape Mendocino region. While the smaller quake occurred about 200 kilometers south of the others, scientists suspect they are all related, says seismologist Jerry Eaton of the U.S. Geological Survey in Menlo Park, Calif.

The stress sparking these quakes comes from tectonic movement of the seafloor in the region. Inching slowly southeastward, a block of ocean crust called the Gorda plate is colliding with two separate bodies: the North American plate on the east and the Pacific plate on the south. Caught in this complex crash, the Gorda plate is both compacting and slipping beneath North America in an act called subduction. The Gorda quakes interest scientists because mounting evidence suggests that subduction just to the north of this region may generate huge earthquakes along the Pacific Northwest coast (SN: 2/17/90. p.104).
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Title Annotation:earthquakes off the coast of Oregon and California
Publication:Science News
Date:Sep 7, 1991
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