Underwater Plate Cuts 400-Mile Gash

(From the N.Y. Times) The magnitude 8.8 earthquake that struck off the coast of Chile early Saturday morning occurred along the same fault responsible for the biggest quake ever measured, a 1960 tremor that killed nearly 2,000 people in Chile and hundreds more across the Pacific. Both earthquakes took place along a fault zone where the Nazca tectonic plate, the section of the earth’s crust that lies under the Eastern Pacific Ocean south of the Equator, is sliding beneath another section, the South American plate. The two are converging at a rate of about three and a half inches a year.

Earthquake experts said the strains built up by that movement, plus the stresses added along the fault zone by the 1960 quake, led to the rupture on Saturday along what is estimated to be about 400 miles of the zone, at a depth of about 22 miles under the sea floor. The quake generated a tsunami, with small surges hitting the West Coast of the United States and slightly larger ones in Hawaii and other parts of the Pacific. A 7.7-foot surge was recorded in Talcahuano, Chile.

Jian Lin, a geophysicist with the Woods Hole Oceanographic Institution, said the quake occurred just north of the site of the 1960 earthquake, with very little overlap. “Most of the rupture today picked up where the 1960 rupture stopped,” said Mr. Lin, who has studied the 1960 event, which occurred along about 600 miles of the fault zone and was measured at magnitude 9.5.

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