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Science - Reuters
Geologists: Alaska Quake Caused by Huge Rupture
Wed Nov 20, 3:36 PM ET
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By Dan Whitcomb

PASADENA, Calif. (Reuters) - A 7.9 magnitude earthquake (news - web sites) in Alaska earlier this month was triggered by one of the largest fault ruptures in at least 150 years, rivaling the one that caused the great 1906 San Francisco quake, geologists said on Wednesday.

AP Photo Photo
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Slideshow Slideshow: 7.9 Quake Shakes Alaska

The Nov. 3 quake struck a remote and sparsely populated area near Denali National Park and caused only minor injuries, though it damaged three major highways and forced officials to temporarily shut down the Trans-Alaska pipeline.

A team of geologists who spent a week studying the temblor determined that it was largely caused by a rupture of a 130-mile section of the Denali fault, with horizontal shifts of up to nearly 26 feet.

Kerry Sieh a geology professor at the California Institute of Technology in Pasadena, said that rupture ranks in size alongside those that caused two great earthquakes (news - web sites) along California's San Andreas Fault, including the 1906 San Francisco quake and another major quake in 1857.

Sieh said the Denali and San Andreas are both "strike-slip" faults, meaning that blocks on either side of the fracture move sideways relative to one another. Over millions of years, he said, the shifts have moved southern Alaska miles westward relative to the rest of the state.

He said the shifts have produced a set of large aligned valleys through the middle of the snowy Alaska range, from the Canadian border in the east to Mount McKinley in the west.

Along much of its length, Sieh said, the Denali fault crosses large glaciers and during the quake broke up through the glaciers, offsetting large crevasses and rocky ridges within the ice. The earthquake also shook loose thousands of snow avalanches and rock falls.

He said that in some places enormous blocks of rock and ice fell onto glaciers and valley floors, skidding a kilometer or more out over ice, stream, and tundra.

The team of investigators included geologists from several organizations, including Caltech's Division of Geological and Planetary Sciences, the U.S. Geological Survey (news - web sites), Central Washington University, and the University of Alaska.

The rugged range is traversed by just two highways, and so the scientists used helicopters to access the fault ruptures in the remote and rugged terrain.

The scientists were surprised to discover that the fault rupture moved only east from the epicenter and left the western half of the great fault unbroken, Sieh said, prompting them to speculate that the quake could have been the first in a series of large events that will eventually include breaks farther west toward Mount McKinley and Denali National Park.

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