- PASADENA, Calif. (Reuters)
-- A 7.9 magnitude earthquake 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 Wednesday.
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- 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.
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- 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 (9
meters).
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- 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 along California's San Andreas Fault,
including the 1906 San Francisco quake and another major quake in 1857.
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- 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.
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- 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.
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- 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.
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- He said that in some places enormous blocks of rock and
ice fell onto glaciers and valley floors, skidding a kilometer (0.62 mile)
or more out over ice, stream, and tundra.
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- The team of investigators included geologists from several
organizations, including Caltech's Division of Geological and Planetary
Sciences, the U.S. Geological Survey, Central Washington University, and
the University of Alaska.
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- 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.
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- 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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