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
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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.
