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11/20/2002 08:00:00

Rupture of Denali fault responsible for 7.9-magAlaskan earthquake of November 3

Geologists just back from a reconnaissance of the 7.9-magnitude Alaska earthquake of November 3 confirm that rupture of the Denali fault was the principal cause of the quake.

According to Caltech geology professor Kerry Sieh, Central Washington University geological sciences professor Charles Rubin, and Peter Haeussler of the U.S. Geological Survey, investigations over a week-long period revealed three large ruptures with a total length of about 320 kilometers. The principal rupture was a 210-kilometer-long section of the Denali fault, with horizontal shifts of up to nearly 9 meters (26 feet). This places the rupture in the same class as those that produced the San Andreas fault's two historical great earthquakes in 1906 and 1857. These three ruptures are the largest such events in the Western Hemisphere in at least the past 150 years.

Like California's San Andreas, the Denali is a strike-slip fault, which means that the blocks on either side of the fracture move sideways relative to one another. Over millions of years, the cumulative effect of tens of thousands of large shifts has been to move southern Alaska tens of kilometers westward relative to the rest of the state. These shifts have produced a set of large aligned valleys that arch through the middle of the snowy Alaska range, from the Canadian border on the east to the foot of Mount McKinley on the west. Along much of its length the great fracture traverses large glaciers. Surprisingly, the fault broke up through the glaciers, offsetting large crevasses and rocky ridges within the ice.

At the crossing of the Trans-Alaska pipeline, approximately in the center of the 320-kilometer rupture, the horizontal shift was about 4 meters. Fortunately, geological studies of the fault prior to construction led to a special design that would have allowed for shifts greater than this without failure of the pipeline.

The earthquake shook loose thousands of snow avalanches and rock falls in the rugged terrain adjacent to the fault. Although most of these measured only a few tens of meters in dimension, many were much larger. 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, 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.

Before departing for the field, the geologists had learned from seismologists the basic character of the rupture. Within a day of the quake, Caltech seismologist Chen Ji had determined that the shift along the fault was principally horizontal, but that the initial 20 seconds of the eastward-propagating crack was along a fault with vertical motion. This fault was discovered midweek, near the western end of the principal horizontal shift. Along this 40-kilometer-long fault, a portion of the Alaska range has risen several meters.

Perhaps the most surprising discovery in the field was that the fault rupture propagated only eastward from the epicenter and left the western half of the great fault unbroken. Several members of the team wonder if, in fact, this great earthquake is the first in a series of large events that will eventually include breaks farther west toward Mount McKinley and Denali National Park.

Contact: Robert Tindol (626) 395-3631