Alaska quake was Denali’s fault

Geologists who surveyed the 7.9-magnitude Alaska earthquake of November 3 have confirmed its principal cause was rupture of the Denali fault.

According to Caltech professor of geology 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 (almost 200 miles). The principal rupture was a 210-kilometer 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 great earthquakes in 1857 and 1906. These three ruptures are the largest such events in the Western Hemisphere in the past 150 years.

Like California’s San Andreas, the Denali is a strike-slip fault, meaning that the blocks on either side of the fracture move sideways relative to one another. Over millions of years, many thousands of large shifts have moved 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.

The earthquake shook loose thousands of snow avalanches and rock falls in the rugged terrain adjacent to the fault, closing numerous roads. Fortunately, no deaths resulted, and injuries and structural damage were limited. At the crossing of the Trans-Alaska pipeline, the horizontal shift was about 4 meters, triggering an automatic protection system. Although the temblor damaged a number of its supports, the pipeline itself reportedly suffered no breaks.

The investigators, who included geologists from Caltech’s Division of Geological and Planetary Sciences, the U.S. Geological Survey, Central Washington University, and the University of Alaska, used helicopters to reach the fault ruptures in the remote and rugged terrain.

Before departing for the field, the geologists had learned the rupture’s basic character from seismologists. 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 was that the fault rupture propagated only eastward from the epicenter and left the western half of the great fault unbroken. Several of the researchers wonder if this earthquake might be 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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