Alaska shook, mountains spoke

From San Francisco, at a meeting of the American Geophysical Union

The largest inland earthquake to strike North America in more than a century shook south central Alaska on Nov. 3, 2002 (SN: 11/16/02, p. 307: Available to subscribers at Shaked Alaska: A sleepy fault wakes and reveals new links). Small pulses in atmospheric pressure detected in Fairbanks soon after the quake suggest that the magnitude 7.9 temblor literally moved mountains, briefly turning them into 3-kilometer-tall granite loudspeakers.

The quake began at a site along the Denali fault about 135 km south of Fairbanks. From there, the slippage sped toward the southeast along the fault at a rate of about 3.3 km per second, says geophysicist Charles R. Wilson of the University of Alaska in Fairbanks. At some spots along the total 320-km slippage, fault surfaces suddenly moved almost 9 meters past each other.

The first seismic waves from the quake arrived in Fairbanks about 27 seconds after the slippage began, says Wilson. About 12 minutes later, an array of sensitive microphones near the city began detecting minuscule variations in atmospheric pressure. During the following 10 minutes, the apparent source of this infrasound–pressure waves with frequencies below those that people can hear–gradually moved from the south to the southeast, tracking the movement of the fault rupture.

All the data suggest that the sudden motion of tall mountains in the Alaska Range, which lies along the Denali fault, disturbed the air and created the infrasonic pulses, says Wilson.


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