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Alaska Quake Thrills Seismologists
9 December 2002 (All day)
SAN FRANCISCO--A huge earthquake that ripped across central Alaska last month has unleashed a cascade of research. The magnitude 7.9 event bore a strong resemblance to past breaks on California's San Andreas fault, promising insights into what might set off the next big shaker there. The Alaskan quake also created new puzzles about how seismic waves trigger unrest thousands of kilometers away.
Seismologists usually don't celebrate massive earthquakes, but the 3 November temblor killed no one and damaged few structures. Even the Alaskan oil pipeline survived by sliding as designed (Science, 15 November, p. 1316). Clues about the quake are pouring in from fieldwork, airborne surveys, seismic records, satellites, and eyewitness accounts. "This earthquake is a really big deal," says seismologist Robert Wesson of the U.S. Geological Survey (USGS) in Denver, Colorado. In response, his exhausted but happy colleagues convened a session of late-breaking research here on 8 December at a meeting of the American Geophysical Union.
The earthquake scarred 342 kilometers of land along three faults in a chain reaction, starting with an upward lurch on a hidden "thrust fault" that may not have budged in the last 10,000 years. The rupture then rifled onto the long, curving Denali fault, where side-to-side slippage reached nearly 9 meters. Few seismic instruments recorded motions close to the remote fault, but researchers found an unusually narrow zone of intense shaking by looking at the positions of thousands of landslides and analyzing 3313 accounts submitted by residents over the Internet.
The reconstruction suggests a parallel with Southern California: The Denali quake is a dead ringer for one that shook 350 kilometers of the San Andreas fault in 1857. Moreover, the quake's origin on a thrust fault is "incredibly significant," says USGS seismologist Peter Haeussler in Anchorage, Alaska, because it may help pinpoint potential triggers by many similar faults that splay off locked sections of the San Andreas.
Denali also spawned swarms of harmless quakes at sites thousands of kilometers away, including Mount Rainier in Washington, Yellowstone National Park, and thermal regions in California. Researchers don't understand this phenomenon, first seen in 1992 after the magnitude 7.4 Landers earthquake in Southern California. Closer to the fault, another odd pattern emerged: Little quakes increased at only one of Alaska's 24 active volcanos and actually subsided at two others in Denali's wake. Volcanologists haven't yet proposed an explanation.