By Andrea Widener
Contra Costa Times

Mon, Dec. 09, 2002

SAN FRANCISCO - Last month's giant Alaska earthquake triggered small temblors at surprisingly distant sites, including ones in California.

Scientists are still sorting out what caused hundreds of small quakes in the eastern Sierra and Southern California and what they mean for local earthquake chances.

These far-removed rumblings reveal the potential reach of an immense quake such as the 7.9 magnitude Denali temblor.

What researchers learn from the Denali earthquake will help them understand the workings of other large, potentially damaging faults, such as the San Andreas system, which runs through large parts of Southern California and the Bay Area.

The findings are being watched especially closely in California, where immense earthquakes are considered inevitable.

"These are some of the basic things that help us understand how faults build up to failure," said University of Alaska professor Jeff Freymueller. "I think we are going to learn a lot."

Earthquakes as immense as magnitude 7.9 are quite rare, so scientists study them intensely to explain how earthquakes work and what might trigger the next one. Early findings about the Denali earthquake were presented Sunday in a special session at the American Geophysical Union's annual meeting in San Francisco.

Alaska has been home to more than its fair share of big quakes; seven of the ten largest known U.S. earthquakes were in Alaska.

The 7.9 earthquake occurred primarily on the large, well-known Denali fault west of Mount McKinley. The fault has been compared in length and earthquake potential to parts of the San Andreas. The Denali fault is on the boundary between the North American continental plate and a smaller plate in southern Alaska that is twisting away from it.

"This earthquake, its magnitude and location were expected," said Peter Haeussler, a U.S. Geological Survey scientist who examined the fault after it ruptured.

But there were many unexpected things about this quake, including the hundreds of small quakes.

Eleven days earlier, a magnitude 6.7 quake shook the ground just west of where the later, bigger earthquake began. That quake is now considered a precursor to the Denali earthquake, but scientists did not recognize it as significant at the time. Precursor quakes happen in less than five percent of big earthquakes, said Roger Hansen, Alaska's state seismologist.

The Denali quake also occurred over several fault segments. Several Bay Area faults also are broken into segments; understanding the interaction of such segments could help scientists forecast the size of future earthquakes.

In some ways, the ground moved more than scientists expected. There was massive shaking -- and lots of movement -- throughout the region surrounding the fault.

"Fairbanks actually moved 5 centimeters (south) -- not enough to explain (Alaska's) warm winter," Freymueller said jokingly.

The hundreds of small earthquakes triggered by the massive underground shaking are not well understood. They were most pronounced in the Yellowstone area, eastern California and Utah. But the 7.9 temblor didn't trigger many of the small earthquakes in Alaska, and scientists don't understand why.