Method to monitor quake fault strength eyed

[October 01, 2009]  LOS ANGELES (AP) -- Scientists are releasing results of a study aimed at gauging the strength of earthquake faults, which could help them pinpoint weak ones at risk of breaking and unleashing temblors.

Earthquakes are caused by a sudden slip on a fault. This occurs because of stress buildup that causes the fault to fail or a weakening of the fault itself.

Until now, scientists have not been able to measure a fault's strength directly, said Taka'aki Taira of the University of California, Berkeley, who led the study.

Taira and his team analyzed 20 years of data at Parkfield, which sits on the mighty San Andreas Fault halfway between Los Angeles and San Francisco. It's the most studied earthquake zone in the world, rigged with sensitive instruments to detect minute changes in the Earth's crust.

The team noted small repeating earthquakes along the San Andreas three months after the magnitude-9 Sumatra temblor in 2004 that spawned a deadly tsunami.

In certain regions of the fault zone, they noticed the fractures were filled with fluid. The migration of fluid decreases friction in the fault zone, weakening the fault and increasing the likelihood of an earthquake, the researchers say.

Similar results were observed after the magnitude-7.3 Landers quake in 1992 that shook the Southern California desert.

The study was published in Thursday's issue of the journal Nature and was funded by the National Science Foundation.

Seismologist Susan Hough of the U.S. Geological Survey called the results intriguing but said more work needs to be done to determine the connection between powerful temblors around the world and their impact on the strength of faults elsewhere.

"You need to see this data after multiple earthquakes," she said.

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Nature: http://www.nature.com/nature/

[Associated Press; By ALICIA CHANG]

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