By Andrew Bridges
AP Science Writer
Wednesday, May 9, 2001; 7:29 p.m. EDT

A buildup of stress in the Earth's crust after an earthquake can trigger a sequence of quakes on neighboring faults – sometimes years later, say scientists who studied the two largest quakes to hit Southern California in the last decade.

Seven years and a dozen miles separated the magnitude-7.3 Landers and magnitude-7.1 Hector Mine earthquakes northeast of Los Angeles.

But the researchers say stress in the Earth's brittle upper crust following 1992 Landers quake likely triggered the second.

The first earthquake, followed hours later by the magnitude-6.5 Big Bear earthquake, killed one person and caused $100 million in damage. The 1999 Hector Mine quake resulted in few injuries or damage since it struck a more remote area.

The report, published in Thursday's issue of the journal Nature, suggests that careful monitoring of stress levels in the Earth's crust following earthquakes can help scientists in their quest to predict quakes.

"It's a recognition that earthquakes do occur in clusters and sequences. And if we can understand those sequences maybe we have a chance at understanding when the next ones will occur," said Andrew Freed, a geophysicist at the Carnegie Institution in Washington, D.C., and the paper's co-author.

The estimated magnitude-7.8 earthquake that laid waste to much of San Francisco in 1906 relieved so much stress that the region experienced few quakes for decades.

But others, such as 1989's Loma Prieta magnitude-7.1 earthquake near San Francisco that killed 69, may portend seismic activity to come, Freed said.

In big earthquakes, Freed said, large amounts of stress are induced on the Earth's lower crust and upper mantle. Those regions cannot sustain the stress over time and slowly snap back.

As those regions relax, the stress is taken up by the upper crust, eventually causing earthquakes that can hit, domino-like, months or years later, Freed said.

That stress is relayed between faults, often over miles and years, is not in dispute. Other models point to the movement of groundwater as the prime relayer of the built-up stress.

"The basic tenets of the theory are pretty sound: When you move a fault, you increase stress," said Susan Hough, a U.S. Geological Survey seismologist.

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On the Net:

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

Carnegie Institution: http://www.ciw.edu/