California is known for surf, sun and sand. Less desirable are the earthquake faults that lie beneath the coastal state. Scientists have long been working to understand the complex fault zone in an effort to prepare the region's inhabitants for deadly tremors. Now research presented this week at the fall meeting of the American Geophysical Union in San Francisco that will be published in the December 12 issue of Science indicates that earthquakes along one type of fault in the area could trigger temblors along other faults.

Researchers gained new insight into how earthquakes can interact from a magnitude 7.9 quake along Alaska's Denali Fault in November 2002 that involved a chain reaction of fault shaking. Greg Anderson of the United States Geological Survey and his colleagues analyzed four faults in southern California--the Sierra Madre, the Cucamonga, the San Andreas and the San Jacinto faults--to try to predict how they might affect one another. The fault systems fall into two types: the Sierra Madre-Cucamonga combination is what is known as a thrust fault, in which the two sides get pushed together, whereas the San Andreas and San Jacinto faults are the strike-slip variety, in which the two sides move beside each other in different directions.

The researchers used the geometry of the region and data from previous events to build a computer model to predict future earthquake behavior. They found that a large earthquake along the northern section of the San Jacinto fault could cascade down to the Sierra Madre-Cucamonga system, with the potential to cause a 7.5 magnitude earthquake on the edge of the Los Angeles metropolitan region. The team notes that "many heavily populated regions contain thrust and strike-slip fault networks, so understanding the mechanisms by which faults in such networks interact is critical for improved estimates of seismic hazard and risk in those areas."

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