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."