for National Geographic News
Two new studies describe California's earthquake zones as more complexand possibly more lethalthan previously thought.
Using sophisticated computer modeling, researchers from the U.S. Geological Survey found that a large earthquake along the northern region of the San Jacinto fault could trigger a cascading rupture of the Sierra Madre-Cucamonga system, potentially causing a major earthquake of magnitude 7.5 to 7.8 near the Los Angeles metropolitan region.
In a separate study, scientists monitored tiny "microearthquakes" along a section of the notorious San Andreas Fault, which runs from Mexico to San Francisco, to create images of what the inside of the fault looks like. The results show that the fault zone has four secondary faults that may connect to a deeper and older part of the fault system. The fault zone is also composed of multiple cracks or possibly pockets of fluid.
"[Our research] suggests a complex fault zone where stresses could be distributed in complicated ways," said Andres Chavarria, a senior graduate student in seismology at Duke University in Durham, North Carolina, and the lead author of the second study. "Some parts of the fault zone may accumulate more stresses than others, therefore becoming more susceptible to larger earthquakes."
The reports are published in the December 5 and December 12 issues of the research journal Science.
A magnitude 7.9 earthquake occurred in a remote part of Alaska on November 3, 2002, when a rupture along a relatively small fault immediately triggered a much greater rupture along a main fault, known as the Denali fault.
The event prompted some experts to speculate that such a chain reaction could happen in the Los Angeles area, where the earthquake faults are structured similarly to those in Alaska.
The USGS researchers studied the Sierra Madre, Cucamonga, San Andreas, and San Jacinto faults in southern California to see how they may interact.
The Sierra Madre-Cucamonga system is a "thrust" fault; the sides are being pushed towards each other. The San Andreas and San Jacinto faults are "strike-slip" faults; the two sides are moving in opposite directions, horizontally, along the fault plane.
Using the geometry of the faults in the region, physics of the earthquake slip and data from previous ruptures, the researchers created computer models for future earthquakes. The results show that while the rupture of a thrust fault system is unlikely to trigger rupture of the strike-slip faults, it could happen the other way around.
"Under certain, very rare circumstances, a large earthquake on the northern San Jacinto fault near Riverside and San Bernardino could trigger a cascading rupture of the Sierra Madre-Cucamonga fault system, which could have a total magnitude of 7.5 to 7.8," said Greg Anderson, a geophysicist formerly with the USGS and now working for UNAVCO, Inc. in Boulder, Colorado. Anderson led a team of three researchers in the study.
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