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.

Earthquake Interactions

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.

An earthquake of magnitude 7.5 to 7.8 is considered "major" (between "strong" and "great") and could cause severe damage to the Los Angeles metropolitan area.

"The shaking and damage from such an event could possibly exceed that of the 'Big One' on the San Andreas Fault," said Anderson. "But it's important to keep in mind that this event is very rare." Scientists refer to the major earthquakes along these known faults as the "Big One." The San Jacinto fault has large earthquakes every 100 to 300 years. The Cucamonga fault has large earthquakes on average every 500 to 1,000 years. The central Sierra Madre fault has large earthquakes perhaps once every several thousand years.

"So we know that at most a small fraction of the large earthquakes on the San Jacinto fault might grow into the much larger event," said Anderson.

The San Andreas Fault

The Duke University scientists studied the structure of the notorious San Andreas Fault near Parkfield, California, using seismic waves measured from a 7,100-foot-deep drill hole.

The hole, known as the San Andreas Fault Observatory at Depth Pilot Hole, is part of the $20 million National Science Foundation Earthscope initiative to monitor the processes controlling earthquake generation in a seismically active fault.

By studying readings from a chain of 32 seismometers installed in the hole, Chavarria and colleagues determined that the San Andreas Fault, which produced the devastating 1906 San Francisco earthquake, is perhaps more complex than previously thought.

"Using very unique recordings of microearthquakes in the pilot hole, we have been able to obtain images of the San Andreas Fault at depth that show a system of four secondary faults adjacent to the main San Andreas," said Chavarria.

The scientists created those images by tracing the complex paths that earthquake waves took after scattering off the possible underground structures.

"Going deeper in the earth we are closer to the source of earthquakes and therefore we can understand these phenomena better," said Chavarria.

Next summer, scientists will extend the drill hole into the fault zone to construct a major earthquake "observatory."

"This knowledge will allow seismologists to better determine which processes happen right before an earthquake starts," said Chavarria. "This will help us to better understand the risks associated [with] earthquakes as well to better prepare for such events."