2004 Calif. quake's aftershocks studied

ATLANTA, Nov. 25 (UPI) -- U.S. scientists say they've determined the 2004 earthquake along the San Andreas fault in California had 11 times more aftershocks than previously thought.

Georgia Institute of Technology researchers said they used a technique normally employed to detect weak tremors and discovered the magnitude 6 earthquake along the Parkfield section of the fault exhibited nearly 11 times more events during the first first three-day period following the main quake.

"That's surprising because this is a well-instrumented place and almost 90 percent of the activity was not being determined or reported," Assistant Professor Zhigang Peng said.

Peng and graduate research assistant Peng Zhao discovered the earliest aftershocks occurred in the region near the main event. Then with time, the aftershocks started migrating.

"Basically, the big event happens due to sudden fault movement, but the fault doesn't stop after the main event," Peng said. "It continues to move because the stress has been perturbed and the fault is trying to adjust itself. We believe this so-called fault creep is causing most of the aftershocks."

The research appears online ahead of print in the journal Nature Geoscience.