Ground-Shaking Blamed for Aftershocks

By ALICIA CHANG
AP Science Writer

June 7, 2006, 1:02 PM EDT

LOS ANGELES -- New research suggests ground-shaking from an earthquake, not underground stress, can trigger far-away aftershocks.

That may help scientists forecast where aftershocks will strike next. It also counters the popular view that aftershocks tend to cluster near the epicenter.

The new theory grew out of an extensive study of data from almost 20 years of California earthquakes.

Earthquakes occur in clusters. A sizable temblor is followed by numerous smaller jolts called aftershocks that usually hit within minutes or even days after the main event and can cause injuries and damage.

Aftershocks generally become weaker and dwindle over time and distance. Many scientists believe aftershocks come from changes in underground stress patterns caused by movement of the Earth's crust. They tend to be localized and occur along the same segment of the ruptured fault or along nearby offshoots.

But the new analysis published in Thursday's issue of the journal Nature suggests ground-shaking -- not static stress -- is responsible for aftershocks.

Researchers found that aftershocks were more likely to occur in regions rocked by strong shaking and that relatively small temblors can sometimes spark aftershocks miles away, putting a wider swath at potential risk.

"It changes where you expect to find aftershocks," said lead researcher Karen Felzer, a geophysicist at the U.S. Geological Survey in Pasadena.

The notion that shaking can spawn distant aftershocks first surfaced after a 1992 magnitude-7.3 quake split the Mojave Desert east of Los Angeles. Numerous aftershocks and increased seismic activity occurred in places far from the rupture site.

In the Nature study, researchers analyzed thousands of small and medium-sized quakes (between magnitudes 2 and 6) and aftershocks that hit Southern California between 1984 and 2002.

They found evidence of remotely triggered aftershocks minutes after a fault break. In some cases, a small temblor was able to produce aftershocks as far as 30 miles from the fault.

The researchers believe it was ground-shaking that caused those events. How this happens is still unclear.

Felzer also analyzed quake data from Northern California and Japan and found the same pattern.

John Vidale, a University of California, Los Angeles geophysicist who had no role in the study, said it's plausible ground-shaking can influence faraway aftershocks. But Vidale said he's not convinced this occurs near the ruptured fault.

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On the Net:

Nature journal: http://www.nature.com

U.S. Geological Survey: http://www.usgs.gov
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