Swarms of tremors could point to quake danger Earthquakes

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Swarms of small tremors deep beneath the ground after two recent quakes in Monterey County may be adding stress to a seismically locked segment of the San Andreas fault and could presage a major earthquake, two Berkeley scientists suggest.

The scientists detected an abrupt increase in tremor activity beneath the tiny San Luis Obispo County town of Cholame, just after two large quakes struck on the San Andreas fault in nearby Parkfield and San Simeon in Monterey County.

That rise in activity, say Robert Nadeau and Aurélie Guilhem of the UC Berkeley Seismological Laboratory, indicates that stress is increasing inside the San Andreas at Cholame, 45 miles southeast of the epicenter of the 1857 Fort Tejon quake at Morgan Peak. The temblor, one of the greatest earthquakes in California history, caused little damage because the region was virtually uninhabited at the time, but the fault has generated no significant movement since then and is considered locked for 185 miles to the south.

Cholame stands just at the northern end of that locked segment of the San Andreas, and the swarm of tremors shuddering faintly beneath the community is continuing today, well after the earthquakes struck only a few miles to the northwest.

Nadeau and Guilhem reported on those tremors and their possible significance today in the journal Science. Nadeau said that increased tremor activity has also been seen in other active fault zones around the world - with the same possible significance.

Deep earth tremors, unrelated to those often felt inside volcanoes that are about to erupt, were first detected less than 10 years ago, and their cause has proved both mysterious and controversial. They occur beneath the San Andreas well below the depth where the fault ruptures in earthquakes and have been detected in swarms more than 20 miles down, where the Earth's brittle crust encounters the more viscous rocks of the mantle.

Nadeau and Guilhem have found records of more than 2,100 tremors in seismometers lowered into deep boreholes near Parkfield, the self-described "Earthquake Capital of the World, where a magnitude-6 temblor struck in 2004. Another quake with a magnitude of 6.5 had struck San Simeon the year before and killed two people.

The most intense swarm of tremors was recorded immediately beneath Cholame, the Berkeley quake scientists said.

"We've shown that earthquakes can stimulate tremors next to a locked (fault) zone, but we don't yet have evidence that this tells us anything about future quakes," Nadeau said in a statement. "But if earthquakes trigger tremors, the pressure that stimulates tremors may also stimulate earthquakes."

Kerry E. Sieh, a seismologist who left the California Institute of Technology recently to direct the Earth Observatory at Singapore, a major new earthquake and volcano research center there, called the report on tremors "very interesting and quite exciting."

"Overall," he said in an e-mail, "I think Nadeau and Guilhem may be on to something very important for medium-term earthquake forecasting."

David Shelly, a geophysicist at the U.S. Geologic Survey in Menlo Park who has studied deep tremors in quake-prone Japan and along the Cascadia subduction zone off the northwestern Pacific coast, said Nadeau's report provides valuable new information on the still poorly understood nature of the deep tremors.

And although he too has suggested that they may be related to "increased seismic hazard," he said it is debatable that the swarms beneath Cholame are signaling that stress is building up beneath the locked segment of the San Andreas there.

"Tremors are exciting, and they may well be reflecting what's going on deep beneath faults," he said. "But they need a lot more monitoring before we can understand how their behavior affects the increasing stresses that build up inside faults before they rupture."