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Study: Tiny tremors may foretell big earthquakes

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Beth Winegarner
Examiner Staff Writer
March 15, 2007

Stanford researchers have discovered a new kind of tiny temblor that may foreshadow major seismic events, such as earthquakes.

A new study by Stanford geophysicists Greg Beroza and David Shelly released Thursday examines minuscule tremors deep under the earth’s crust that appear to simultaneously relieve pressure on one part of a fault line while increasing the load on another part.

Unlike typical earthquakes, which last a few seconds each, these tremors can shake the ground for hours or even days, according to study spokesman Mark Schwartz.

“These tremors are happening in places and ways that could encourage an earthquake on shallower parts of the fault, but so far it’s premature to say it’s goingto help us predict earthquakes,” Beroza said.

Sensors in fault lines in California, Japan and Canada have detected the tremors, which are so small they can’t be measured on the Richter scale. The tiny movements have been detected in the California town of Cholame, just northeast of Cambria, but not yet in the Bay Area, according to Beroza.

The findings come at a time when Bay Area residents have been unsettled by a series of small temblors in the East Bay, including several that hit Berkeley in December and a recent rash in Lafayette, Walnut Creek and San Pablo.

While those quakes aren’t the same as the tremors Stanford scientists studied, they all occurred along the Hayward fault, whose structure is similar to those where such minuscule shaking could predict bigger earthquakes to come, according to Robert Nadeau, an assistant seismologist at UC Berkeley.

“We can’t see anything, but more sensitive equipment may find them if they’re there,” Nadeau said.

Like the southern Japan and San Andreas faults, the Hayward fault has a “lock zone” — an area where plates are stuck fast and any small movement might set off a chain reaction leading to a larger quake.

The Hayward fault has a 27 percent chance of seeing a major earthquake in the next 30 years, according to a 2003 study from the United States Geological Survey. The San Andreas fault’s chances are pegged at 21 percent, according to William Ellsworth, a geophysicist with the survey.

While researchers believe large earthquakes happen every 100 years or so on the San Andreas fault — the last was the 1906 quake— the Hayward fault sees major action every 150 years, and its most recent “big one” occurred in 1868, Ellsworth said.

This article originally appeared in the San Francisco Examiner.

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Written by Beth Winegarner

March 15, 2007 at 10:47 PM

Posted in environment, Menlo Park

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