Using Tremors to Gauge the Strain

Overdue. Underground vibrations near the epicenter of the 1857 Fort Tejon earthquake may signal that another big one is on the way.

Mysterious tremors normally associated with volcanic activity have been detected deep on the San Andreas fault in California. Geologists think these vibrations might provide clues about the next big quake.

Some of the first signs that Mount St. Helens was reawakening in September were gentle vibrations that lasted for several minutes at a time. Scientists suspect this type of tremor is caused by cracks opening and closing as magma moves below the volcano. In recent years, similar vibrations have been detected in the Pacific northwest, where the Pacific plate slides below North America, and in a similar subduction zones in Japan. Since then, scientists have discovered that slow slipping deep within such faults is often accompanied by more and stronger tremors. These slow slip events may be straining the upper part of the subduction zone, priming it for a major earthquake.

Intrigued by the connection between tremors and slip events, seismologists Robert Nadeau and David Dolenc of the University of California, Berkeley, went looking for tremors on the San Andreas fault. They studied archived data from a network of sensitive seismic detectors in central California near Parkfield. From 2000 to 2003, they identified 110 tremors between 20 and 40 kilometers deep on the fault--far below the upper 15 kilometers of the crust where earthquakes occur, they report online in Science on 9 December.

Nadeau has also recently found a correlation between the deep tremors and small earthquakes, magnitude 2 or smaller, closer to the surface. This could be increasing strain on the fault, upping the odds of a major rupture. The tremors were located near Cholame, California, near the epicenter of the magnitude 7.8 Fort Tejon quake in 1857. Geologists estimate that segment of the fault has a major earthquake every 140 years, making it overdue for a big one.

The pattern of tremors "might be a clue to when faults in California are more or less dangerous," says geophysicist John Vidale of the University of California, Los Angeles, but it's not likely to provide precise predictions of earthquakes. Vidale would like to see measurements of how much deformation occurs above the deep tremors using GPS or radar imagery. Such measurements, he says, would provide a better picture of how much strain is being loaded onto the fault.

Related sites
Berkeley Seismological Laboratory
A short history of the 1857 Fort Tejon Earthquake (USGS)
More about the 1857 Fort Tejon Earthquake (USGS)
Abstract of Science report by Nadeau and Dolenc

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