Stanford Study: Small earthquakes provide clues about seismic activity

By Erica Holt, Bay City News Service

July 13, 2006

Seismologists are one step closer to understanding earthquakes, as a result of a Stanford-led study that found a direct relationship between hard-to-pinpoint "non-volcanic tremors'' and "silent earthquakes,'' which may some day be helpful in seismic forecasting, Stanford geophysics professor Greg Beroza said Wednesday.

The so-called non-volcanic tremors are faint seismic vibrations that originate deep inside active fault zones, but are "much, much too weak to feel,'' Beroza said. "Waves come in continuously for minutes to hours'' whereas waves from an earthquake are impulsive.

First detected in Japan in 2001, the source of the non-volcanic tremors was impossible to locate until Beroza and his colleagues from Stanford and the University of Tokyo found a clear relationship in Japan between the tremor signals and a different seismic movement -- silent quakes.

Stanford graduate student David Shelly is the lead author of the study, which appears in Thursday's issue of Nature.

Beroza said that in his study, researchers found in Japan that low-frequency earthquakes, parts of the non-volcanic tremor that can be measured and located, "correlate nearly 100 percent'' with silent quakes or slow slips.

Slow slips can be detected with Global Positioning System technology. They release so little energy over a long period that they don't rock the ground or show up on seismology equipment. Some scientists think the slow slips may be useful in earthquake prediction someday because they are detectible and may trigger large earthquakes, according to Stanford geophysics professor Paul Segall.

Beroza said he is "convinced they are related because they happen at the same time,'' but there is no empirical proof just yet.

The non-volcanic signals have been recorded in seismically active sections of Japan, the Pacific Northwest and California's San Andreas Fault.

According to the Beroza, silent quakes and tremors occur about once every 14 months in the Pacific Northwest Cascadia subduction zone. The unusual predictability of the events may be useful to determine how much stress is regularly put onto the locked part of the fault, where a devastating earthquake may strike sometime in the next 300 years.

"We don't know what these tremors are,'' Beroza said. "The San Andreas Fault isn't a subduction zone, like Japan or the Pacific Northwest, and a silent earthquake accompanying the tremors there hasn't been detected. It shows that we still have a lot to learn.''

Scientists have discovered a correlation between non-volcanic tremors and small earthquakes on a section of the San Andreas Fault that was struck by magnitude 7.8 temblor 150 years ago in Central California, according to Beroza. One study revealed that a change in the number of tremors might correspond to a similar change in tiny earthquakes weeks later.

According to Segall, while research on small slip and non-volcanic tremors deepens scientists' understanding of earthquakes, they still have a long way to go before it may be possible to predict earthquakes.

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