Mysterious Tremors Detected On San Andreas Fault

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                                   Mysterious tremors detected on San Andreas Fault
           




 
Alicia Chang,
Ap Science Writer
– Thu Jul 9, 2009
LOS ANGELES

– Scientists have detected a spike in underground rumblings on a section of California's San Andreas Fault that produced a magnitude-7.8 earthquake in 1857.

What these mysterious vibrations say about future earthquakes is far from certain. But some think the deep tremors suggest underground stress may be building up faster than expected and may indicate an increased risk of a major temblor.

Researchers at the University of California, Berkeley, monitored seismic activity on the fault's central section between July 2001 and February 2009 and recorded more than 2,000 tremors. The tremors lasted mere minutes to nearly half an hour.

Unlike earthquakes, tremors occur deeper below the surface and the shaking lasts longer.

During the study period, two strong earthquakes hit — a magnitude-6.5 in 2003 and a magnitude-6.0 a year later. Scientists noticed the frequency of the tremors doubled after the 2003 quake and jumped six-fold after 2004.

Tremor episodes persist today. Though the frequency of tremors have declined since 2004, scientists are still concerned because they are still at a level that is twice as high as before the 2003 quake.

The team also recorded unusually strong rumblings days before the 2004 temblor.

Results of the research appear in Friday's issue of the journal Science. The work was funded by the U.S. Geological Survey and National Science Foundation.

"The fact that the tremors haven't gone down means the time to the next earthquake may come sooner," said Berkeley seismologist and lead researcher Robert Nadeau.

Nadeau first discovered tremors deep in the San Andreas Fault in 2005. Before that, the phenomenon was thought only to occur in Earth's subduction zones, where one tectonic plate dives beneath another.

USGS seismologist Susan Hough found the latest observations intriguing, but said it's too soon to know what they mean.

"We don't have enough data to know what the fault is doing in the long term," said Hough, who had no part in the research.


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On the Net:

Science:
http://www.sciencemag.org

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                              Tremors may indicate risk of big California quake: journal
           





Fri Jul 10, 2009
LOS ANGELES
(AFP)

– An increase in tremors deep under California's San Andreas fault may be the harbinger of a major earthquake, according to a study out Friday in the journal Science.

Seismologist Robert Nadeau of the University of California at Berkeley reached this conclusion after analyzing tremors along a segment of the San Andreas Fault near Parkfield, California.

Nadeau found that after the 2003 6.5-magnitude San Simeon quake and the 2004 6.0-magnitude Parkfield quake -- both located mid-way between San Francisco and Los Angeles -- tremors became more frequent and underground stress increased at the end of a "locked segment" of the San Andreas fault.

A "locked segment" is as a portion of a fault that has not moved in years and is at high risk of a major earthquake.

The researchers believe that the increase in tremors could mean that stress is accumulating faster than in the past along that segment of the fault, "which ruptured in the moment magnitude 7.8 Fort Tejon earthquake of 1857," read the article in Science.

"We've shown that earthquakes can stimulate tremors next to a locked zone, but we don't yet have evidence that this tells us anything about future quakes," Nadeau said.

"But if earthquakes trigger tremors, the pressure that stimulates tremors may also stimulate earthquakes."

Seismologists believe there is a 70 percent probability that a devastating earthquake will strike California in the next 30 years.

The San Andreas fault runs through much of the western state of California, the most populous in the United States.