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

U.S. Geological Survey: http://www.usgs.gov

San Andreas Fault at Wallace Creek

Kite Aerial Photography

The San Andreas fault, which is more than 700 miles (1100 kilometers) in length, is the boundary between the Pacific and North American tectonic plates. The portion of the fault running through the Carrizo Plain National Monument, south of Parkfield (see map) is unique because the surface expression of the fault trace is very well defined. The Carrizo Plain is an arid environment and therefore the fault has not been significantly eroded in this region.

These photos were taken using a technique called Kite Aerial Photography (KAP). KAP has a long history, beginning in France in 1889 when Arthur Batut lifted his camera above Labruguière. In 1906, George Lawrence flew his panoramic camera above San Francisco to document damage resulting from the great earthquake.

Interactive, 360 degree panorama of the San Andreas Fault

On January 9, 1857, the M 7.9 Fort Tejon earthquake occurred just north of the Carrizo Plain. At Wallace Creek, in the Carrizo Plain, the fault moved 30 feet (9m), forming the offset stream channel seen in the interactive photo above. The rupture zone extended nearly 220 mi (350km) from near Parkfield at the northwest end to the vicinity of San Bernardino, east of Los Angeles. Click image for an interactive panorama

Photo showing the kite that is suspending the camera over the San Andreas Fault.

Photos by Scott Haefner, USGS.

View looking southeast along the surface trace of the San Andreas fault in the Carrizo Plain, north of Wallace Creek. Elkhorn Rd. meets the fault near the top of the photo. Click image for a larger version

Closeup shot of the same area above. The cross-cutting feature is a road cut going through the fault. Click image for a larger version

See Also

• Geologic Guide to the San Andreas Fault at Wallace Creek
• 1857 Fort Tejon Earthquake