Low-pitched rumbles from deep under California's San Andreas Fault may offer a way to predict future earthquakes, geologists said Thursday.
They said they were recording continuous tremors that sounded almost like "chatter" from deep beneath the surface, much deeper than most quakes. Some began just before a relatively large quake last September, they report in Friday's issue of the journal Science.
"This is new information from an area deep down under the fault we have not been able to look at before," said Robert Nadeau, a seismologist at the Berkeley Seismological Laboratory at the University of California. "If these tremors are precursory to earthquakes, there is potential here for earthquake forecasting and prediction."
The vibrations come from a depth of 12 to 25 miles (20 to 40 kilometers) beneath the surface, near the boundary between Earth's crust and the hot mantle. Most quakes start at a depth of just 9 miles (15 kilometers).
Parallels to Japan and Pacific Northwest
The newly detected tremors start at a point about 15 miles (22 km) southeast of Parkfield, near a town called Cholame. They resemble measurements made at subduction zones in Japan and the Pacific Northwest.
A subduction zone is where one of Earth's tectonic plates is slipping under another. These areas are known for intense earthquake activity.
On Sept. 28, the Parkfield segment of the San Andreas Fault ruptured in a magnitude-6 quake with its epicenter close to the tremor region. There may be a relationship, Nadeau said.
The geologists also identified 110 tremors lasting four minutes or more during the three years that ended with the magnitude-6.5 San Simeon earthquake on Dec. 22, 2003.
'Earthquake Capital'
Parkfield bills itself as the "Earthquake Capital of the World" because starting 20 years ago scientists began studying the area to see if they could predict a large quake that they believe is due to hit there.
Experts believe that Cholame was the epicenter of the last big quake to hit Southern California, the magnitude-7.8 Fort Tejon quake of 1857. Nadeau said big earthquakes hit this area on average every 140 years, so another is due.