"Analyzing these tremors may help us to better understand deeper processes associated with earthquakes, therefore improving our knowledge of them, including our forecasting capabilities," said Andres Chavarria, a seismologist at Duke University in Durham, North Carolina.
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California's two largest earthquakes—Fort Tejon in 1857 and San Francisco in 1906—occurred on the San Andreas fault, and scientists expect the next "big one" to occur on this fault.
"It certainly is the big dog," Nadeau said.
In recent decades, seismologists have taken particular interest in the Parkfield segment of the San Andreas Fault because of the moderate-size earthquakes (around magnitude 6) that occur there at fairly regular intervals.
Seismologists have installed a dense network of instruments in the Parkfield area to learn more about the earthquakes. Researchers have placed seismometers in bore holes 100 to 200 yards (90 to 180 meters) deep, away from noise sources that typically mask seismic signals from small earthquakes.
"The high sensitivity and low noise level of seismic signals recorded by this network were also extremely helpful for the tremor analysis we performed," Nadeau said.
On September 28, nine months after the scientists ended their analysis, another magnitude 6 earthquake struck the area close to the tremor region.
"Many kinds of instruments were able to record [this event], providing an invaluable source of data for earthquake studies," Chavarria, the Duke University seismologist, said.
Predicting Quakes
The discovery in 2002 of non-volcanic tremors in Japan represented a major advance in seismology. Fluids from diving ocean bottom rocks were believed to be critical for generating the tremors in the subduction zones of Japan and the North American Pacific Northwest.
However, the San Andreas Fault does not subduct. As a result, no source of fluids from the rock of a descending tectonic plate is available.
"Consequently, no one expected tremors to occur beneath the San Andreas Fault and nobody really looked for them systematically," Nadeau said.
Indeed, the tremors may have been occurring all along, while scientists simply failed to detect them. The new findings may stimulate the search for tremors more generally throughout California. Researchers hope that tremors will prove to be a useful tool for forecasting larger earthquakes.
"We still have only a rudimentary understanding of the process that creates the tremor and its significance for earthquakes," said William Ellsworth, chief scientist for the Earthquake Hazards Team at the U.S. Geological Survey in Menlo Park, California.
"This discovery opens an important new window into the workings of the San Andreas Fault that will undoubtedly become a major research focus in the coming years," he added.
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