Scientists have detected an increase in mysterious underground tremors along a stretch of the San Andreas fault, signaling stress that could boost the likelihood of a major earthquake.
Seismic tools buried in deep holes near the town of Parkfield, 175 miles south of San Jose, have found that the number of tremors along the fault has increased up to 80 percent over four years, according to University of California-Berkeley seismologist Robert Nadeau and graduate student Aurélie Guilhem.
The study, published in Friday’s issue of the journal Science, offers no precise forecast of a rupture along this restless region. But it may bring scientists one step closer toward the long-sought goal of predicting potentially devastating quakes. The same pressure that stimulates tremors may also stimulate quakes.
Quakes are brief and fairly shallow events; tremors are ongoing, low-level rumblings from perhaps 10 to 20 miles beneath the surface. In Parkfield, each tremor may last three to 21 minutes.
“Tremors are very sensitive to stress changes. They can signal that there are deep stress changes going on that we hadn’t detected before,” said Nadeau. “They may give us useful information about the likelihood of earthquakes — if not now, someday.”
Little has been known about the behavior of tremors, but this study shows that they are far more widespread than once thought. For this study, Nadeau and Guilhem pinpointed the location of nearly 2,200 tremors recorded between 2001 and 2009 by seismometers implanted along the fault as part of UC-Berkeley’s Seismic Network. The increase was noted since 2004, about the time of the 6.0-magnitude Parkfield quake.
Scientists are now searching for tremors in the San Juan Bautista area of Monterey County, as well as Southern California. With current tools, it is difficult to measure tremors in urban regions like the Bay Area because of the many sources of man-made rumbling, such as auto traffic and trains.
It is well established that earthquakes are a source of tremors. Scientists have found that underground stress increased after the 6.5-magnitude San Simeon quake in 2003 and the Parkfield quake.
But there was an unusually strong episode of tremors in the weeks leading up to the earthquakes, which suggests that tremors could precede future quakes.
After both quakes, persistent episodes of tremors have continued — an indication that this stretch of the San Andreas fault did not release stress, as once imagined, but instead has turned into a region of new stress and deformation.
“What’s surprising is that the activity has not gone down to its old level,” before the quakes, said Nadeau.
Most tremors are found on the edge of a “locked” zone, a segment of a fault that hasn’t moved in years and is at high risk of a major earthquake. The new report strengthens that association, said Nadeau.
The recently increased rate of tremors may indicate that stress is accumulating more rapidly than in the past along this segment of the San Andreas fault, which is at risk of breaking like it did in 1857 to produce the great 7.8 magnitude earthquake.
“What we learn here may apply to other tremors that are starting to be discovered,” said Nadeau.
Contact Lisa M. Krieger at 408-920-5565.
