"There's no way that you could ever feel thisit's just barely above the background noise level. But the most sensitive seismometers can detect them," Shelly said.
Since their discovery in 2002, nonvolcanic tremors have been observed in seismic hotspots such as the U.S. Northwest, California's famous San Andreas fault, and Japan's Nankai trough.
(Read "Deep Tremors Detected Along San Andreas Fault" [December 2004].)
Scientists don't completely understand where and how the tremors originate, but recent data suggest that they may be related to silent earthquakes.
Tremor frequency supports this theory. The tremorsat least those observed in the Japanese troughare intermittent.
Extended quiet periods, the researchers say, are broken by active stretches of days or weeks during which tremors are nearly constantthese active periods may indicate a silent quake in progress.
"There's a very strong connection between a high level of tremor activity and the silent earthquakes. They tend to go together," Shelly explained.
Tom Jordan is the director of the Southern California Earthquake Center and is unaffiliated with the Nature study.
He says the tremors are "a little bit mysterious, and there's an interesting correlation between [silent earthquakes] and nonvolcanic tremors."
"The $64,000 question is, To what extent do these types of signals relate to the large megathrust earthquakes, and can they be used to predict the occurrence of large earthquakes? We don't know that yet."
Data do show that silent earthquakes occurred only 15 minutes before the largest-ever recorded earthquakea magnitude 9.5 shocker that killed more than 2,000 Chileans in May 1960.
Tsunamis spawned by that quake caused death and destruction from Hawaii to Japan.
Theoretically, silent earthquakes could increase the load on faults and produce larger surface quakes.
"When the silent quakes happen, they happen on a part of the fault adjacent to where the large earthquakes occur," Shelly said.
"So when a silent quake happens, it may increase the stress on the part of the fault that's locked and make it more likely to slip in a larger quake."
Gregory Beroza, professor of geophysics at Stanford and study co-author, said, "It's possible that [silent quakes] foreshadow powerful seismic temblors of magnitude 8 and larger.
"Therefore, knowing when a silent event has occurred could contribute to seismic hazard forecasting. We believe that nonvolcanic tremor signals may be useful in monitoring silent quakes, which aren't that easy to detect."
Though the work is promising, it is only a single step forward in earthquake forecastingan area that has proven extremely challenging.
"We're all intrigued by interesting research in this area," Jordan, of the Southern California Earthquake Center, said.
"But we don't want to be optimistic that this would lead to a silver bullet of earthquake prediction."
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