You can't feel them, hear them or see them, but swarms of tiny, noisy tremors occurring sporadically miles below the Earth's surface are actually slow-moving earthquakes that may help predict a catastrophic temblor.
Just six years ago, scientists didn't even know the earth was weakly shaking in parts of California, Washington and Japan. But in an important study on the phenomena published today in the journal Nature, Stanford researchers say the tremors are caused by the same mechanism as major quakes, slipping on the fault plates, and that they could signal times of increased seismic risk.
"It's an important new window into the understanding of how earthquakes work," said William Ellsworth, a seismologist with the U.S. Geological Survey who was not involved in the study. "This is a newly discovered way that faults move."
These quake swarms, which can't be felt, shouldn't be confused with the small earthquakes that have been reported in the Bay Area recently.
The new study gives much more weight to the theory that the deep tremor is caused by seismic slips. Previously, some scientists had felt the fault quivers were caused by the movement of fluids in the ground.
"One of the important conclusions from this paper is that this tremor is composed of lots of tiny earthquake-like slip events that are all superimposed together. And so you get this complex pattern of waves, which are hard to interpret," Ellsworth said.
But, he said, "they've cracked the code."
The researchers were able to show that the multiple slips of a fault created a complex noise pattern, a chattering of the earth.
For years, scientists had brushed those sounds off as the wind rustling through trees near their seismologic equipment -- or, perhaps, a train going by.
But now they know it's the earth, doing something it could take decades for them to fully understand.
In fact, scientists concede they still know little about what they're calling "non-volcanic tremors" and "low-frequency earthquake swarms."
Do they occur along every fault -- and in other states and countries? Why are they so much smaller and slower than traditional quakes? And, perhaps most important, could they actually trigger a Big One?
"They could be related to the kind of earthquakes everybody's worried about, but no one's documented that with real data," said Gregory Beroza, a professor of geophysics at Stanford University and one of the study's authors. "It's an intriguing possibility."
Scientists know that the tiny quakes relieve pressure on faults at the point at which they occur -- 20 to 30 miles beneath Shikoku, Japan, and about 15 to 20 miles below the Central California town of Cholame. But bigger quakes occur higher on the faults, and the tremors seem to increase the stress on those parts of the faults, suggesting that these otherwise tiny, harmless earthquake swarms could ultimately cause the faults to erupt.
"A tremor can be thought of as hundreds of thousands of these tiny, slow quakes," Beroza said.
"The probability of a big quake goes up every time these tremors occur," said Robert Nadeau, a seismologist at UC Berkeley who discovered tremors in recordings taken from seismometers adjacent to Cholame and nearby Parkfield. But scientists don't know how much the tremors increase the likelihood of a major quake.
The hope is that one day, as scientists come to understand tremors better, they might be able to gauge exactly when a catastrophic quake will occur, Nadeau said. Now, however, earthquake forecasts just offer the probability that a sizable temblor will hit a certain fault in the next 20 or 30 years.
Scientists say that low-frequency earthquake swarms are extremely small, releasing about as much energy as a magnitude 1 or 2 earthquake. They can't be felt and are only detected through sensitive seismologic equipment.
The tremor sequences are very slow-moving, lasting days or even weeks at a time, unlike typical quakes, which are over in seconds.
And they're somewhat predictable. In the Cascadia "subduction zone" in northwestern Washington, where one plate of earth slides beneath another, they've been occurring every 13 to 15 months, Beroza said. "Wouldn't it be nice if ordinary earthquakes were so predictable?"
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