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Scientists have found that silent rumblings in the Earth may give them a tool to predict large earthquakes. Using satellite data, researchers have identified ground movements that, they say, could be precursors of larger quakes.

Every day, experts say, thousands of earthquakes occur around the world. But nearly all of them are harmless and cannot be felt.

An earthquake occurs when two sides of a fault slip past each other, releasing stored energy called seismic waves. At the Earth's surface this typically causes shaking or movement in the ground. The results are destructive if the motions are very strong.

Paul Segall, an earthquake scientist from Stanford University in California, said "In a conventional earthquake, which I will call a high frequency earthquake, that motion will occur very rapidly," said Paul Segall. "The fault will slip one side with respect to the other at one meter per second. A typical earthquake will take 10 seconds, 15 seconds depending on its size. A very large earthquake like the Sumatra earthquake in 2004 actually lasted for several hundred seconds."

Segall and colleagues have identified what they think are silent quakes that may foreshadow strong, catastrophic temblors.

Unlike regular earthquakes, Segall says, the silent ones cannot be detected by conventional instruments.

"A silent earthquake is an earthquake where the fault slips but slips much too slowly to generate shaking," he said. "It sends out no seismic waves. And these events will last anywhere from 36 hours, 48 hours up to several days, even months or years. We have not known about them in the past because we did not have instruments that were capable of measuring them."

Segall and colleagues published their work on silent earthquake detection in the journal Nature.

Silent slips in the Earth were first discovered in Japan several years ago using global positioning satellite data. They have also been observed in Alaska, Mexico, and the Pacific Northwest.

In Hawaii, Segall found for the first time a series of temblors that followed the silent slips.

"What we found was that they generated swarms of little earthquakes, more or less like you see aftershocks of a conventional earthquake," noted Paul Segall. "These silent earthquakes generated swarms of magnitude two and three earthquakes and that's never been seen before."

Although tremors of these magnitudes are generally not felt, they are detected by ground sensors. Segal speculates that the silent earthquakes are adding stress to the fault, causing minor tremors that could eventually trigger a destructive one.