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In 2004, a magnitude 6.0 earthquake ripped through southern California on the San Andreas fault. It struck near the sparsely populated town of Parkfield. There were no injuries or fatalities. During the three months before the main shock, finely tuned instruments lining the fault picked up hints of quivering. If a new study is right, these faint tremors could be a first crucial step toward predicting earthquakes. In a new analysis of seismic data collected before and after the quake, David Shelley of the United States Geological Survey in Menlo Park, Calif., has found what he thinks is a precursor to the quake.
Shelley's analysis uncovered a swarm of tremors barely more than a whisper above normal seismic noise. They formed a pattern, moving steadily from north to south about 25 kilometers (15.5 miles) below Earth's surface. At that depth, some six miles below where the main shock would later hit, faults become mysterious places. They can slip several feet without causing more than a seismic shimmy. Since 2002, scientists have started finding tiny tremors linked up with these "slow slip events." Such events could transfer stress from the deep parts of a fault toward the surface, where rocks stay locked until rupturing violently.
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