'Silent quakes' may signal big temblors-study

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SAN FRANCISCO, July 6 (Reuters) Locating so-called ''silent earthquakes'' may help scientists better predict the probability of severe temblors, a Stanford University professor said.

Silent earthquakes -- slow-moving events tracked by satellites measuring subtle changes on the earth's surface because they do not broadcast shock waves -- appear to build pressure on fault zones, contributing to weak magnitude-two and magnitude-three earthquakes, said Stanford geophysicist Paul Segall.

Locating silent earthquakes could help better understand how small temblor activity develops and that could help scientists better gauge the likelihood of more powerful quakes, Segall said, referring to findings his research team has published that will appear today in the journal Nature.

Data from silent earthquakes may help scientists develop a ''stress gauge'' for particular fault zones, Segall said.

''Each time you have a silent event it's like you are tweaking the system a little, pushing on it a little harder,'' he said. ''If we could see an increase in the rate of little earthquakes in response to these silent events then we could calibrate the gauge.'' ''That would be potentially a way of making a probabilistic forecast'' for large earthquakes, Segall said. ''We could say that during this few-week period the probability is higher ...

and hopefully be able to put some numbers on the probability.'' Seismologists estimate severe earthquakes strike Japan every 200 years and the Pacific Northwest region of the United States every 500 years. Scientists in the past six years have found southwestern Japan and parts of the Pacific Northwest to be experiencing silent earthquake activity, Segall said.

''It's possible that each time a slow event occurs, and as we get later and later in the cycle, or closer and closer to the really big one, these slow events should start to get bigger, because the area that's getting closer to failure will have grown larger,'' Segall said.

Reuters DH VP0513

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