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San Andreas Fault watchers find fresh quake warnings

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Strange things are happening along the San Andreas, the premier geologic fault in California. * The Mohave Desert has grown by several acres in the last few months, a result of stretching of the earth's crust in that region.

* Radon, an exotic gas that Russian scientists say is a "precursor" of earthquakes, is bubbling out of wells along the San Andreas in increasing quantities.

* Alterations in the earth's magnetic field, and perhaps its gravitational field, in the area suggest deep-seated changes in the geological forces associated with the fault.

These unusual developments have increased scientists' concern about the possibility of a large earthquake on a long-dormant stretch of the San Andreas extending from San Bernardino south to the Salton Sea.

"This part of the San Andreas concerns me the most," says Kerry E. Sieh of the California Institute of Technology. He has been reconstructing the chronology of past earthquakes along several stretches of the fault, which reaches from San Francisco to Mexico.

While extremely suggestive, the recent observations are not sufficient to predict a large earthquake in the near future, explains C. Barry Rayleigh of the US Geological Survey (USGS) after a symposium on the San Andreas at the annual meeting of the American Association for the Advancement of Science here.

"We certainly don't want to scare people unnecessarily, but it might not be a bad idea to take the antique china off the shelf and pack it securely," Dr. Rayleigh comments.

For the past decade seismologists have been searching for precursors: special signs that will tell them the approximate size, center, and time of occurrence of major earthquakes, before they happen. While the experts have drawn up a long list of potential precursors, none has so far proved reliable. Earthquakes appear too variable to provide the scientists with a simple warning sign.

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