``Good evening. Tonight's forecast calls for a 40 percent chance of rain and a 10 percent chance of a big earthquake.'' Californians may be tuning into such bulletins soon as seismologists put their computer models to work to issue warnings after ominous rattles of the Earth. The seismologists have gathered enough data on major fault systems in California to be able to predict, to some degree, the probability of a quake of 6.5 or more on the Richter scale within certain brief periods. After a foreshock of magnitude of 4.0 or greater is registered, the model will go into work, determining whether there is, say, a 5, 10 or 25 percent chance of a quake within the next three days. The developers of the model are ready to put it into use but are discussing with the state how the predictions will be used _ whether they will be released publicly as earthquake warnings or provided to state agencies so they can prepare. The model is a mathematical formula that uses data on fault activity and the probability of major shocks, Lucile M. Jones of the U.S. Geological Survey said at a meeting Wednesday of the Seismological Society of America. Seismologists long have known that big quakes generally are preceded by less powerful shocks, but the new system is more precise, Jones said. Jones said a study of the major faults in California indicates, for example, that in the relatively quiet Carrizo Plain southwest of Fresno the chance that a 5-magnitude quake will be a foreshock to an 8-magnitude quake is 24 percent. Jones, who developed the formula with Duncan C. Agnew of the University of California-San Diego, said scientists will probably never be able to predict with absolute certainty. But the percentages she can deliver are enough to let people take action to prevent damage or injury, Jones said. ``As scientists we feel that sharing information is always better than withholding information,'' she said. The state could use the information to cancel vacations for disaster personnel, Jones said. Fire stations could move trucks outside. Daycare centers could leave gates unlocked in case they need to evacuate quickly. Residents could fill bathtubs with water for drinking, put away dishes and prepare emergency supplies. ``Each individual group can start making those sorts of decisions,'' she said. ``We aren't at a really high level (of accuracy), but people have been trying to get us to make these statements because there are useful things to do. And as we get better, the probabilities are going to head up.'' Geologist Jim Davis of the California Division of Mines and Geology said he is talking with the Office of Emergency Services about whether the agency should announce earthquake warnings. Davis said he didn't think quake forecasts would unduly alarm people or otherwise cause harm. The prediction system has been worked out so far only for California, whose seismic activity has been closely studied by scientists.