PROBABILITY GAIN DUE TO FORESHOCKS FOLLOWING QUIESCENCE TESTED BY SYNTHETIC CATALOGS

The database obtained from the observations of the Italian Seismologic al Network over more than 15 yr was analyzed by a simple algorithm to determine the rate of occurrence of major events after the precursors called foreshocks: moderate earthquakes following a period of quiescen ce. The number of observed foreshocks depends, among other factors, on the spatial (R(1)) and temporal (T-1) ranges of the quiescence used t o define the foreshocks. These parameters can be optimized to get the highest value of the success rate of the precursor for a given data se t. In our case, the optimization process led to a probability gain (G) higher than 100. In order to estimate the factors that influence the value of G, we tested our method on synthetic catalogs obtained from t he real one by randomizing the origin times of the events, or the spat ial coordinates, or both of them. The analysis showed that the apparen tly high value of the probability gain obtained optimizing the paramet ers. R(1) and T-1 is the result of (a) shortness of the catalog, (b) u neven spatial distribution of seismicity, and (c) real variation in ti me of the occurrence rate.