Foreshocks and time-dependent earthquake hazard assessment in southern California

The probability that an earthquake in southern California (M ≧ 3.0) will be followed by an earthquake of larger magnitude within 5 days and 10 km (i.e., will be a foreshock) is 6 ± 0.5 per cent (1 S.D.), and is not significantly dependent on the magnitude of the possible foreshock between M = 3 and M = 5. The probability that an earthquake will be followed by an M ≧ 5.0 main shock, however, increases with magnitude of the foreshock from less than 1 per cent at M ≧ 3 to 6.5 ± 2.5 per cent (1 S.D.) at M ≧ 5. The main shock will most likely occur in the first hour after the foreshock, and the probability that a main shock will occur decreases with elapsed time from the occurrence of the possible foreshock by approximately the inverse of time. Thus, the occurrence of an earthquake of M ≧ 3.0 in southern California increases the earthquake hazard within a small space-time window several orders of magnitude above the normal background level.