INFLUENCE OF STATIC STRESS CHANGES ON EARTHQUAKE LOCATIONS IN SOUTHERN CALIFORNIA

EARTHQUAKES induce changes in static stress on neighbouring faults tha t may delay, hasten or even trigger subsequent earthquakes(1-10). The length of time over which such effects persist has a bearing on the po tential contribution of stress analyses to earthquake hazard assessmen t, but is presently unknown. Here we use an elastic half-space model(1 1) to estimate the static stress changes generated by damaging (magnit ude M greater than or equal to 5) earthquakes in southern California o ver the past 26 years, and to investigate the influence of these chang es on subsequent earthquake activity. We find that, in the 1.5-year pe riod following a M greater than or equal to 5 earthquake, any subseque nt nearby M greater than or equal to 5 earthquake almost always ruptur es a fault that is loaded towards failure by the first earthquake. Aft er this period, damaging earthquakes are equally likely to rupture loa ded and relaxed faults. Our results suggest that there is a short peri od of time following a damaging earthquake in southern California in w hich simple Coulomb failure stress models could be used to identify re gions of increased seismic hazard.