A search for evidence of secondary static stress triggering during the 1992 Mw7.3 Landers, California, earthquake sequence

Secondary triggering of aftershocks is widely observed and often ascribed to secondary static stress transfer. However, small to moderate earthquakes are generally disregarded in estimates of Coulomb stress changes (Delta CFS), either because of source parameter uncertainties or a perceived lack of importance. We use recently published high-quality focal mechanisms and hypocenters to reassess the role of small to moderate earthquakes for static stress triggering of aftershocks during the 1992 M(w)7.3 Landers, California, earthquake sequence. We compare the Delta CFS imparted by aftershocks (2 <= M <= 6) onto subsequent aftershocks with the total Delta CFS induced by the M>6 main shocks. We find that incremental stress changes between aftershock pairs are potentially more often positive than expected over intermediate distances. Cumulative aftershock stress changes are not reliable for receivers with nearby aftershock stress sources because we exclude unrealistic aftershock stress shadows that result from uniform slip models. Nonetheless, 27% of aftershocks receive greater positive stress from aftershocks than from the main shocks. Overall, 85% of aftershocks are encouraged by the main shocks, while adding secondary stress encourages only 79%. We infer that source parameter uncertainties of small aftershocks remain too large to convincingly demonstrate (or rule out) that secondary stress transfer induces aftershocks. An important exception concerns aftershocks in main shock stress shadows: well-resolved secondary stress from detected aftershocks rarely compensates negative main shock stress; these aftershocks require a different triggering mechanism.