Anomalously large complete stress drop during the 2016 M_w 5.2 Borrego Springs earthquake inferred by waveform modeling and near-source aftershock deficit

Abstract

The 2016 M_w 5.2 Borrego Springs earthquake occurred in the trifurcation area of the San Jacinto Fault Zone and generated more than 23,000 aftershocks. We analyze source properties of this earthquake along with 12,487 precisely located aftershock hypocenters to obtain an unusually detailed view of the rupture process and energy budget for this moderate earthquake. Source time functions are obtained using an empirical Green's function approach and are inverted for a slip distribution on the fault plane. The rupture propagated unilaterally to the northwest over a distance of 1.8 km, resulting in clear directivity signals. Two asperities are identified and the maximum slip is 2.54 m, resulting in a static stress drop of 78.2 MPa. Over 97% of the aftershocks occur more than 1 rupture length from the slip area. We conclude that the Borrego Springs earthquake had a complete stress drop and estimate the seismic efficiency to be 15–26%.