Anomalously large complete stress drop during the 2016 M_w 5.2 Borrego Springs earthquake inferred by waveform modeling and near-source aftershock deficit
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%.
Additional Information© 2017 American Geophysical Union. Received 4 MAR 2017; Accepted 5 MAY 2017; Accepted article online 8 MAY 2017; Published online 17 JUN 2017. We thank Zhongwen Zhan, Yehuda Ben-Zion, Marine Denolle, and an anonymous reviewer for their constructive comments. This research was supported by USGS/NEHRP grant G16AP00147; NSF award EAR-1550704; and by the Southern California Earthquake Center, which is funded by NSF Cooperative Agreement EAR-1033462 and USGS Cooperative Agreement G12AC20038. We used GMT from Wessel et al.,  to make the figures. We have used waveforms and parametric data from the Caltech/USGS Southern California Seismic Network (SCSN); doi:10.7914/SN/CI; stored at the Southern California Earthquake Center. doi:10.7909/C3WD3xH1. There are no real or perceived financial conflicts of interests for any author.
Supplemental Material - grl55912-sup-0001-2017GL073338_SI.docx