Locally and remotely triggered aseismic slip on the central San Jacinto Fault near Anza, CA, from joint inversion of seismicity and strainmeter data

We study deep aseismic slip along the central section of the San Jacinto Fault, near the Anza Seismic Gap, in southern California. Elevated strain-rates following the remote Mw7.2 April 4, 2010 El Mayor-Cucapah and the local Mw5.4, July 7, 2010 Collins Valley earthquakes were recorded by Plate Boundary Observatory borehole strain-meters near Anza, and were accompanied by vigorous aftershock sequences. We introduce a method to infer the distribution of triggered aseismic slip from combined seismicity and geodetic data, based on a rate-and-state friction model that maps observed changes in seismicity rates into stress changes. We invert for the cumulative slip in the ten-day period following each mainshock. Synthetic tests show that the effect of aftershock interactions on the inferred slip distribution is negligible. The joint dataset is more consistent with a model in which aseismic slip on a principal fault triggers seismicity on adjacent faults, than with one in which aseismic slip and seismicity are co-planar. Our results indicate that aseismic slip primarily occurs along the rim of two seismicity clusters adjacent to Anza Gap, as well as beneath the Anza Gap itself, at depths larger than 10 km. The triggered aseismic slip generated by the two mainshocks has little overlap, a pattern also found in sequences of large earthquakes occurring on a same fault. Stresses inferred from seismic activity leading to the Collins Valley mainshock suggest that this earthquake was triggered by stresses imposed by the El Mayor-Cucapah remote triggered aseismic slip, which persisted for more than two months. c ⃝2017 American Geophysical Union. All Rights Reserved.