Afterslip And Spontaneous Aseismic Slip On The Anza Segment Of The San Jacinto Fault Zone, Southern California

Understanding the interplay of seismic and aseismic slip is key in seismic hazard evaluation. It is particularly important to know if the same or nearby fault segments can host different slip modes and understand the transition between modes of slip. We investigate this in the trifurcation area of the Anza segment of the San Jacinto fault where deep creep driving seismicity below the geodetic locking depth has been proposed. We focus on periods following local moderate-sized earthquakes by combining the occurrence of highly correlated families of earthquakes (i.e., closely spaced or partially overlapping events) that we describe here as near-repeating earthquakes, seismicity, and borehole strainmeter data. We find that all M-w > 4.5 earthquakes between 2010 and 2020 triggered afterslip with coplanar families of near-repeating earthquakes and off-fault microseismicity. These observations include newly detected afterslip following the June 10, 2016 M-w 5.2 and April 4, 2020 M-w 4.9 local earthquakes. Afterslip geometries defined by the near-repeating earthquake families are consistent with strain change observations. We conclude that families of near-repeating earthquakes, similar to low-frequency earthquakes within tremor, can be useful indicators of aseismic slip transients and can reveal faulting complexities during aseismic slip. Further, we identify the first evidence of spontaneous aseismic slip in the Anza region from near-repeating earthquake families on two minor faults in 2015. Taken together, our observations support a model where deep microseismicity is located in a transitional region at the bottom of the seismogenic zone with spatially heterogeneous frictional properties that produce frequent aseismic slip transients near Anza.