부산대학교 도서관

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 Mw > 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 Mw 5.2 and April 4, 2020 Mw 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. Plain Language Summary: Slip on faults occurs as a spectrum ranging from continuous sliding, where motion is too slow to radiate seismic waves (aseismic creep), to rapid movement during earthquakes (seismic slip). Within the fault slip spectrum lies the transient release of stress through aseismic slip. We investigate the interplay of these various slip modes near the Anza segment of the San Jacinto fault by combining observations of seismicity and borehole strainmeter data. We test the hypothesis that transient slip drives intervening small strong patches to failure generating near‐repeating earthquakes (i.e., highly correlated events located very close in space). We find that all moderate‐sized (Mw > 4.5) local earthquakes between 2010 and 2020 triggered transient slip events and coplanar families of near‐repeating earthquakes. We use the locations of near‐repeating earthquake families to define transient slip geometries and find that they are largely consistent with strain change observations. Transient slip occurs on main fault strands and minor structures. We also identify the first evidence of spontaneous transient slip in the Anza region from near‐repeating earthquake families and strainmeter data. Key Points: Near‐repeating earthquakes, like low‐frequency earthquakes in tremor, can indicate intermittent periods of aseismic slipLarge afterslip followed all four moderate‐sized earthquakes in Anza from 2010 to 2020; three events have afterslip moments greater than the mainshockAfterslip in the Anza region likely occurs on several fault segments both on and off the mainshock fault [ABSTRACT FROM AUTHOR]