Variability of fault slip behavior along the San Andreas Fault in the San Juan Bautista Region

An improved understanding of the time history of fault slip at depth is an essential step toward understanding the underlying mechanics of the faulting process. Using a waveform cross‐correlation approach, we document spatially and temporally varying fault slip along the northernmost creeping section of the San Andreas Fault near San Juan Bautista (SJB), California, by systematically examining spatiotemporal behaviors of characteristically repeating earthquakes (CREs). The spatial distribution of pre‐1998 SJB earthquake (1984–1998) fault slip rate inferred from the CREs reveals a ~15 km long low creep or partially locked section located near the 1998 Mw 5.1 SJB earthquake rupture. A finite‐fault slip inversion reveals that the rupture of the 1998 SJB earthquake is characterized by the failure of a compact ~4 km2 asperity with a maximum slip of about 90 cm and corresponding peak stress drop of up to 50 MPa, whereas the mean stress drop is about 15 MPa. Following the 1998 earthquake, the CRE activity was significantly increased in a 5–10 km deep zone extending 2–7 km northwest of the main shock, which indicates triggering of substantial aseismic slip. The postseismic slip inferred from the CRE activity primarily propagated to the northwest and released a maximum slip of 9 cm. In this 5–10 km depth range, the estimated postseismic moment release is 8.6 × 1016 N m, which is equivalent to Mw 5.22. The aseismic slip distribution following the 1998 earthquake is not consistent with coseismic stress‐driven afterslip but represents a triggered, long‐lasting slow earthquake. Key Points A 15 km long partially locked section near the 1998 San Juan Bautista earthquakeThe 1998 earthquake rupture is characterized by the failure of a 4 km2 asperityThe moment magnitude of the aseismic slip following the 1998 earthquake is 5.22