Using the match-and-locate method to characterize foreshocks of the July 2019 M_W6.4 Ridgecrest, California earthquake

The July 2019 M_W6.4 Ridgecrest, California earthquake and its distinct foreshocks were well recorded by local and regional stations, providing a great opportunity to characterize its foreshocks and investigate the nucleation mechanisms of the mainshock. In this study, we utilized the match-and-locate (M&L) method to build a high-precision foreshock catalog for this M_W6.4 earthquake. Compared with the sequential location methods (matched-filter + cross-correlation-based hypoDD), our new catalog contains more events with higher location accuracy. The M_W6.4 mainshock was preceded by 40 foreshocks within ~2 h (on July 4, 2019 from 15:35:29 to 17:32:52, UTC). Their spatiotemporal distribution revealed a complex seismogenic structure consisting of multiple fault strands, which were connected as a throughgoing fault by later foreshocks and eventually accommodated the 2019 M_W6.4 mainshock. To better understand the nucleation mechanism, we determined the rupture dimension of the largest M_L4.0 foreshock by calculating its initial rupture and centroid points using the M&L method. By estimating Coulomb stress change we suggested that the majority of foreshocks following the M_L4.0 event and M_W6.4 mainshock occurred within regions of increasing Coulomb stress, indicating that they were triggered by stress transfer. The nucleation process before the M_L4.0 event remains unclear due to the insufficient sampling rate of waveforms and small magnitude of events. Thus, our study demonstrates that the M&L method has superior detection and location ability, showing potential for studies that require high-precision location (e.g., earthquake nucleation).