Absolute Location of 2019 Ridgecrest Seismicity Reveals a Shallow M-w 7.1 Hypocenter, Migrating and Pulsing M-w 7.1 Foreshocks, and Duplex M-w 6.4 Ruptures

摘要

The 2019 Ridgecrest, California, sequence includes an M-w 6.4 earthquake on 4 July and an M-w 7.1 mainshock 34 hr later. We perform absolute location of M-w >= 1.0 Ridgecrest events using multiple velocity models, station corrections, and a location algorithm robust to velocity model and arrival-time error. The obtained seismicity is mainly similar to 3-12 km deep, with few shallower events. The M-w 6.4 hypocenter is similar to 12 km deep, compatible with hypocentral depths of most M-w >= 6 earthquakes in southern California. The M-w 7.1 hypocenter, however, is unusually shallow at similar to 4 km. The immediate post-M-w 6.4 seismicity defines a deep, similar to 12 km long, southeast-northwest structure containing the M-w 6.4 hypocenter and a shallower, orthogonal, similar to 18 km long northeast-southwest structure. These structures have little or no intersection, making the M-w 6.4 event a double earthquake, rupturing first the deeper and then the shallower structure. The ensuing, pre-M-w 7.1 seismicity extends the southeastnorthwest structure northwestwards to within similar to 3 km of the future M-w 7.1 hypocenter and illuminates a new crossing structure, whereas small clusters of events within 2 km of the future M-w 7.1 hypocenter activate 3-4 times in pulses from a few hours after the M-w 6.4 event through M-w 7.1 initiation. This pre-M-w 7.1 seismicity suggests M-w 7.1 rupture initiation activated as an event in the pulsing clusters, and early M-w 7.1 rupture growth was primed by stress changes from the M-w 6.4 rupture and its aftershocks. Moreover, shallow M-w 7.1 nucleation, for which spontaneous rupture growth into a large earthquake is not expected, may have required this incitation by the M-w 6.4 events, a significant complication for hazard estimation. Otherwise, M-w 7.1-like rupture might not have occurred until much later, perhaps with nucleation at greater depth. The Ridgecrest seismicity defines additional structures around and crossing the main M-w 6.4 and 7.1 rupture zones, but some of this seismicity likely shows delayed activity on pre-existing faults due to stress changes from the main events and not rupture complexity during the larger events.