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

Abstract

ABSTRACT The 2019 Ridgecrest, California, sequence includes an Mw 6.4 earthquake on 4 July and an Mw 7.1 mainshock 34 hr later. We perform absolute location of Mw≥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 ∼3–12  km deep, with few shallower events. The Mw 6.4 hypocenter is ∼12  km deep, compatible with hypocentral depths of most Mw≥6 earthquakes in southern California. The Mw 7.1 hypocenter, however, is unusually shallow at ∼4  km. The immediate post-Mw 6.4 seismicity defines a deep, ∼12  km long, southeast–northwest structure containing the Mw 6.4 hypocenter and a shallower, orthogonal, ∼18  km  long northeast–southwest structure. These structures have little or no intersection, making the Mw 6.4 event a double earthquake, rupturing first the deeper and then the shallower structure. The ensuing, pre-Mw 7.1 seismicity extends the southeast–northwest structure northwestwards to within ∼3  km of the future Mw 7.1 hypocenter and illuminates a new crossing structure, whereas small clusters of events within 2 km of the future Mw 7.1 hypocenter activate 3–4 times in pulses from a few hours after the Mw 6.4 event through Mw 7.1 initiation. This pre-Mw 7.1 seismicity suggests Mw 7.1 rupture initiation activated as an event in the pulsing clusters, and early Mw 7.1 rupture growth was primed by stress changes from the Mw 6.4 rupture and its aftershocks. Moreover, shallow Mw 7.1 nucleation, for which spontaneous rupture growth into a large earthquake is not expected, may have required this incitation by the Mw 6.4 events, a significant complication for hazard estimation. Otherwise, Mw 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 Mw 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.