Absolute location of 2019 Ridgecrest seismicity reveals duplex Mw6.4 ruptures, migrating and pulsing Mw7.1 foreshocks, and unusually shallow Mw7.1 nucleation

Submitted to Bulletin of the Seismological Society of America - Request Preprint

To be presented at the 2020 SSA Annual Meeting Albuquerque: session Observations From the 2019 Ridgecrest Earthquake Sequence
Tuesday, 28 April 2020, 2:30 PM, Room 115, Talk: 5:30 PM

Visualization of results:

Movie S1 3D animation of absolute relocation of M≥1.0 Ridgecrest seismicity with interpreted faulting structures

[Updated 2020.01.15 with violet bar indicating (small) overlap between Mw6.4 rupture surfaces, and deepened 6.4SW rupture structure.]

Events plotted in color corresponding to origin time from the first Mw6.4 foreshock until the Mw7.1 event; later events to 20190708.03h20m shown as light gray filled circles, all events plotted with transparency. Faulting structures described in the paper are schematically indicated by shaded surfaces; overlap of 6.4NW and 6.4SW structures show by thick violet bar. Preferred, manually picked, Mw6.4 and Mw7.1 hypocenters are labelled and shown with light gray and black outline. Seismic stations available before (after) July 7, 2019 shown as large (small) tetrahedrons. Dark gray lines show surface ruptures from the Provisional Map of Surface Rupture for the Ridgecrest Sequence (8/22/2019) from EERI. Relocations performed with NonLinLoc in the 3D, tomographic model for the Coso-Ridgecrest area of Zhang and Lin (2014).

Abstract:

The 2019 Ridgecrest, California sequence includes an Mw6.4 earthquake on July 4 and an Mw7.1 mainshock 34 hours later. We perform absolute location of M≥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 ~3-12km deep, with few shallower events. The Mw6.4 hypocenter is ~12km deep, compatible with hypocentral depths of most M≥6 earthquakes in southern California. The Mw7.1 hypocenter, however, is unusually shallow at ~4km.

The immediate post-Mw6.4 seismicity defines a deep, ~12km long, SE-NW structure containing the Mw6.4 hypocenter, and a shallower, orthogonal, ~18km long NE-SW structure. These structures have little or no intersection, making the Mw6.4 event a double earthquake, rupturing first the deeper and then the shallower structure.

The ensuing, pre-Mw7.1 seismicity extends the SE-NW structure northwestwards to within ~3km of the future Mw7.1 hypocenter and illuminates a new crossing structure, while small clusters of events within 2km of the future Mw7.1 hypocenter activate 3-4 times in pulses from a few hours after the Mw6.4 event through Mw7.1 initiation.

This pre-Mw7.1 seismicity suggests Mw7.1 rupture initiation activated as an event in the pulsing clusters, and early Mw7.1 rupture growth was primed by stress changes from the Mw6.4 rupture and its aftershocks. Moreover, shallow Mw7.1 nucleation, where spontaneous rupture growth into a large earthquake is not expected, may have required this incitation by the Mw6.4 events, a significant complication for hazard estimation. Otherwise, Mw7.1-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 Mw6.4 and Mw7.1 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.



Zhang, Q., and G. Lin (2014), Three-dimensional Vp and Vp/Vs models in the Coso geothermal area, California: Seismic characterization of the magmatic system, J. Geophys. Res.: Solid Earth 119 4907–4922. https://doi.org/10.1002/2014JB010992



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