Absolute Location of 2019 Ridgecrest Seismicity Reveals Duplex M6.4 Ruptures, Migrating and Pulsing M7.1 Foreshocks and Unusually Shallow Mw7.1 Nucleation. Did the M7.1 Rupture Require Incitation by M6.4-Like Rupture?

Session: Observations From the 2019 Ridgecrest Earthquake Sequence
Type: Oral
Date: 4/28/2020
Time: 05:15 PM
Room: 115
Description: 

The 2019 Ridgecrest, California sequence includes an M_w6.4 earthquake on July 4 and an M_w7.1 mainshock 34 hours later. We examine 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 M_w6.4 hypocenter is ~12km deep, compatible with hypocentral depths of most M≥6 earthquakes in southern California. The M_w7.1 hypocenter, however, is unusually shallow at ~4km.

The immediate post-M_w6.4 seismicity defines a deep, ~12km long, SE-NW structure containing the M_w6.4 hypocenter and a shallower, orthogonal, ~18km long NE-SW structure. These duplex structures have little or no intersection, making the M_w6.4 event a double earthquake, rupturing first the deeper and then the shallower structure.

The ensuing, pre-M_w7.1 seismicity extends the SE-NW structure northwestwards to within ~3km of the future M_w7.1 hypocenter and illuminates a new crossing structure, while small clusters of events within 2km of the future M_w7.1 hypocenter activate several times in pulses from a few hours after the M_w6.4 event through M_w7.1 initiation.

This pre-M_w7.1 seismicity suggests M_w7.1 rupture initiation was triggered by the pulsing clusters, and early M_w7.1 rupture growth was incited by stress changes from the M_w6.4 event and its aftershocks. Moreover, M_w7.1 nucleation at shallow depth, where spontaneous rupture growth into a large earthquake is not expected, may have required incitation by the M_w6.4 event. Otherwise, M_w7.1-like rupture might not have occurred until long in the future, either through incitation by another M_w6.4-like event or with nucleation at greater depth. Such a dependency of very large event rupture on a large inciting event is a significant complication for hazard estimation.

3D visualizations of absolute relocation of M≥1.0 Ridgecrest seismicity with interpreted faulting structures: http://alomax.net/projects/Ridgecrest_2019

Presenting Author: Anthony Lomax

Authors