Poster #096, Earthquake Forecasting and Predictability (EFP)

Foreshock Cascade to Failure in the M 6.4 July 4, 2019 Ridgecrest, California Earthquake

William L. Ellsworth
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Poster Presentation

2020 SCEC Annual Meeting, Poster #096, SCEC Contribution #10244 VIEW PDF
Our current understanding of how earthquakes nucleate leaves open critical questions about the physical processes that occur before dynamic rupture including whether or not foreshocks have other than weak statistical value as a precursor. Two end-member hypotheses that describe the underlying mechanisms are the preslip and cascade model, which take opposing views on the role of aseismic deformation in the nucleation process. Here we examine the foreshocks to the Mw 6.4 July 4, 2019 Ridgecrest, California earthquake. We used template matching to identify the events, cross correlation and first-motion timing with hypoDD to determine hypocentroids for all event and the initial hypocenters for... the M > 2 foreshocks and the initiation point of the mainshock.

The foreshock sequence initiated 2.5 hours before the M 6.4 July 4 mainshock on a northwest-southeast striking fault. The initial events cluster at the base of the sequence at 12 km depth. Thirty minutes before the mainshock a Mw 4.0 -- the largest foreshock -- nucleated on the edge of area ruptured by the first foreshocks and ruptured unilaterally upward and to the northwest. This event was followed by its own aftershock sequence, all located on the periphery of the Mw 4.0 event, including the Mw 6.4 which nucleated on the edge of the Mw 4.0 rupture. The foreshocks form a cascade to failure, following the same pattern of closely packed ruptures observed in precision seismological analyses of the Mw 7.6 1999 Izmit, Turkey and Mw 7.1 1999 Hector Mine, California earthquakes. The absence of repeating earthquakes in all three of the foreshock sequences argues against significant aseismic slip in the initiation process as a silent driver of the foreshock process.

Constraining aseismic slip is notoriously difficult. For the M 6.4 July 4 Ridgecrest earthquake we detected nothing before rupture initiation on either seismometers or a borehole tensor strainmeter located 18 km from the epicenter. We can, however, place a limit on undetectable aseismic slip during the half hour between the M 4.0 and mainshock. A slip-related strain with an amplitude greater than 0.2 nanostrain would have been observed at this distance, corresponding to an upper limit of aseismic slip equivalent to Mw 3.5. Consequently, if aseismic slip occurred it played at most a minor role in foreshock sequence and we can rule out slip acceleration as the time to failure approached.