Detailed nucleation process and mechanism of the July 2019 Mw 6.4
Ridgecrest, California earthquake
Abstract
Foreshocks provide valuable information on the nucleation process and
mechanism of impending earthquakes. In this study, we utilized the
Match&Locate method to build a high-precision foreshock catalog for the
July 2019 Mw 6.4 Ridgecrest, California earthquake. The Mw 6.4 mainshock
was preceded by 40 foreshocks within ~2 hours (on July
4, 2017 from 15:35:29 to 17:32:52, UTC). Their spatiotemporal
distribution reveals a complex seismogenic structure consisting of
multiple fault strands, which were connected as a throughgoing fault by
later foreshocks and eventually accommodated the 2019 Mw 6.4 mainshock.
To better understand the nucleation mechanism, we conducted a series of
analysis for the foreshocks including repeating earthquake
identification, rupture directivity inversion, and Coulomb stress change
estimation. We identified a pair of small earthquakes with close
magnitude, high waveform similarity, and high cross-spectral coherence
at the early nucleation stage. However, we cannot confirm if they are
repeating earthquakes due to their low magnitude and insufficient
sampling rate. Thus, the initial nucleartion mechanism is unclear to us.
Following the largest ML 4.0 foreshock, we found the majority of its
aftershocks and the Mw 6.4 mainshock occurred within regions of
increasing Coulomb stress, indicating that they were triggered by stress
transfer. Our study suggests that the nucleation of the Mw 6.4 mainshock
can be prominently explained by cascade triggering even though we cannot
exclude the possible existence of a minor aseismic slip process at the
early stage.