Inverse Migration of Seismicity Quiescence During the 2019 Ridgecrest Sequence

Seismicity quiescences are best observed during sequences with multiple mainshocks and often emerge with some delay after an initial phase of aftershock triggering. The mechanisms controlling this delay are poorly understood, due to a lack of systematic and precise estimation of the duration of the triggering phase which is made complicated by the rapidly changing magnitude of completeness after the mainshock. Here, we analyze how such a delayed quiescence developed during the 2019 Ridgecrest (California) sequence, using a refined earthquake data set obtained by template matching and relative relocation. A method that allows estimating the aftershock occurrence rate after correcting for the change in detection (completeness magnitude) is applied. We find that an immediate seismicity shutdown was triggered by the second (M7.1) mainshock at the tips of the north-east trending cross-cut fault that hosted most of the slip of the first (M6.4) mainshock. This shutdown progressed toward the central part of this fault over the following days. The overall shutdown of the aftershock sequence of the M6.4 by the M7.1 rupture is consistent with static Coulomb stress modeling. This inverse migration is a unique observation, best explained by an enhanced spatial stress change variability in this central part where the cross-cut fault intersects the main fault of the M7.1 shock, hence a strongly damaged zone there. This further confirms previous observations of an enhanced structural disorder in this central part of the cross fault, showing that structural disorder has a first-order control on the seismicity dynamics at the scale of days.

Automated summary

By analyzing the 2019 Ridgecrest earthquake sequence, researchers found a delayed quiescence in seismic activity triggered by the second mainshock, with the shutdown of aftershocks following a static Coulomb stress model. This suggests that seismic activity dynamics are influenced by structural disorder.