Controls of Dynamic and Static Stress Changes and Aseismic Slip on Delayed Earthquake Triggering: Application to the 2019 Ridgecrest Earthquake Sequence

Jeena Yun , Alice-Agnes Gabriel , Dave A May, Yuri Fialko 

Dynamic earthquake triggering often involves a time delay relative to the peak stress perturbation. In this study, we investigate the physical mechanisms responsible for delayed triggering. We compute detailed spatiotemporal changes in dynamic and static Coulomb stresses at the 2019 Mw 7.1 Ridgecrest mainshock hypocenter, induced by the Mw 5.4 foreshock, using 3D dynamic rupture models. The computed stress changes are used to perturb 2D quasi-dynamic models of seismic cycles on the mainshock fault governed by rate-and-state friction. We explore multiple scenarios with varying hypocenter depths, perturbation amplitudes and timing, and different evolution laws (aging, slip, and stress-dependent). Most of the perturbed cycle models show a mainshock clock advance of several hours. Instantaneous triggering occurs only if the peak stress perturbation is comparable to the strength excess during quasi-static nucleation. While both aging and slip laws yield similar clock advances, the stress-dependent aging law results in a systematically smaller clock advance. The sign of the stress perturbation in regions of accelerating slip controls whether the mainshock is advanced or delayed. Mainshocks can be triggered even when the future mainshock hypocenter is within a stress shadow, due to stress transfer from the foreshock sequence. Our results imply that the Ridgecrest mainshock fault was already on the verge of runaway rupture prior to the Mw 5.4 foreshock. These results highlight the contribution of both foreshocks and aseismic deformation to earthquake triggering and emphasize the importance of considering the physics of fault-system-wide processes when assessing triggering potential.