Group B, Poster #036, Seismology

What prevented the instantaneous dynamic triggering of the 2019 Mw 7.1 Ridgecrest mainshock by the Mw 5.4 foreshock?

Jeena Yun, Yuri Fialko, & Alice-Agnes Gabriel
Poster Image: 

Poster Presentation

2022 SCEC Annual Meeting, Poster #036, SCEC Contribution #12248 VIEW PDF
The July 2019 Mw 7.1 Ridgecrest earthquake was preceded by multiple foreshocks including the two largest Mw 6.4 and Mw 5.4 events, the latter being particularly close to the mainshock hypocenter both spatially and temporally. One might expect that the stress field at the nucleation site of the mainshock was strongly perturbed by the foreshocks. Preliminary models suggest dynamic stress changes of several MPa (e.g., Jin and Fialko, 2020). It is of interest to understand why the large stress perturbations due to various foreshocks failed to instantaneously trigger the mainshock that was already on the verge of a runaway rupture. In this study, we aim to investigate the physical factors and pro...cesses that prevented the instantaneous triggering of the Mw 7.1 mainshock by the Mw 5.4 foreshock. We first refine the static and dynamic Coulomb Failure Stress changes (dCFS) at the mainshock hypocenter caused by the foreshock using the 3D dynamic rupture and seismic wave propagation modeling software SeisSol (Krenz et al., 2021). We simulate a diverse set of possible geometries of the nucleation site of the mainshock (depths of 3 - 7 km; strikes of 320˚ - 340˚) to account for uncertainties in the relative orientation of the rupture planes of the foreshock and the mainshock nucleation site. We find negative static Coulomb stress changes for a relatively large (> 5 km) presumed hypocenter depth of the mainshock. At a smaller depth (< 5 km), the static Coulomb stress changes are positive. The static Coulomb stress changes are of the order of a few hundreds of kPa, and increase with a decreasing strike angle of the fault that nucleated the Mw 7.1 rupture. The computed dynamic dCFS are as large as several MPa and decrease with decreasing hypocenter depth, and the more northerly strike angle. We plan to use the dynamic stress perturbations predicted by our numerical models in a simulation of rupture nucleation at the mainshock hypocenter. Our goal is to constrain a set of model parameters such as the characteristic slip weakening distance, the effective normal stress, and the velocity-dependence parameters that give rise to the observed behavior. Our modeling results may provide insights into the dynamics of slip nucleation and foreshock-mainshock sequences elsewhere.