Regional seismic velocity changes following the 2019 Mw7.1 Ridgecrest California earthquake from autocorrelations and P/S converted waves

Yang Lu, & Yehuda Ben-Zion

Under Review 2021, SCEC Contribution #10928

We examine regional transient changes of seismic velocities generated by the Mw7.1 2019 Ridgecrest earthquake in California, using autocorrelations of moving time windows in continuous waveforms recorded at regional stations. We focus on travel time differences in a prominent phase generated by an interface around 2 km depth, associated with transmitted Pp waves and converted Ps waves from the ongoing microseismicity. Synthetic tests demonstrate the feasibility of the method for monitoring seismic velocity changes. Taking advantage of the numerous aftershocks in the early period following the mainshock, we obtain a temporal resolution of velocity changes up to 20 min in the early post-mainshock period. The results reveal regional coseismic velocity drops in the top 1-3 km with an average value of ∼2 percent over distances up to 100 km from the Ridgecrest event. These average velocity drops are likely dominated by larger changes in shallow materials and are followed by rapid recoveries on timescales of days. In the nearby Coso geothermal region, the observed coseismic velocity drops are up to ∼8 percent. The method allows monitoring temporal changes of seismic velocities with high temporal resolution, fast computation, and precise spatial mapping of changes. The results suggest that significant temporal changes of seismic velocities of shallow materials are commonly generated on a regional scale by large events.

Citation
Lu, Y., & Ben-Zion, Y. (2021). Regional seismic velocity changes following the 2019 Mw7.1 Ridgecrest California earthquake from autocorrelations and P/S converted waves. Geophysical Journal International, (under review).