Regional seismic velocity changes following the 2019 Mw 7.1 Ridgecrest, California earthquake from autocorrelations and P/S converted waves

Abstract

Summary We examine regional transient changes of seismic velocities generated by the Mw 7.1 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 per cent over distances up to 100 km from the Ridgecrest event. These average velocity drops are likely dominated by larger changes in the shallow materials, and are followed by rapid recoveries on timescales of days. Around the north end of the Ridgecrest rupture and the nearby Coso geothermal region, the observed coseismic velocity drops are up to ∼8 per cent. 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.

The Seismic Signature of California's Earthquakes, Droughts, and Floods

“…The velocity drop is ≈1.5% for CI.HEC and CI.JRC2 and ≈2.5% for CI.WES. These are reasonable values compared to other studies of these earthquakes (Boschelli et al., 2021; Lu & Ben‐Zion, 2022) or greater than others that used stations more distance from the source (Mao et al., 2020; Taira et al., 2015). Because of the relatively low temporal resolution, we likely largely underestimate the maximum drop experienced during and quickly after the shaking (Bonilla et al., 2019; Shokouhi et al., 2017).…”

Monitoring Seasonal Shear Wave Velocity Changes in the Top 6 m at Garner Valley in Southern California With Borehole Data

“…Monitoring studies have revealed widespread velocity changes in the subsurface (e.g., Lu & Ben‐Zion, 2022; Niu et al., 2003; Wang et al., 2017; Wu et al., 2009). In particular, the correlation of subsurface velocity changes with underground water level has led to the growth of hydrological monitoring work using seismological tools (e.g., Mao et al., 2022; Rodríguez Tribaldos & Ajo‐Franklin, 2021).…”

Elastic Contrast, Rupture Directivity, and Damage Asymmetry in an Anisotropic Bimaterial Strike‐Slip Fault at Middle Crustal Depths

“…Therefore, accumulated microfractures produced during multiple earthquakes may potentially have caused significant reduction in stiffness of the more damaged QF rock, leading to changes in the material contrast across the shear zone. However, at middle crustal depths, healing/sealing processes in damaged rocks during post‐ and interseismic periods (e.g., Johnson, Song, Vel, et al., 2021) and closure of microfractures under high confining pressure may facilitate nearly complete recovery of elastic stiffness (e.g., Li et al., 2006; Lu & Ben‐Zion, 2022; Meyer et al., 2021; Wu et al., 2009). Thus, the long‐term effect of damage on rupture dynamics in faults/shear zones at depth remains an open question.…”