Poster #208, Seismology

Internal structure of the Garlock fault zone from Ridgecrest aftershocks data recorded by dense linear arrays

Hongrui Qiu, Benxin Chi, & Yehuda Ben-Zion
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Poster Presentation

2021 SCEC Annual Meeting, Poster #208, SCEC Contribution #11428 VIEW PDF
The left-lateral strike-slip Garlock Fault runs northeast–southwest along the southern ends of Sierra Nevada and north margins of the Mojave Desert of Southern California. Although no significant ruptures have occurred in the past century, the Garlock fault is capable of producing major earthquakes as indicated by paleoseismic evidence. However, basic aspects of the seismological structure (e.g., velocity contrast, damage zone properties) of the Garlock fault have not been imaged so far. The 2019 Ridgecrest sequence in the southern part of the Walker Lane shear zone led to rapid deployments of seismic arrays across and around the surface ruptures. Several linear arrays with ~100 m spacing an...d various apertures (Catchings et al., SRL, 2020) recorded continuously at 500 Hz for about a one-month period (July-August of 2019). In this study, we analyze seismic waveforms recorded by a ~8-km-long linear array (B4) ~5 km north of the Garlock fault crossing surface ruptures of the Mw 7.1 Ridgecrest earthquake and a ~2-km-long linear array (A5) centered on the surface trace of the Garlock fault. Clear P waves reflected from the Garlock fault are observed at array B4 and identified for 7 events with depths ranging from 4 to 10 km. The polarity of fault zone reflected waves suggests that P waves travel faster in the crustal block north of the Garlock fault. This is in agreement with large-scale tomography models, and results of P-wave delay times of local and teleseismic events along with fault zone head waves resolved at array A5. We hand-pick good quality reflected signals and image a vertical dipping angle of the fault interface between 2-6 km via Kirchhoff migration using a velocity model recently developed for the region (White et al., GJI, 2021). In addition to the velocity contrast across the Garlock fault, we find a ~300-m-wide low-velocity zone to the south of the fault surface trace beneath array A5 that significantly delays P waves, amplifies S waveforms, and produces clear fault zone trapped waves (FZTWs). The high-quality FZTWs will be inverted for parameters of the trapping structure at the site. Updated results will be presented in the meeting.