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

Hongrui Qiu, Benxin Chi, & Yehuda Ben-Zion

Submitted September 11, 2022, SCEC Contribution #12185, 2022 SCEC Annual Meeting Poster #028

Along the northern edge of the Mojave Desert, the Garlock fault intersects the San Andreas fault and is the second largest (~300 km long) fault in Southern California. It can host M>7 earthquakes that pose significant seismic hazard to densely populated communities. However, the subsurface structure of the Garlock fault is not well understood due to the sparse seismic network and lack of seismic activity nearby. The 2019 Ridgecrest earthquake sequence in the Eastern California Shear Zone led to a rapid deployment of several dense linear arrays with ~100 m spacing and apertures of a few kilometers, which cross the Ridgecrest rupture zone and also the Garlock fault. The recorded seismic data is used here to illuminate the internal structure of the Garlock fault. We analyze data of the 2019 Ridgecrest aftershock sequence recorded by two dense seismic arrays that cross the Ridgecrest rupture zone (B4) and Garlock fault (A5). Analyses of fault zone head waves and P-wave delay times at array A5 indicate that the Garlock fault separates two distinctive crustal blocks with P waves traveling ~5% faster in the northern block. The across-fault velocity contrast agrees with large-scale tomography models and generates clear P-wave reflections in waveforms recorded by array B4. Kirchhoff migration of P waves reflected by the Garlock fault yields an image of a near-vertical reflector between 2-6 km depth. The P-wave delay times indicate a ~300-m-wide transition zone near the Garlock fault surface trace beneath array A5, offset to the side with faster seismic velocity. The results provide important constraints for improved derivations of earthquake source parameters and simulations of ruptures and ground motion associated with the Garlock fault.

Key Words
Garlock fault; Fault zone imaging; Fault zone head waves; Fault zone reflected waves; Bimaterial interface

Qiu, H., Chi, B., & Ben-Zion, Y. (2022, 09). Internal structure of the Garlock fault zone from Ridgecrest aftershocks recorded by dense linear seismic arrays. Poster Presentation at 2022 SCEC Annual Meeting.

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