Imaging the Garlock Fault Zone With a Fiber: A Limited Damage Zone and Hidden Bimaterial Contrast

Creators: Atterholt, James¹; Zhan, Zhongwen¹; Yang, Yan¹; Zhu, Weiqiang^{1, 2}

1. California Institute of Technology
2. University of California, Berkeley

Abstract

The structure of fault zones and the ruptures they host are inextricably linked. Fault zones are narrow, which has made imaging their structure at seismogenic depths a persistent problem. Fiber‐optic seismology allows for low‐maintenance, long‐term deployments of dense seismic arrays, which present new opportunities to address this problem. We use a fiber array that crosses the Garlock Fault to explore its structure. With a multifaceted imaging approach, we peel back the shallow structure around the fault to see how the fault changes with depth in the crust. We first generate a shallow velocity model across the fault with a joint inversion of active source and ambient noise data. Subsequently, we investigate the fault at deeper depths using travel‐time observations from local earthquakes. By comparing the shallow velocity model and the earthquake travel‐time observations, we find that the fault's low‐velocity zone below the top few hundred meters is at most unexpectedly narrow, potentially indicating fault zone healing. Using differential travel‐time measurements from earthquake pairs, we resolve a sharp bimaterial contrast at depth that suggests preferred westward rupture directivity.

Copyright and License

Acknowledgement

We would like to thank Dr. Robert Clayton and Dr. Martin Karrenbach for their help in performing the active source survey. We would also like to thank Dr. John Vidale, Dr. Elizabeth Cochran, Dr. Hongrui Qiu, Dr. Heather Savage, Dr. Yihe Huang, Dr. Ahmed Elbanna, and Dr. Ares Rosakis for their insightful conversations and suggestions which greatly improved the paper. James Atterholt was supported by the National Science Foundation (NSF) Graduate Research Fellowship Program (GRFP) number DGE-1745301. The study was made possible by the Gordon Moore Foundation and the National Science Foundation (NSF) through the Faculty Early Career Development (CAREER) award number 1848166. We would also like to thank the California Broadband Cooperative for allowing access to the fiber used for the DAS array in this experiment. Finally, we would like to thank the editor Dr. Fenglin Niu for facilitating the review process and two anonymous reviewers for their suggestions.

Contributions

Conceptualization: James Atterholt, Zhongwen Zhan.
Data curation: James Atterholt, Zhongwen Zhan.
Formal analysis: James Atterholt.
Funding acquisition: James Atterholt, Zhongwen Zhan.
Investigation: James Atterholt.
Methodology: James Atterholt, Zhongwen Zhan, Yan Yang, Weiqiang Zhu.
Resources: Zhongwen Zhan.
Software: James Atterholt, Yan Yang, Weiqiang Zhu.
Supervision: Zhongwen Zhan.
Validation: James Atterholt.
Visualization: James Atterholt.
Writing – original draft: James Atterholt.
Writing – review & editing: James Atterholt, Zhongwen Zhan, Yan Yang.

Data Availability

The virtual and active source shot gathers used to produce the model shown in Figure 3 and the earthquake data used to produce the profiles shown in Figures 3 and 4 are available at (Atterholt, 2023). The broadband data used to construct the velocity profiles shown in Figure 6 were obtained from the Southern California Seismic Network (SCEDC, 2013). The program used to compute the travel-times for the parametric evaluation of damage zone property sensitivity, Pykonal (White et al., 2020). Ray paths for the inversion of the velocity contrast across the fault were computed using the TauP (Crotwell et al., 1999) functionality in Obspy (Krischer et al., 2015). Figure 1 was made using The Generic Mapping Tools (GMT), version 6 (Wessel et al., 2019).

Supplemental Material

Supporting Information (PDF).

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JGR Solid Earth - 2024 - Atterholt - Imaging the Garlock Fault Zone With a Fiber A Limited Damage Zone and Hidden.pdf

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