Internal structure of the San Jacinto fault zone at the Ramona Reservation, north of Anza, California, from dense array seismic data

Journal Article · Fri Oct 23 00:00:00 EDT 2020 · Geophysical Journal International

DOI:https://doi.org/10.1093/gji/ggaa482· OSTI ID:1899388

Qin, Lei  ^[1]; Share, Pieter-Ewald  ^[2]; Qiu, Hongrui ^[3]; Allam, Amir A. ^[4]; Vernon, Frank L. ^[2]; Ben-Zion, Yehuda ^[5]

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1. Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089, USA
2. Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92093, USA
3. Department of Earth, Environmental and Planetary Sciences, Rice University, Houston, TX 77005, USA
4. Department of Geology and Geophysics, University of Utah, Salt Lake City, UT 84112, USA
5. Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089, USA, Southern California Earthquake Center, University of Southern California, Los Angeles, CA 90089, USA

SUMMARY We image the internal structure of the San Jacinto fault zone (SJFZ) near Anza, California, with seismic data recorded by two dense arrays (RA and RR) from ~42 000 local and ~180 teleseismic events occurring between 2012 and 2017. The RA linear array has short aperture (~470 m long with 12 strong motion sensors) and recorded for the entire analysed time window, whereas the RR is a large three-component nodal array (97 geophones across a 2.4 km × 1.4 km area) that operated for about a month in September–October 2016. The SJFZ at the site contains three near-parallel surface traces F1, F2 and F3 from SW to NE that have accommodated several Mw > 6 earthquakes in the past 15 000 yr. Waveform changes in the fault normal direction indicate structural discontinuities that are consistent with the three fault surface traces. Relative slowness from local events and delay time analysis of teleseismic arrivals in the fault normal direction suggest a slower SW side than the NE with a core damage zone between F1 and F2. This core damage zone causes ~0.05 s delay at stations RR26–31 in the teleseismic P arrivals compared with the SW-most station, and generates both P- and S-type fault zone trapped waves. Inversion of S trapped waves indicates the core damaged structure is ~100 m wide, ~4 km deep with a Q value of }20 and 40 per cent S-wave velocity reduction compared with bounding rocks. Fault zone head waves observed at stations SW of F3 indicate a local bimaterial interface that separates the locally faster NE block from the broad damage zone in the SW at shallow depth and merges with a deep interface that separates the regionally faster NE block from rocks to the SW with slower velocities at greater depth. The multiscale structural components observed at the site are related to the geological and earthquake rupture history at the site, and provide important information on the preferred NW propagation of earthquake ruptures on the San Jacinto fault.

• Research Organization: Univ. of Southern California, Los Angeles, CA (United States); Univ. of California, San Diego, CA (United States)
• Sponsoring Organization: USDOE Office of Science (SC)
• Grant/Contract Number: SC0016520; SC0016527
• OSTI ID: 1899388
• Alternate ID(s): OSTI ID: 1852274
• Journal Information: Geophysical Journal International, Journal Name: Geophysical Journal International Vol. 224 Journal Issue: 2; ISSN 0956-540X
• Publisher: Oxford University Press
• Country of Publication: United Kingdom
• Language: English

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