Poster #030, San Andreas Fault System (SAFS)

Structural properties of the Southern San Andreas fault around Thousand Palms, California, from analysis of large-N seismic array data

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Poster Presentation

2020 SCEC Annual Meeting, Poster #030, SCEC Contribution #10688
We present preliminary results from a large-N seismic nodal array deployment spanning the Banning and Mission Creek strands (BF and MCF) of the Southern San Andreas fault (SSAF) near the Thousand Palms Oasis Preserve, CA. A number of key characteristics of the SSAF remain poorly known, including the fault attitude at depth, the extent of fault-related damage, partitioning of slip between active fault strands, and along-strike variations in shallow creep. Analyses of seismic waveforms from local, regional and teleseismic earthquakes recorded across the array are used to better understand the subsurface fault structure and properties.

The nodal array was deployed in the beginn...
ing of March 2020 and consisted of 322 Zland 3-component 5 Hz nodes recording at 500 samples per second. Most of the nodes continuously recorded data into April. The array configuration included two 100+ node 2-D subarrays with aperture of 0.6-1 km centered on the BF and MCF. Each subarray included a grid of regularly spaced nodes, as well as several nodes scattered around that grid. In addition, we deployed a ~4 km-long 100+ node quasi-linear profile crossing both strands and connecting the two subarrays. The inter-node spacing varied from ~15 m around the fault traces to ~100 m away from the fault traces. Four nodes were installed farther NE to help constrain local seismicity in the region.

During acquisition, the array recorded 25 M>1 local events that occurred on or near the SSAF within 50 km from the array, several moderate to large regional earthquakes, including the M5.7 Salt Lake City and M6.5 Central Idaho events, and numerous teleseismic earthquakes. A preliminary delay time analysis of regional and teleseismic P arrivals recorded by the linear array reveals at large-scale a seismically slower SW side and faster NE side in the study area. Maximum slowness is observed ~400 m SW of the BF trace. Rapid changes in P amplitude, frequency and phase observed ~500 m NE of the BF likely reflect a terminus of a broad damage zone associated with the BF. Characteristic waveform changes are also observed across the MCF and reveal, in addition to a large decrease in delay times to the NE, the MCF as a major crustal discontinuity bounding the seismically fast crustal structure to the NE. The continuing analyses use P and S arrival times, amplitudes and internal fault zone phases (reflected, head and trapped waves) from local earthquakes to further study the SSAF structure.