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General seismic architecture of the Southern San Andreas fault zone around the Thousand Palms Oasis from a Large-N nodal array

Pieter-Ewald Share, Hongrui Qiu, Benxin Chi, Frank L. Vernon, Yuri Fialko, Ellis Vavra, Amir A. Allam, & Yehuda Ben-Zion

Published August 16, 2021, SCEC Contribution #11629, 2021 SCEC Annual Meeting Poster #093

We present results from a large-N seismic nodal deployment spanning the Banning and Mission Creek strands (BF and MCF) of the southern San Andreas fault (SSAF) near the Thousand Palms Oasis Preserve, California. Motivating this research are several poorly understood characteristics of the SSAF, including fault attitude at depth, the extent of fault-related damage, partitioning of slip and activity between the BF and MCF, and along-strike variations in shallow creep. We help inform these topics through analyses of ambient and earthquake seismic wavefields recorded across the array.
Data were acquired in Spring 2020 using 322 Zland 3-component 5 Hz nodes configured as two 100+ node 2-D subarrays with apertures of 0.6-1 km centered on the BF and MCF and a ~4 km-long 100+ node quasi-linear profile crossing both strands and connecting the two subarrays. Recorded P wavefields from local SSAF earthquakes include rapid changes in phase, amplitude and frequency content over narrow 20-40 m zones (~node spacing) that coincide with the surface mapped BF and MCF. These variations indicate that the BF and MCF are high impedance contrast interfaces. Reverse-time migration results of P reflections highlight these interfaces as steeply northeast dipping (the imaging resolution with the recorded data is limited to the top 3-4 km). Another prominent but more diffuse change in SSAF structure, as quantified by P and S wavefield changes from local, regional and teleseismic events, occurs ~1 km northeast of the BF. Based on average to minimal delay of arrival times (P and S) and low ambient noise levels (<30 Hz) this may represent a relatively intact faster velocity block sandwiched between broad MCF and BF damage zones. Maximal P arrival delay (+0.2 s) and highest ambient noise levels (>2 times medial level) are consistently observed (with some dependence on event azimuth) southwest of the BF. Much of this is likely related to the thick Coachella Valley sediments on that side. Immediately northeast of the MCF, slightly elevated P arrival delays and ambient noise levels coincide with maximal S arrival delay (+0.3 s) and the detection of candidate S trapped waves. These indicate a broad high VP/VS MCF zone with active damage production potentially infiltrated by fluids feeding the nearby oases. Continued analysis and synthesis of these and other data will lead to higher resolution information on SSAF structure and activity.

Share, P., Qiu, H., Chi, B., Vernon, F. L., Fialko, Y., Vavra, E., Allam, A. A., & Ben-Zion, Y. (2021, 08). General seismic architecture of the Southern San Andreas fault zone around the Thousand Palms Oasis from a Large-N nodal array. Poster Presentation at 2021 SCEC Annual Meeting.

Related Projects & Working Groups
San Andreas Fault System (SAFS)