Internal structure of the San Jacinto fault zone at Jackass Flat from data recorded by a dense linear array

Hongrui Qiu, Yehuda Ben-Zion, Zachary E. Ross, Pieter-Ewald Share, & Frank L. Vernon

Submitted August 23, 2016, SCEC Contribution #6783, 2016 SCEC Annual Meeting Poster #219

We analyze seismograms recorded by a dense linear array crossing the San Jacinto Fault Zone (SW to NE) at Jackass Flat (JF) southeast of the trifurcation area. The array has 9 stations with ~20-30 m separation in the center and total aperture of ~400 m. Data of events recorded in 2012-2014 are used to search for fault zone head and trapped waves. Delay times between phases generated by ~3500 local earthquakes are utilized to estimate velocity variations across the array. We use automatic P picks for each source-receiver pair to calculate the ratio of each station pick to the array-wide average pick. Statistical analysis of all data indicates an initial increase followed by flattening and then decrease of relative slowness from SW to NE. The results suggest that all JF array stations are inside a fault damage zone. We perform a systematic search for fault zone head waves using an automatic detection algorithm and visual inspection. A set of events shows a clear moveout between head and direct P waves within the JF array with increasing moveout from SW to NE. However, data generated by different events and observed at a given station show no moveout with increasing source-receiver distance, implying that the head waves are generated by a local interface (perhaps the edge of the damage zone or basin) rather than a deep fault interface. Fault zone trapped waves are identified with automatic detection and visual inspection. The generating events have broad spatial distribution, implying a relatively shallow trapping structure. The trapped waves are observed clearly only by a few stations on the SW part of the array where the relative slowness is near maximum. Sets of waveforms recorded across the entire array, with candidate-trapped waves at the SW stations, are inverted with a genetic algorithm for fault zone parameters. The most likely parameters for the trapping structure are depth of 2-3 km, width of 100-200 m, Q value of 10-20, and velocity reduction of 40-50% from the host rock. Combining the results with the surface geology, the seismogenic fault is inferred to be located between the SW end stations JFS3 and JFS4.

Qiu, H., Ben-Zion, Y., Ross, Z. E., Share, P., & Vernon, F. L. (2016, 08). Internal structure of the San Jacinto fault zone at Jackass Flat from data recorded by a dense linear array. Poster Presentation at 2016 SCEC Annual Meeting.

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