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Internal structure of the San Jacinto fault zone at Blackburn Canyon from a dense linear deployment across the fault

Yehuda Ben-Zion, Pieter-Ewald Share, Amir A. Allam, Fan-Chi Lin, & Frank L. Vernon

Published August 5, 2016, SCEC Contribution #6526, 2016 SCEC Annual Meeting Poster #185

We image the internal structure of the Clark section of the San Jacinto fault zone (SJFZ) at Blackburn Canyon using teleseismic and local earthquake waveforms recorded during about 1 month by a linear array consisting of 125 three-component 5 Hz geophones with an aperture of 2.4 km. The instrument spacing is 10 m in a zone 400 m wide centered on the surface trace of the fault, and ~30 m to the NE and SW of that zone. Analysis of 4 teleseismic events indicates an abrupt change in the horizontal first motions of P phases around the central station, BS55, coinciding with the surface trace. Variations in P arrival times suggest a zone of slowness from station BS55 to ~350 m to the NE, with maximum slowness ~130 m from BS55. Automatic algorithms are used to detect P fault zone head waves (FZHW) and S fault zone trapped waves (FZTW) generated by local events. Over 200 FZHW candidate phases are detected for 57 M>1 events located SE of the array along the SJFZ. Inspection of the picks reveals probable FZHW in most stations within ~1 km wide zone to the SW of station BS32 (~350 m NE from the central station BS55). FZTW are detected for 36 M>1 events located in a very broad region around the array. When considering only stations in the central 400 m zone, 76% of the FZTW detections are made for stations in the 200 m wide zone NE of BS55. A 100 m wide internal region with 10 stations (B40 to B50), centered 100 m NE of BS55, has the largest amplitude and lowest frequency for waveforms with FZTW, and most likely overlies the core active damage zone of the Clark fault. Based on these initial findings and geological constraints, the main Clark fault at depth is likely closest to station BS55, damage is asymmetric to the NE with most damage confined to a ~100 m zone, and the damage zone terminates at a bimaterial interface close to station BS32. Further analysis is needed to confirm the discussed results and conclusions.

Ben-Zion, Y., Share, P., Allam, A. A., Lin, F., & Vernon, F. L. (2016, 08). Internal structure of the San Jacinto fault zone at Blackburn Canyon from a dense linear deployment across the fault. Poster Presentation at 2016 SCEC Annual Meeting.

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