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Constraints on Fault Damage Zone Properties and Normal Modes from a Dense Linear Array Deployment along the San Jacinto Fault Zone

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

Published August 11, 2016, SCEC Contribution #6655, 2016 SCEC Annual Meeting Poster #201

We present earthquake data and statistical analyses from a month-long deployment of a linear array of 134 Fairfield three-component 5 Hz seismometers along the Clark strand of the San Jacinto fault zone in Southern California. With a total aperture of 2.4km and mean station spacing of ~20m, the array locally spans the entire fault zone from the most intensely fractured core to relatively undamaged host rock on the outer edges. We recorded 36 days of continuous seismic data at 1000Hz sampling rate, capturing waveforms from 751 local events with Mw>0.5 and 43 teleseismic events with M>5.5, including two 600km deep M7.5 events along the Andean subduction zone. For any single local event on the San Jacinto fault, the central stations of the array recorded both higher amplitude and longer duration waveforms, which we interpret as the result of damage-related low-velocity structure acting as a broad waveguide. Using 271 San Jacinto events, we compute the distributions of three quantities for each station: maximum amplitude, mean amplitude, and total energy (the integral of the envelope). All three values become statistically lower with increasing distance from the fault, but in addition show a nonrandom zigzag pattern which we interpret as normal mode oscillations. This interpretation is supported by polarization analysis which demonstrates that the high-amplitude late-arriving energy is strongly vertically polarized in the central part of the array, consistent with Love-type trapped waves. These results, comprising nearly 30,000 separate coseismic waveforms, support the consistent interpretation of a 450m wide asymmetric damage zone, with the lowest velocities offset to the northeast of the mapped surface trace by 100m. This asymmetric damage zone has important implications for the earthquake dynamics of the San Jacinto and especially its ability to generate damaging multi-segment ruptures.

Key Words
Damage Zone, Normal Modes, Dense Deployment

Allam, A. A., Lin, F., Share, P., Ben-Zion, Y., Vernon, F. L., Schuster, G., & Karplus, M. (2016, 08). Constraints on Fault Damage Zone Properties and Normal Modes from a Dense Linear Array Deployment along the San Jacinto Fault Zone. Poster Presentation at 2016 SCEC Annual Meeting.

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