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Fault-zone trapped waves: High-Resolution Characterization of the Damage Zone on the Parkfield San Andreas Fault at Depth

Yong-Gang Li, Peter E. Malin, & Elizabeth S. Cochran

Published February 2012, SCEC Contribution #1321

This chapter presents that highly damaged rocks within the San Andreas fault (SAF) at Parkfield form a low-velocity waveguide to trap seismic waves. The amplitudes and dispersion feature of trapped waves are sensitive to the geometry and physical properties of the fault zone due to the constructive interference conditions of these waves. We use fault-zone trapped waves (FZTWs) generated by earthquakes and explosions and recorded at a cross-fault surface array and borehole seismographs at the San Andreas Fault Observatory at Depth (SAFOD) site to document fault zone structure and rock damage at seismogenic depths with high-resolution. Observations and 3-D finite-difference simulations of these FZTWs at dominant frequencies of 2-10 Hz show the downward tapering SAF characterized by a 30–40-m wide fault core with the maximum velocity reduction up to ~50% embedded in a 100–200-m wide zone with velocities reduced by 25-40% in average from wall-rock velocities. The width and velocity reduction of the damage zone at 3 km depth delineated by FZTWs are verified by the direct measurements in SAFOD drilling and logging studies at this depth [Hickman et al., 2007]. The results indicate the localization of severe rock damage on the SAF likely reflects pervasive cracking caused by historical earthquakes on it. The magnitude of damage varies with depth and along the fault strike due to rupture distributions and stress variations over multiple length and time scales. The damage is not symmetric across the main slip plane but extends farther on the southwest side of the main fault trace. Based on the depths of earthquakes generating prominent FZTWs, we estimate that the low-velocity damage zone along the SAF at Parkfield extends at least to depths of ~7-8 km.

Li, Y., Malin, P. E., & Cochran, E. S. (2012). Fault-zone trapped waves: High-Resolution Characterization of the Damage Zone on the Parkfield San Andreas Fault at Depth. In Li, Y. (Eds.), Imaging, Modeling and Assimilation in Seismology, (Series in Global Change and Earth System Science, pp. 43) Boston, : DeGruyter