Parkfield fault-zone guided waves: Low-velocity damage zone on the San Andreas fault at depth near SAFOD site at Parkfield by fault-zone trapped waves

Yong-Gang Li, Peter E. Malin, & John E. Vidale

Published 2007, SCEC Contribution #1139

We deployed a dense linear array of 45 seismometers across and along the San Andreas fault near the SAFOD site at Parkfield in 2003 to record fault-zone trapped waves generated by near-surface explosions and microearthquakes located within the fault zone. Observations and 3-D finite-difference simulations of the fault-zone trapped waves generated by microearthquakes at different depths show a ~100-200-m-wide low-velocity zone (LVZ) along the SAF reaching to depths of 6-to-7 km or more, within which shear velocities are reduced by 20-40% in average from wall-rock velocities with the maximum velocity reduction of 40-50% in a 30-40-m-wide fault core zone, indicating the localization of severe damage on the SAF. The damage zone is not symmetric but extends farther on the southwest side of the main fault trace. The width and velocities of this zone delineated by trapped waves are confirmed by the results of the SAFOD drilling and logs that show high porosity and multiple slip planes in a ~200-m-wide LVZ across the SAF with velocity reduction of ~20-30% at ~3.2 km depth, within which a ~35-m-wide severely damaged zone is seen around the main slip [Hickman et al., 2005]. Recently, the down-hole seismic seismograph installed in the SAFOD mainhole at ~3 km depth recorded prominent fault-zone guided waves with long-duration dispersive wavetrains after S-arrivals from microearthquakes occurring below [Ellsworth and Malin, 2006]. Finite-difference simulations of these waves show a ~30-40-m-wide core zone with velocities as low as 50% of the intact rocks embedded in the wider fault zone with intermediate velocity reduction, indicating that the damage zone on the SAF at Parkfield extends to the depth of at least several kilometers below 3 km.

Citation
Li, Y., Malin, P. E., & Vidale, J. E. (2007). Parkfield fault-zone guided waves: Low-velocity damage zone on the San Andreas fault at depth near SAFOD site at Parkfield by fault-zone trapped waves. Scientific Drilling Journal,(Special Issue, No. 1), 73-77. doi: 10.2204 /iodp.sd.s01.09.2007.