Seismic Study of the San Andreas Fault in California and the Longmen-Shan Fault Ruptured in the M8 Wenchuan Earthquake in China on May 12, 2008

Yong-Gang Li

Published 2008, SCEC Contribution #1359

Knowledge of the fine-scale internal structure of active faults holds the key to understanding the physics of earthquakes and estimation of earthquake risk. The spatial extent of fault weakness, and the loss and recouping of strength across the earthquake cycle are critical ingredients in understanding of fault mechanics and earthquake physics. The results from the fault-zone trapped waves (FZTWs) recorded at our portable arrays deployed at the San Andreas Fault (SAF) near Parkfield for local earthquakes and aftershocks of the 2004 M6 Parkfield earthquakes indicate that the active fault zone undergoes strong dynamic stresses and pervasive cracking during a major earthquake. We have delineated a distinct low-velocity waveguide on the SAF using FZTWs and interpreted it as being a remnant of break-down zone that accumulated damage from historical earthquakes, including the latest M6 event in 2004. This zone co-seismically weakens and consequently heals. The damage zone is approximately 200-m wide and likely extends to the depth of ~7 km with the velocity reduction of 20-50% from wall-rock velocities. We further study the damage magnitude and extent on the deep portion of the SAF using the borehole data recorded at seismographs installed in the SAFOD mainhole passing the SAF at ~3-km depth and in the pilot hole. The data set includes three-component seismograms from ~350 well-located aftershocks of the 2004 M6 Parkfield earthquake which provide us more constraints on variations in rock damage along the fault strike and with depth.
In view of the success with trapped wave studies in resolving the physical characteristics of these major rupture zones in California, it is logical to apply the above method for the investigation of the Longmen-Shan Fault which ruptured in the M8 Wenchuan earthquake on May 12, 2008. The knowledge of mapping the 3-D spatial patterns on the active faults in California using fault-zone trapped waves will help with the design of the seismic experiment,data analysis, modeling and interpretation for investigation of the Longmen-Shan Fault in China.
A comparison of the results from the Longmen-Shan Fault with those obtained at the San Andreas Fault will allow us to further understand the mechanical properties of active fault zones,including the magnitude and extension of rock damage associated with big earthquakes on them, and earthquake processes and hazards globally.

Citation
Li, Y. (2008). Seismic Study of the San Andreas Fault in California and the Longmen-Shan Fault Ruptured in the M8 Wenchuan Earthquake in China on May 12, 2008. Los Angeles, : Chinese Scholars Association - Southern California.