Fault-Zone Trapped Waves at a Dip Fault: Documentation of Rock Damage on the Thrusting Longmen-Shan Fault Ruptured in the 2008 M8 Wenchuan Earthquake

Yong-Gang Li, Jin-Rong Su, & Tian-Chang Chen

Published February 2012, SCEC Contribution #1669

This Chapter presents observations and 3-D finite-difference simulations of fault-zone trapped waves (FZTWs) recorded at the south Longmen-Shan fault (LSF) with varying dip angles, which ruptured in the 2008 M8 Wenchuan earthquake in Sichuan, China. Results from the FZTWs show a distinct low-velocity zone (LVZ) composed by severely damaged rocks at seismogenic depths. Through numerical investigations of trapping efficiency for a dip fault, we imaged a damaged zone several hundred meters wide along the thrusting LSF, within which seismic velocities are reduced by ~30-60% from wall-rock velocities with the maximum velocity reduction in the fault core at shallow depth. We interpret this remarkable LVZ as being a break-down zone accumulating damages caused by dynamic rupture in historical major earthquakes, mainly in the 2008 M8 earthquake, which eventually forms a distinct low-velocity waveguide to trap seismic waves. Because the amplitude and dispersion feature of FZTWs are sensitive to the source location with respect to the waveguide, these waves allow us to delineate the geometry of fault-zone damage along with the principal slip of the Wenchuan mainshock at seismogenic depth based on locations of those aftershocks generating prominent FZTWS. By examining the changes in the dispersion features of FZTWS recorded at the same station for similar earthquakes occurring before and after the 2008 Wenchuan earthquake, we estimate that seismic velocities within the LVZ along the south LSF was reduced by ~10-15% likely due to the co-seismic damage of fault rocks (with rigidity weakening) during the 2008 M8 mainshock. This value is larger than the damage magnitude of fault rocks caused by the 1992 M7.4 Landers, 1999 M7.1 Hector Mine and 2004 M6 Parkfield earthquakes in California [Li et al., 1998, 2003, 2006, 2007; Vidale and Li, 2003], probably due to the different sizes of slip and stress drop, and faulting mechanisms in these earthquakes.

Citation
Li, Y., Su, J., & Chen, T. (2012). Fault-Zone Trapped Waves at a Dip Fault: Documentation of Rock Damage on the Thrusting Longmen-Shan Fault Ruptured in the 2008 M8 Wenchuan Earthquake (Series in Global Changes and Earth System Science). Boston, : De Gluyter.