Structural Relationship Between the 2008 M8 Wenchuan and 2013 M7 Lushan Earthquakes Viewed by Fault-Zone Trapped Waves

Abstract

We analyzed waveform data recorded at seismic network and temporal stations for aftershocks of the 2013 M7 Lushan earthquake and the 2008 M8 Wenchuan earthquake in both time and frequency. Prominent fault-zone trapped waves (FZTWs) with large amplitudes and long post-S coda durations are identified in seismograms at stations along the surface trace of the Xinkaidian fault (XF), but not at stations along the Dachuan-Shuangshi fault (DSF) to west and the Dayi fault (DF) to east within a thrust-folding region with multiple imbricated faults in Lushan area. Because the amplitude and dispersion of FZTWs are sensitive to the geometry and physical property of the fault-zone as well as the location of source and station relative to the fault zone, the recorded FZTWs infer a low-velocity waveguide existing along the XF. We interpret that this waveguide is formed by severely damaged rocks within the XF, which most likely ruptured at seismogenic depth in the 2013 M7 Lushan earthquake although the rupture did not reach the earth surface. 3D finite-difference simulations of these FZTWs show that seismic velocities within the XF rupture zone are reduced by ~40–50% from wall-rock velocities to form a remarkable low-velocity waveguide trapping seismic waves. We find that the FZTWs generated by Lushan aftershocks are also recorded at network stations located within Wenchuan rupture zones along the Yingxiu-Beichuan Fault (YBF) and Pengxian-Guanxian Fault (PGF) although there exists a ~45-km “seismic gap” between the Lushan and Wenchuan epicentral zones. Vice versa, FZTWs generated by Wenchuan aftershocks are recorded not only at stations within Wenchuan rupture zones but also at stations along the XF in Lushan area. Observations and finite-difference simulations show that a continuous waveguide extends along multiple faults of the south (LMSF) system. However, the waveguide effect (corresponding to the rock damage magnitude and velocity reduction) varies with the greater trapping efficiency along Wenchuan and Lushan rupture zones than the un-ruptured fault in the “seismic gap.” Therefore, we consider that the 2008 M8 event and the 2013 M7 event consist of an earthquake sequence along the LMSF system, and the Lushan event is the largest aftershock of the Wenchuan earthquake.