Citation: Zheng Y, Ma H S, Lü J, et al. Source mechanism of strong aftershocks (M s ≥5.6) of the 2008/05/12 Wenchuan earthquake and the implication for seismotectonics.Dozens of >M5, hundreds of >M4, and much more >M3 aftershocks occurred after the 2008/05/12 Wenchuan earthquake, which were well recorded by permanent and portable seismic stations. After relocated with P arrival, the >M3 aftershocks show two trends of distribution, with most of the aftershocks located along the north-east strike consistent with Longmenshan fault system, yet there is a north-west trend around the epicenter. It seems that substantially more aftershocks occur in regions with crystalline bedrocks. Then we collected waveform data from National Digital Seismograph Network and regional seismograph network of China, and employed "Cut and Paste" method to obtain focal mechanisms and depths of the big aftershocks (M≥5.6). While most of those aftershocks show thrust mechanism, there are some strike slip earthquakes in the northern-most end of the rupture. Focal mechanisms show that the events located on the southern part of central Beichuan-Yingxiu Fault (BY) are mainly thrust earthquakes, which is consistent with initial mechanism of the main shock rupture. In the north part the aftershocks along the BY are also dominated by thrust slip, which is quite different from the right slip rupture of the main shock. Around Qingchuan-Pingwu Fault, the focal mechanisms are dominated by right-slip rupture with large depths (~18 km). So we suspected that in the north part the main shock might rupture on two faults: Beichuan Fault and Qingchuan-Pingwu Fault. The complex pattern of aftershock mechanisms argues for presence of a complicated fault system in the Longmenshan area.focal mechanism, seismotectonics, Wenchuan earthquake, cut and paste.

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Surface wave dispersion imaging approach is crucial for multi-channel analysis of surface wave (MASW). Because the resolution of inversed S-wave velocity and thickness of a layer are directly subjected to the resolution of imaged dispersion curve. The τ-p transform approach is an efficient and commonly used approach for Rayleigh wave dispersion curve imaging. However, the conventional τ-p transform approach was severely affected by waves amplitude. So, the energy peaks of f-v spectrum were mainly gathered in a narrow frequency range. In order to remedy this shortage, an improved τ-p transform approach was proposed by this paper. Comparison has been made between phase shift and improved τ-p transform approaches using both synthetic and in situ tested data. Result shows that the dispersion image transformed from proposed approach is superior to that either from conventionally τ-p transform or from phase shift approaches.

The MASW method is robust in determine shear wave velocity of shallow site because the dispersive properties of Rayleigh wave was dominated by shear wave velocity of subsurface. Using this method, an assumption that the earth model is one dimensional and horizontal layered must be put to simplify the real earth model without considering the lateral variation. However, it is not always the truth. In order to obtain a two dimensional S-wave velocity profile of shallow site, a CMP cross-correlation (CMPCC) method was proposed by Hayashi and Suzuki (2004) to approximate two dimensional S-wave velocity profile with one dimensional inversion procedure. In order to verify its approximate resolution, a horizontal stepped layer model and a dipping layer model were chosen. The synthetic wave fields of the two models were calculated by staggered grid finite difference method. Result shows that this method can only be used to approximate horizontally stepped layer model and cannot be used to approximate dipping layer model.