S-wave splitting in upper crust near the scientific drilling WFSD-3 at the Wenchuan earthquake fault zone

In order to cooperate with the Wenchuan Fault Scientific Drilling Program (WFSD),Institute of Geophysics, China Earthquake Administration deployed 15 microearthquake instruments in Tianchi township, Mianzhu city of Sichuan Province and set up 17 short-period seismic stations near the Guanxian-Anxian fault. This paper studies anisotropic characteristics of the upper crust based on the seismic data recorded by thes microearthquake and short period seismic arrays and permanent stations near the WFSD-3. The dominate polarization directions of fast S-wave and delay times of slow S-wave are derived by analyzing S-wave particle motion in the S-wave window. The results show that the polarization directions of fast S-wave in most of the research area are in NE, consistent with the strike of the Longmenshan fault. However, the Tianchi microearthquake array shows partition characteristics, which is in NE for eastern part and NW for western part, respectively. S-wave splitting is mainly caused by stress-aligned microcracks in the rock and the structures of the rock and mineral. The polarization directions of fast S-waves show mainly NE, consistent with the strike of the faults, which reflects crustal seismic anisotropy in the study area is mainly controlled by the structure. In some places, the polarization directions are NW, consistent with principal compressive stress, which demonstrates crustal seismic anisotropy is also affected by the local stress field. The change of the regional stress field is studied by analyzing the normalized delay times changing time. The normalized delay times were discrete in 2012-2013 and tended to converge in 2014, implying the stress field of the region had a tendency becoming stable after the strong earthquake.SCI(E)ARTICLE51690-17026