Features of the Anxian - Guanxian fault zone in Longmenshan area of Sichuan Province: A case study of No. 3 hole of Wenchuan Earthquake Fault Zone Scientific Drilling (WFSD-3)

Wenchuan Earthquake Fault Zone Scientific Drilling (WFSD) was organized jointly by Ministry of Science and Technology, Ministry of Land and Resources and China Bureau of Seismology. The WFSD project plans to drill five scientific wells on the hanging wall of the co-seismic surface rupture zone, including the Yingxiu-Beichuan fault and the Anxian-Guanxian fault in Longmenshan area. The five drilling holes include WFSD-1, WFSD-2, WFSD-3, WFSD-3P and WFSD-4. The WFSD-3 and WFSD-3P drill holes are located on the hanging wall of the Anxian-Guanxian fault. Basic studies were carried out for the WFSD-3 drilling cores in such aspects as petrology, structural geology and borehole logging. The results show that the fault-rocks in the WFSF-3 contain fault breccia, cataclastic rock and fault gouge. The pseudotachylite does not appear in this well. Twenty-six fault zones were detected in this well, including the three main major fault zones of FZ634, FZ1215 and FZ1250, with the FZ1250 being the possible primary slip zone during the 2008 Wenchuan earthquake. The dip angles of the trench, WFSD-3P and WFSD-3 are respectively about 60°, 46° and 38°. The variation of the dip angle indicates that the Anxian-Guanxian fault is a listric thrust fault with low dip angle.