The Principle Slip Zone of the 2008 Wenchuan earthquake: a thrust fault oblique cutting the Yingxiu-Beichuan fault zone

Abstract：The May 2008 Ms 8.0 disastrous Wenchuan earthquake occurred in the eastern margin of Tibetan Plateau, producing 270 and 80 km-long different kinematics features coseismic surface ruptures along the Yingxiu-Beichuan and Anxian-Guanxian faults, respectively. Drilling in active fault zones, especially after large earthquakes, is an effective way to study earthquake mechanisms, in order to better understand the fault mechanisms during the earthquake process as well as physical and chemical characteristics of rocks. The Wenchuan earthquake Fault Scientific Drilling project was carried out on November 6, 2008 (178 days after the Wenchuan earthquake). Five boreholes were ultimately drilled along the Yingxiu-Beichuan and Anxian-Guanxian faults, ranging from 600 m to 3000 m in depth. This paper focuses on the cores of the first hole (WFSD-1). Petrological and structural analysis of the cores allowed the identification of the distribution and combination of fault-related rocks in the Yingxiu-Beichuan fault, and the Principle Slip Zone (PSZ) location of the Wenchuan earthquake was determined, which provided a basis for further research on earthquake mechanism.The detail analysis of the cores reveals the existence of a fault zone, which is about 100 m wide from 575m to 759 m in depth and consists of gouge, cataclasite, and fault breccia. From the top to the bottom there exist mainly cataclasite zone, fault gouge and fault breccias mixed zone, breccias zone and fault gouge zone. This fault zone lies between the Neoproterozoic Pengguan complex and Triassic Xujiahe sedimentary rocks, which is the Yingxiu-Beichuan fault zone, mainly distributed in Xujiahe Formation. The Principle Slip Zone (PSZ) of the Wenchuan earthquake was found at ~589 m with ~1 mm fresh fault gouge, which lies in the upper part of Yingxiu-Beichuan fault zone, as evidenced by logging data, and clay mineral analysis, in combination with the microstructure of the gouges. There exist similar structures between Pengguan complex and Triassic Xujiahe Formation in Hongkou outcrop with the thickness of about 190m, lying at the bottom of the Yingxiu-Beichuan fault zone. Hence, The Wenchuan earthquake fault slip cut the Yingxiu-Beichuan fault zone obliquely, indicating that Wenchuan earthquake fault might be a new high-angle thrust fault with a dip angle of 62°-65°, which did not totally follow the slip zone of ancient earthquake and might be related to the earthquake magnitude. It is the result of the long term earthquake activity and evolution from about 10-15 million years ago to the present, which formed the 100~190 m-wide Yingxiu-Beichuan fault zone.