The Structure Characteristics Of The Wenchuan Earthquake Fault Zone And Its Relationship With Seismic Activity

Longmen Shan fold-and-thrust belt locate at the eastern margin of the Tibetan Plateau, which comprising the Yingxiu-Beichuan fault, Guanxian-Anxian fault and Maoxian-Wenchuan fault. The Wenchuan earthquake (Ms 8.0) struck the Longmen Shan area, which produced a large co-seismic surface rupture zone along the Yingxiu-Beichuan and Guanxian-Anxian fault zones, and aroused the attention to tectonic structures and fault activity. In order to investigate the faulting mechanism and physical chemistry change during the earthquake, and to get the information of aftershoke, the Wenchuan Earthquake Fault Scientific Drilling (WFSD) Project was rapidly carried out. The WFSD-1 was drilled 1200-m-deep on the Yingxiu-Beichuan fault, with about 80 degree-tilt angle perpendicular to the direction of the surface rupture zone of 2008 Mw 8.0 Wenchuan earthquake. The WFSD-1 penetrated the Longmenshan fault, and core samples were recovered from between 6- and 1201.15-m depth. Based on the research of WFSD-1 core and the co-seismic surface rupture zone, as well as the temperature measurements in WFSD-1 borehole, the composition features and structures of the Longmenshan fault belt, together with the infault gouge and its relationship with seismic activity are discussed.According to our reaserch, we identify that FZ590 most probably corresponds to the slip surface of the Wenchuan earthquake, which is the principal slip zone (PSZ) during Wenchuan arthquake and produced approximately 2cm fresh fault gouge; frictional heat anomaly is one of the most important factor to indentify the principal slip zone. From the fracture density profile of WFSD-1 core and the logging data, about 450-m Yingxiu-Beichuan fault zone is recognized, with a general trend of N40o-43oE, NW tendency; 60o-65odip angle. The fracture density profile shows a symmetrical structure of fracture characteristic with the principal slip zone (PSZ) as the central, which is composed of many small sub-faults (damage zone) that consist of fault breccia, cataclasite and/or fault gouge, and small amounts pseudotachylite in some faults. The thickness of the gouge in the fault zone ranges from several millimeters to 25 centimeters, consistenting with the fault characteristics recorded in the surface rupture zone. Only several millimeters to approximately 2 centimeters gouge can be formed in one large earthquake, from the results of the Taiwan Chelungpu-fault Drilling Project (TCDP) and WFSD Project, so we can infer that each layer of gouge in Yingxiu-Beichuan fault zone might be produced by at least 1 to 13 large earthquakes. The total thickness of the gouge in this area is about 150 cm, indicating at least 183 earthquake events, and suggesting that strong earthquakes repeatedly occurred along the Yingxiu-Beichuan fault zone.Based on the distribution and micro-layers with different grain size in gouge, each earthquake does not completely slip along the PSZ of the older earthquake, but rather along the edge of the gouge. According to the gouge statistics of the whole fault zone, seismic events have the obvious tendency towards the foot wall, and the thickness of gouge is proportional to the activity of the fault, indicating that the width of fault zone is directly related to the number and evolution history of earthquakes, This fault-grow process reflects the the relationship between fracture stress migration and the growth of mountain. Consequently, we suggest that crustal shortening related to repeated great seismic events, may be a key component of the geodynamics that drive ongoing mountain building on the eastern Tibetan Plateau.