Co-seismic thrusting slip and shortening structures produced by the 2008 M?? 7.9 Wenchuan earthquake, China

The magnitude (Mw) 7.9 Wenchuan earthquake occurred on 12 May 2008 in the Longmen Shan region, the transition zone between the Tibetan Plateau and the Sichuan Basin, China, resulting in extensive damage throughout central and western China. Official estimates of casualties released by the Chinese Government as of 31 August 2008 include 69,197 confirmed deaths, 374,176 injured, and 18,209 missing persons. To understand the seismic faulting mechanism and surface deformation features associated with the earthquake, including rupture length and slip distribution, our survey group traveled to the epicentral area 2 days after the earthquake and undertook 10 days of fieldwork, collecting primary data related to rupture structures and slip distribution. Based on this preliminary survey, we carried out additional detailed fieldwork on the co-seismic surface rupture over the past 3 months. Here we report the main results of our field investigations and discuss the co-seismic rupturing mechanism, including its implications for seismotectonics in the eastern marginal zone of the Tibetan Plateau. Field data reveal that the Wenchuan earthquake produced a 285-km-long surface rupture zone along the Longmen Shan Thrust Belt upon the eastern margin of the Tibetan Plateau, with a dominantly thrusting slip accompanied by a right-lateral component in the northern segment of the rupture zone, and a left-lateral component in the southernmost segment. The co- seismic ruptures mainly occurred along the pre-existing Yingxiu-Beichuan, Guanxian-Anxian, and Qingchuan faults, which are the main faults of the Longmen Shan Thrust Belt. The displacements measured in the field are approximately 0.5-5 m in the vertical, generally 1-3 m, accompanied by an average left-lateral component of <2 m in the 60-km-long southernmost segment of the rupture zone, and an average right-lateral component of <1 m in the 100-km-long central-northern segment. The rupture length and maximum displacement are the largest among intracontinental thrust-type earthquakes reported to date. Our results document and confirm that i) the Wenchuan earthquake occurred on the pre-existing active faults of the Longmen Shan Thrust Belt, which controlled the spatial distribution of co-seismic surface rupture and displacement and the rupture processes of the earthquake; ii) the long rupture length and large thrusting slip resulted from compressive stress associated with eastward extrusion of the Tibet Plateau as it accommodates the ongoing penetration of the Indian Plate into the Eurasian Plate; and iii) present-day shortening strain upon the eastern margin of the Tibetan Plateau is mostly released by seismic slip along thrust faults within the Longmen Shan Thrust Belt.