Precise Location Of The Wenchuan Earthquake Sequence

A MS8.0earthquake occurred in Wenchuan County, Sichuan Province, on May12,2008. The earthquake ruptured the Longmenshan fault zone on the eastern margin of Tibetan plateau. The epicenter locates at south west side of Yingxiu Town (N31.0°, E103.4°; USGS) with a depth of14km.numerous aftershocks are followed with the main shock. According to China earthquake networks center, there are2146aftershocks with magnitude2.0and above up to November30,2008, in which there are770aftershocks with magnitude2.0～2.9,752with magnitude3.0～3.9,548with magnitude4.0～4.9,65with magnitude5.0-5.9,11with magnitude6.0-6.9.In this paper1028aftershocks from May12,2008to July1have been located precisely using a method of Multiparameter Seismic Location. The parameters of apparent emergence angles and arrival times are extracted according to the wave data with EVT format. The objective function in the method of Multiparameter Seismic Location with apparent emergence angles and arrival time was founded. Monte Carlo method was used to realize its optimization. Apparent emergence angle has the very strong sensitivity to depth. The results of Multiparameter Seismic Location are able to guarantee the accuracy in the directions of latitude and longitude, especially to enhance the depth resolution effectively.The results of Multiparameter Seismic Location show that the earthquake sequence distributes densely along the central Longmenshan fault zone taking on a spatial belt with NE direction. The earthquake belt formed by the aftershocks may be divided into two segments by the Nanba. South of the Nanba, the epicenters distribute in the north west side of Yingxiu-Beichuan fault zone; north of Nanba, the epicenters distribute in the both sides of Pingwu-Qingchuan fault. The average depth of earthquake sequence is12.8km, distributing in a range of4～20km (up crust), its superiority distribution are in8-18km, coming together the detachment plane (belt) with a depth of20km approximately. The depth distributing shows segmented fault. The southern and northern segments are thrust and strike-slip respectively. The aftershocks locate intensively at the fault ramp, which is the region of stress accumulating (with depth about8-18km) and the main shock occurred at this location.The India plate pushes the Tibet plateau to the north to force the middle part (Bayankala plate) moving to southeast. Low speed structure of the lower crust has provides relaxed boundary condition to the transversal motion, the Bayankala plate moves along detachment surface to the southwest, it is strongly resisted by the South China plate (the Sichuan basin), the stress is accumulated at the fault ramp in long-term. It is the reasons that the Wenchuan earthquake occurred.