Hypocentral location and source mechanism of the MS7.0 Lushan earthquake sequence

Based on the digital waveforms of China National Seismic Network and Sichuan Seismic Network, The Lushan earthquake sequence (M-L >= 2.0) was relocated precisely by HypoDD, 36 best double-couple focal mechanisms (M-L >= 4.0) were determined by CAP method. The geometric fault parameters and slip direction were estimated by using distribution of the Lushan earthquake sequence and regional stress field. We analyzed the characteristics of focal depth, focal mechanisms and source rupture to determine the seismogenic structure. The main shock was relocated at 30.30 degrees N, 102.97 degrees E; the depth of the initial rupture and centriod is about 15 km and 14 km, respectively; the nodal plane parameters of the best double-couple focal mechanism, strike 209 degrees, dip 46 degrees and slip 94 degrees; strike 23 degrees, dip 44 degrees and slip 86 degrees, respectively. The focal mechanism of main shock is pure thrust type, and most of the focal mechanisms of the aftershocks show the same rupture property as the main shock. The aftershocks (M-L >= 2.0) distributed in two sides of the main shock, the length of sequence is about 30 km, the focal depth is centrally distributed in 5 similar to 27 km; the focal depth of the aftershocks (M-L >= 3.5) is centrally distributed in 9 similar to 25 km and show the focal fault dip toward northwest. We determined the focal fault parameter strike 207 degrees, dip 50 degrees and slip 92 degrees using distribution of sequences and regional stress field, most aftershocks distributed in the 10 km vicinity of the focal plane. Combine the distribution of earthquake sequence, the focal depth of the main shock with the result of rupture process, we infer that the main rupture extend along the two sides of the fault from the initial rupture point, the southern rupture is a little longer than the northern part, the slip focus on the vicinity of the initial rupture point and the rupture is not up to the surface. Based on the results of this paper, the distribution of seismic intensity and field investigation, we speculate preliminarily the focal fault is the front fault of Longmenshan, but we do not rule out the possibility that there is an unknown base fault nearby the east of main-shock.