We relocated M8.0 Wenchuan earthquake and 2706 aftershocks with M≥2.0 using double-difference algorithm and obtained relocations of 2553 events. To reduce the influence of lateral variation in crustal and upper mantle velocity structure, we used different velocity models for the east and west side of Longmenshan fault zone. In the relocation process, we added seismic data from portable seismic stations close to the shocks to constrain focal depths. The precisions in E-W, N-S, and U-D directions after relocation are 0.6, 0.7, and 2.5 km respectively. The relocation results show that the aftershock epicenters of Wenchuan earthquake were distributed in NE-SW direction, with a total length of about 330 km. The aftershocks were concentrated on the west side of the central fault of Longmenshan fault zone, excluding those on the north of Qingchuan, which obviously deviated from the surface fault and passed through Pingwu-Qingchuan fault in the north. The dominant focal depths of the aftershocks are between 5 and 20 km, the average depth is 13.3 km, and the depth of the relocated main shock is 16.0 km. The depth profile reveals that focal depth distribution in some of the areas is characterized by high-angle westward dipping. The rupture mode of the main shock features reverse faulting in the south, with a large strike-slip component in the north.Wenchuan earthquake, Longmenshan fault zone, earthquake relocation, regional velocity model, first arrival On May 12, 2008 at 14:28 local time, a massive earthquake of M8.0 occurred in Longmenshan tectonic zone near Wenchuan County, Sichuan Province (31.0°N, 103.4°E). The earthquake is one of the deadliest and strongest earthquakes to hit the Chinese mainland since 1949. It struck an area of 100000 km 2 and claimed more than 80000 lives. Such a severe disaster has rarely been seen in China in terms of its power, range, and damaging effect.The Longmenshan fault zone, which is the east border of Qinghai-Tibet Plateau, consists of the back-range fault (F1), the central fault (F2), and the front-range fault (F3) from west to east [1][2][3][4][5] (Figure 1). Its south and middle segments (102.0°-104.5°E) feature reverse faulting, where earthquakes occurred frequently, and the largest one (M6 1 / 2 ) in history occurred in Wenchuan in 1657.The seismicity in north segment is not very active [5,6] . The initial rupture of the Wenchuan earthquake originates just in the seismically active central south part, and results of source rupture process show that the rupture length is 300-450 km, though certain differences exist among the results [7] .The epicenter (initial rupture point) of Wenchuan earthquake is located on the central fault of Longmenshan fault zone. The initial location results indicate that the aftershocks cover the back-range, the front-range, and the central faults, with focal depths ranging between 0 and 50 km. Preliminary analysis reveals large focal depth error, which results from complexity of the crustal velocity structure in Longmenshan fault zone. The fault zone acts...
