Tomography of the three dimensional P-wave velocity structure in the source region of the M_S7.0 Lushan, Sichuan, earthquake and its surrouding areas

Using 28188 P-wave travel time data from 2026 regional seismic events recorded by digital seismic network and transportable seismic stations of Sichuan Province after Lushan M_S7.0 earthquake, we conducted seismic tomography to obtain the middle-upper crustal 3D P-wave velocity structure in the souce region of Lushan earthquake and its surrounding areas. The findings indicate that the spatial distribution of velocity anomalies in the shallow upper crust is closely related to the surface geological structure, terrain and lithology. Chengdu basin shows low-velocity anomalies associated with the Quaternary sedimentary area; the slight rising area of Qianwei-Leshan in central Sichuan basin and the adjacent district to the west of the Longmenshan fault of Sichuan-Qinghai block are characterized by high-velocity anomalies related to tectonic uplift; the area near Baoxing and Kangding with basic volcanic rocks and volcanic clastic rocks presents obvious high-velocity anomaly. The epicenter of the Lushan earthquake is located near the boundary of the high velocity and low velocity, and obvious low velocity anomaly exists below this region. This low velocity anomaly body may be associated with the presence of fluid. The fluid effect leads to the weakening of seismogenic layer in the interior of middle-upper crust and may trigger the Lushan earthquake. The aftershock areas of Lushan earthquake and Wenchuan earthquake are 50 km apart. At the depth range around the earthquake source, this 50 km seismicity gap exhibits high-velocity anomaly. Combined with the typical dislocation geomorphic structure at the southwest section of the Longmenshan fault, the deep and shallow structures are complex beneath this seismic gap, namely, the Dayi-Qionglai active fault segment that is not ruptured previously, has potential seismic hazards that deserve further study.