Study on the deep seismic wide-angle reflection/refraction profile in the middle of Longmenshan fault zone

Ray tracing is an important method to study seismic wave propagating in the lateral inhomgeneous media of the crust. Partial derivatives of theoretical trival time with respect to the velocity at the meshing point of the model are derived. Simultaneous iterative reconstruction tomography (SIRT for short) method is used to invert the data of a deep seismic wide-angle reflection/refraction profle across the middle of Longmenshan fault zone, which can cover the shortage of time consuming and non-quantitative error analysis for trial-and-error method. Observed data for each shot are firstly inverted to get a one-dimensional velocity model, and then a simple two-dimensional velocity model is obtained by interpolating method. The two-dimensional model is adjusted by fitting observed and theoretical travel times, which makes the new model more close to the real one. SIRT method is further used to travel time inversion, and thus reaches final two-dimensional crustal velocity structure. The inverted result shows that the deposit in the east part of the profile is thicker than that in the middle folded zone and the western plateau, and there is basement rock appearing in the part area of the middle folded zone. The interfaces of the strata in the two sides of the transitional zone are almost horizontal, and in the western side of the transitional zone, there is a discontinuity interface in both the middle and lower crust. In addition, there are also lower velocity layers with different thickness in the transitional zone, and their thickness shows an increase tendency from east to west. It should be mentioned, that the three main faults in Longmenshan fault zone cut deeply the bottom of basement, which was resulted from the upward climbing movement of Songpan-Garze block to the eastern direction along a shovel-like fault and prevented from Yangtze block. There is a maximum shear stress region in the footwall of the fault inside the upper crust at the depth of 15 km, which is just the source location of the M_S8.0 Wenchuan earthquake.