Abstract:The Tianshan Mountains, also known as the Snow Mountains, lie across Eurasia to border the Xinjiang Uygur Autonomous Region and Gansu province in eastern China to the western region of The Republic of Kazakhstan and Kyrgyzstan. In the west, the Tianshan Mountains split from the Ili River valley and reveal green valleys and splendid peaks. These mountains are situated between the Tarim plate and the Junggar plate and reach an altitude of 2 500 km and span 300~500 km. The mid-eastern segments of the Tianshan Mountains were selected as the study area (79°-98°E, 39°-46°N), and we utilized 35 051 natural events from 2001 to 2010 to study the crustal velocity structure. The data contains 418 915 P- and S-wave phases, which were recorded by the Xinjiang regional seismic network. We divided the research area with a mesh grid on the basis of the previous results and the character of the seismic ray distribution. Next, the finite difference tomography method and a programmer were used to invert the P- and S-wave velocity disturbance and the P- and S-wave velocity structure. Analysis of the relationship between tomography and corresponding explanations was conducted on the basis of the relationship between velocity structure and great earthquakes in history. An additional velocity model was used with several iterations of calculation to verify our results. The two models showed very close results at the same tectonic position with the same parameters. Therefore, the data and methods are credible in this paper. During the process of the iteration, the results of relocate events and ray tracing error decreased gradually with the iteration time increase. The range of error was relatively narrow after three iterating three times; thus, the third iteration calculation result was used for resolution in this paper. The resolution of image was better at a depth of 60 km, and the result was verified after testing by checkerboard in the study area. The tomographic results suggest that the deep structures and geodynamics have significant impacts on deformations and earthquake activities in the crust in addition to the mountain building, collision, and dynamics of the entire Tianshan Mountain belt. The fluctuation of the crust velocity structure was dramatic. The low velocity area could be related to the sedimentary layer thickness and the buried depth of the basement. There are obvious high and low velocity gradient zones in the middle and the lower regions of the crust. The distributions of the high and low velocity at 60 km indicate that the dynamic contact mode of the basin and mountains conjunction is limited in the front regions of north and south Tianshan. In addition, the Moho surface depth decreased gradually from the west to the east. At the Junggar basin and Tarim basin conjunction, the Moho surface depth in the N-S direction showed obvious fluctuation. The low-velocity blocks under the basin dipped and squeezed in an opposite direction, and the depth of the Moho layer varied from 48 km to 60 km in the study area. At high and low velocity in a transition band or near the fault that includes medium variation, the region is much more likely to be the seismogenic zone and seismic zone symbol of the strong earthquake. The preliminary results show that the Tianshan Mountain uplift in crust may be related to squeezing in the N-S direction between Junggar basin and Tarim basin.
