The Seismic Tomography On The Velocity Structure Of The Crust And Upper Mantle In The Northeastern Margin Of The Qinghai-Tibetn Plateau

This dissertation first reviews the history of geophysical inversion and seismic tomography and the present status of geophysical inversion and seismic tomography research in China and the world.The research region of this dissertation is 32° - 40° N and 100° - 108° E. In China geological and tectonic map, this region is referred to as the northeastern margin of the Qinghai-Tibetan Block and southeastern margin of the Erdos Platform, where is characterized by intense tectonic activity, many large-scale active faults and frequently occurring earthquakes.Based on the inversion result from deep seismic sounding and natural earthquakes, this dissertation gives out the contour map of Moho interface in the research region. The thickness of crust in this region gradually reduces from southwest to northeast. Fault distribution on the ground surface agree well with the contour of the Moho interface in many areas, which indicate that many faults in this region are ultra-crustal faults and the breaking distance and mode of occurrence of faults display differences only on the Moho interface.The seismic data used in this dissertation are 3 229 local earthquakes (Ms>1.5) and 168 teleseismic events occurred during 1980 to 2002 recorded by 130 seismic stations from seismic networks of Gansu, Qinghai, Ningxia and Sichuan. The seismic phases used are 51 210 local phases with the maximum travel time residual of 3.0 s selected from 55 024 local phases and 2 651 teleseismic phases. The tomographic inversion was calculated using the program codes provided by Dr. Zhao Dapeng. According to the data distribution and several check-board tests, reliable inversion results are obtained for a model with grid size of 0.5° by 0.5° in the horizontal directions and 6 layers of 1, 10, 30, 50, 65 and 80 km deep from the surface to 80 km in the depth.The tomographic inversion results show that: Lower P-wave velocity anomaly exists widely in the lower crust beneath the northeastern margin of the Qinghai-Tibetan plateau. Average P-wave velocity in most of the top of upper mantle...