Ambient Seismic Noise Tomography And 3-D Shear Wave And Passion Ratio Structure Of East-Tibet And Surrounding Areas

The time derivative of cross-correlation functions(CCF) of ambient noise fields recorded by two stations can be approximated as the Green’s Function(GF) between the stations. The CCFs are thus used as Peudo-GFs which are dominated by surface waves to invert for group velocity structure. After extracting group velocity dispersion curves, we can invert to find 3-D shear wave structure of the crust and upper mantle. All the velocity structure results can be account for explaining the structure of crust and upper mantle and plate movements.One-year’s ambient noise fields records from stations of Sichuan and Yunnan networks,together with some stations of the China’s National Seismic Network, are collected to invert for group velocity structure of Rayleigh surface waves in the east Tibet and surrounding area. Ambient seismic noise tomography is used to invert for group velocity structure maps in period 3-50 s. 3-D shear wave velocity structure and passion ratio distribution of 60 km beneath the surface are calculated by the simulated annealing method. On account of the great dependent on initial model of the traditional linear conjugate method, we choose the simulated annealing method to invert 3-D shear wave velocity and passion ratio structure cause its lower dependence on initial model.The group velocity structures are similar to the previous research. Our results indicated that: thickness of the sediment layer character by shear wave velocity of 2.5 km/s is about1.0-1.5 km, the sedimentary strata we suggested is Cenozoic deposit. Velocity of different layers show that Longmen Shan thrust belt, Xiaojiang fault and Red River fault mark the rapid lateral velocity variation between Sichuan Basin and Sichuan-Yunnan diamond block.The crust thickness of our study area is characterized by thick to thin from northwest to southeast. There is no higher passion ratio characterized in the low velocity zone under Sichuan-Yunnan diamond block, we suggest that there is no wide range of melt structure.The simulated annealing method used as a non-linear seismic inversion method, is more precise and flexible. It can give us some results with higher resolution in some precise areas. Velocities of layers together with thickness of the sediment layer and passion ratio are determined by ourselves. We found that the initial model still have impacts on our inversion results, more intensive studies should be continued to develop different effect rules. Thepassion ratio structure shows lower value in east block of Yunnan area which suggests felsic rocks are abundant in upper crust, but there’s no evidence in prominent low-velocity anomalies exhibits in the lower crust beneath Yunnan area. A prominent low-velocity zone in the middle crust of Sichuan-Yunnan diamond block shows normal passion ratio value but molten material in local places can be taken into account.