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Lithospheric S-Wave Velocity Structure of the North-South Seismic Belt of China from Surface Wave Tomography
详细信息   
摘要
This paper uses surface wave data from natural earthquakes and a tomographic method to study the S-wave velocity structure and anisotropy of the lithosphere of North-South Seismic Belt and adjacent regions.The result indicates that the east margin of the seismic belt is not only a zone of rapid change of crustal thickness,but also a remarkable boundary between different crustal velocities.To its west the S-wave velocity of the middle and lower crust is obviously lower than that on the east side,and strong earthquakes generally occur within or at the border of the low velocity regions.The S-wave velocity of middle-lower crust in the east margin of Tibetan Plateau is obviously lower than that of normal continental crust;low velocity zones(LVZ) exist at depths about 25~45 km in the crust of Songpan-Garzê block and the western part of Chuan-Dian block.These low velocity features are connected with the low velocities of the plateau proper,which facilitates the lateral flow of lower crustal material.The anisotropy patterns of the crust is consistent with the lower crust flow model,that is,the lower crust material moves around the eastern Himalayan syntaxis and the eastward movement is diverted to south and northeast when blocked by the rigid Yangtze crust.The result of our surface wave tomography supports the lower crust flow model for crustal movement of Tibetan Plateau.The lithosphere of the North-South Seismic Belt has similar thickness with that of Erdos and Yangtze on its east,but with relatively lower velocity.At the top of upper mantle in the west Chuan-Dian block(from Moho to about 88 km) the shear velocity is abnormally low;in the upper mantle lid of Songpan-Garzê block there is a low velocity layer between about 90 and 130 km depth.The velocity pattern of the upper mantle is remarkably different from that of the crust;there is an NNE-trending high velocity zone between the plateau proper and western Sichuan-Yunnan region,which may hamper the eastward movement of mantle material.The anisotropy of upper mantle is weaker and exhibits an obviously different pattern from the crustal anisotropy.Therefore,it is considered that the modes of tectonic movement of crust and upper mantle of Tibetan Plateau are different,and the weak lower crust provides the condition for decoupling of crust and mantle movement.

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