Shear Wave Velocity Structure Beneath Southeastern Margin Of Tibetan Plateau From Joint Inversion Of Surface Wave Dispersion And Receiver Functions

Southeastern margin of Tibetan Plateau, located in the transition zone between Tibetan Plateau and Yangtze platform, is the result of the collision of Indian and Eurasian Plates and has been uplifted and deformed violently since Quaternary. The topography and crustal thickness of southeastern margin are significantly different from those of Yangtze Platform. However, the models concentrating on the uplifting and deformation mechanism are still under dispute. Southeastern margin is also a region with strong seismic activities due to the complicated tectonic structure, especially the 12 May 2008 Wenchuan earthquake and 20 April 2013 Lushan earthquake, which not only brought large casualties but also heavy economic losses to the southwestern China. The study of shear wave velocity structure in southeastern margin of Tibetan Plateau will help to understand the deformation, uplifting mechanism and crust-mantle coupling problems and to understand the seismogenic environment and the generation mechanism of strong earthquakes.Ambient noise tomography can make up for the deficiency of surface wave tomography based on earthquake data at short period. But limited by the frequency band and inter-station distance, reliable surface wave images at long period can not be easily inverted. Surface wave tomography based on earthquake has advantages at long period. But it is difficult to obtain accurate images at short period, resulting from the attenuation and scattering of high frequency information. Receiver functions are sensitive to the velocity contrast across the discontinuities but fail to invert accurate velocity between adjacent discontinuities. Accordingly, resolution deficiency can be made up by jointly using the three kinds of data. What's more, velocity structure can be better contained to relieve the non-uniqueness problems.In this study,18 months continuous wave data, surface wave events and P wave teleseism events observed at 170 broad-band stations from national and regional networks in southeastern margin of Tibetan Plateau and adjacent area are used to retrieve Rayleigh wave group velocity and phase velocity dispersions at periods from 8 to 80s and P wave receiver functions beneath each stations. Finally,3D shear wave velocity structures of crust and upper-mantle beneath southeastern margin of Tibetan Plateau are constructed by joint inversion of surface wave dispersions and receiver functions. Crustal thickness and Poisson's ratio are calculated by H-? stacking.According to the H-? stacking results, the crust thickness reaches a minimum in South China Orogenic Belt and is under 30 km in parts of the Orogenic Belt. It increases slowly to about 45 km westward in western Yangtze Platform and changes dramatically across Longmenshan-Anninghe-Xiaojiang faults to about 55 km, finally reaches a maximum of about 70 km in Songpan-Ganze Fold Belt. In addition, a crustal thickness mutation belt can also be observed near Lijiang-Xiaojinhe fault, across which crust thickens northward. Superhigh Poisson's ratio greater than 0.30 can be observed near Xianshuihe fault, Longmenshan fault, Anninghe fault, Xiaojiang fault and Jinshajiang fault showing the possibility of partial melting in the crust beneath these areas. High Poisson's ratio can also be found in Panzhihua and Yongsheng regions, which may result from the mineral composition of the magmatism activity belts related to the intrusions of basic and ultrabasic body. The Poisson's ratio in Tengchong area is high, which may caused by the magmatism activity in Tengchong volcanic. Low Poisson's ratio under 0.26 can be seen in South China Orogenic Belt, Yangtze Craton and Qinling Fold Belt, which approximate to an average continental ratio, showing the mineral compositions are mainly neutral or acidic rock mass.The low velocity layers are ubiquitous in the mid-lower crust beneath the southeastern margin of Tibetan Plateau, but they are discontinuous and vary violently in depth, thickness and velocity value. The low velocity layers are restricted by the major faults in study area. The low velocity layer in mid-lower crust is missing eastward across Longmenshan fault and is nearly missing southward across Lijiang-Xiaojinhe fault. This can be interpreted by channel flow model, that the eastward migration of materials in mid-lower crust is inhibited by the rigid Sichuan Basin, while the southward migration is absorbed and shielded by Lijiang-Xiaojinhe fault. This process is also closely related to the crust thickening and surface uplifting of southeastern margin of Tibetan Plateau.From the distribution of earthquakes in shear wave velocity images, earthquakes occurred in brittle layers above low velocity layers. A possible interpretation is that the obstacle to stress concentration is due to the weakness of the LVLs, but concentrating stress on the above brittle medium helps in finally producing earthquakes. The Ms 8.0 Wenchuan earthquake and Ms 7.0 Lushan earthquake all occurred in the transition depth of homogeneous and heterogeneous medium. The two earthquake sequences all occurred in the crust where the low velocity layers are inhibited by the rigid block and where the topography and crustal thickness vary sharply. This may forms a conducive environment for the occurrence of the two earthquakes. The Poisson's ratio varies along Longmenshan fault, which is high near the initial point of Wenchuan earthquake and becomes low northeastward to the initial point along the fault and becomes high again to its northeast, while the ratio is high southwestward the initial point along the fault. This may suggest that friction coefficient is lower in low Poisson's ratio area and causes fault displacement more easily, which finally results in the northeastward unidirectional expansion along Longmenshan fault following the initial rupture of Wenchuan earthquake.