Teleseismic Tomography Of Deep Velocity Structure In The Eastern Edge Of Tibetan Plateau

The theory and the methods of seismic tomography have been introduced systematically, as well as the multistage fast marching method and the subspace inversion algorithm which are used in the FMTOMO seismic tomography software have been elaborated in this thesis.The research aim of the thesis is the deep velocity structure in the eastern edge of Tibetan plateau within the study region of 98°E-108°E,26°N-34°N,0-300km depth. The 3-D P-wave velocity structure has been obtained by using 9194 teleseismic P-wave traveltime residuals data and the FMTOMO seismic tomography software. And then, several results have been got:By observing the horizontal slice of P-wave velocity perturbation from 60km to 140km depth, the P-wave velocity of Mid-Sichuan Block is relative high, while the P-wave velocity of Sichuan-Qinghai Block and Sichuan-Yunnan Diamond Block is relative low. The Longmenshan Fault is the velocity boundary between the Sichuan-Qinghai Block with lower velocity and Mid-Sichuan Block with higher velocity. The lower velocity region of the Sichuan-Yunnan Diamond Block and the higher velocity region of the Mid-Sichuan Block are divided by the Anninghe Fault.From 60km to 140km,102°E-104°E,29°N-31°N, There is lower velocity region in the southwest of Sichuan basin. The possible explanation of this phenomenon is that: From the lower crust to the upper mantle, because of effect of east flow in the Tibetan plateau, when east flow soft material has met the block of hard Sichuan basin nearby the Longmenshan fault, some of the soft materials flow along the southeast direction as well as northwest direction, but a part of soft materials invade the southwest region of the hard Sichuan basin, which created the low P-wave velocity phenomenon.There is a relative high velocity phenomenon along the northwest side of Longmenshan fault in depth range from 60km to 140km. The possible explanation of this phenomenon is that:From the crust to the upper mantle, because of the soft materials which flow east met the impediment function of hard Sichuan basin, a part of soft materials had the thrust movement along the hard Sichuan basin edge. Therefore the relatively high velocity region appeared in this depth range.In the depth range from 190km to 290km, there is a high velocity area in the southwest region of eastern edge of Tibetan plateau. Longmenshan Fault and Anninghe Fault are not the boundary between high velocity region and low velocity region.In the image of vertical slice of P-wave velocity perturbation alone the 30.5°N, the high velocity of Sichuan basin extents to about 200km depth, which means the hard lithosphere of Sichuan basin may extent to this depth.In the image of vertical slice of P-wave velocity perturbation alone the 101.5°E, The entire west Sichuan plateau area is low velocity region in the depth range from lower crust to upper mantle, which also proves that soft materials may exist in the western Sichuan plateau in this depth range.