| The southeast Tibetan Plateau consists of Sichuan-Yunnan block, Yangtze block and Southwest of Yunnan block. Its tectonics and deep structure have been closely related to the collision between the Indian and Asian plate since 45 Ma. The long-term stress concentration in this region strongly deformed the crust, and the deformation has continued up to the present. In addition, our study region lies in the southern segment of the South-North Seismic Zone of China, and its characteristics of seismic activities are high-frequency, large-magnitude and wide-distribution. Because of the special tectonic setting, it has been an important place for studying crustal deformation and deep dynamical process.In this parper, we selected 45 local earthquakes which are distributing uniformly in study area. After manually picking up 3013 P wave travel time data with good constraints, we adopted the VELEST algorithm to derive the optimized 1D crustal P wave velocity model. After testing the stability of positioning, we relocated 108 earthquakes based on the new 1D model. The results show that there are three velocity layers between 0 to 30km, Vp between 5.96km/s to 6.60km/s. At lower crust with depth of 30-40km, the Vp value is between 6.6-6.8km/s.And the average speed is 7.8km/s at the top of upper mantle. According to the relocations, we find the average source depth is 20.3km and it indicates most of earthquakes occur at the middle and upper crust.We have executed twice inversions respectively with two different 1D velocity models for testing the influence of different initial model under the same conditions, such as model parameters, the grid subdivision and the number of iteration. The tomographic results derived from the same initial mode show the they have similar structural characteristics when ploted by absolute velocity value or relative velocity abnormal. In addition, the relative velocity results derived from different models have the same distribution of abnormal but have slight difference in amplitude. But they have greater difference when ploted as absolute velocity value.Joint inversion of seismic data and gravity data has many advantages compared to a single geophysical inversion method. We adopt the sequential inversion method to explore the crustal velocity and density structure. The seismic data sets include two parts. One is the recorded waveform data of China earthquake science array with our manually picking out the body-wave travel time and another is the phase observation reports of China Seismic Network. Finally, we select 26890 P wave travel time data and 15344 S wave travel time data. After several correcting process, we obtain the bouguer gravity anomaly data of study region. The final resolution is 0.5°×0.5°and some conclusions are as follows.The Moho discontinuities derived from velocity result and density result are consistent. The crustal thickness of study region is between 30km～60km and thickens from southeast to northwest gradually. Bouguer gravity shape is mirror corresponding to the landform. The velocity value is 7.6～7.8km/s at the top of the upper-most mantle, but is lower than the average value of global mainland velocity model(8.04～8.10km/s). This feature is related with the thermal process during Cenozoic. As part of Yangtze craton, Sichuan basin shows stable high-velocity and high-density deeper than 10km and its crust thickness is about 45km. There is no low-velocity-zone throughout the whole crust. Emeishan basalt distributes widely in study region, and the inner zone shows higher bouguer gravity anomaly value. Panzhihua region shows relative high-velocity and high-density from shallow crust to about 30～40km. It indicates basic and ultrabasic volcanic intrusive rocks. The distribution of low-velocity and low-density anomaly is related to geologic structure. For example, low-velocity anomalies at shallow crust in Sichuan basin, Simao and Dali are caused by thicker sediment. |
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