3-d Velocity Structure Study In Western Yunnan

Western Yunnan is located in the eastern edge of the Tibetan Plateau, an area with unique geological environment. It is not only an important part of the world famous Alps-Himalayan Orogenic Zone, but also an area suffers intense compression from the collision between Gondwana continent and Eurasian continent. Since the late Paleozoic, Western Yunnan experienced lots of tectonic processes, frequent magmatic activity, varying degrees metamorphism and multi-stage mineralization, in addition, Western Yunnan is an area with highest seismicity in China. Earthquakes in this region have large magnitude and high frequency, which cause great loss in people’s lives and properties. It is of significant importance to study the three dimensional crustal structure of this region to understand the tectonic setting and disaster mechanism. Densification and digitalization of seismic network in this region provides an opportunity to study the velocity structure with bulletin data.We use the method3-D tomography code developed by Zhao to inverse3-d velocity structure in western Yunnan. We collect bulletin data from79seismic stations of Yunnan and Sichuan networks in the period of January2008to December2010. The original data set includes about11,781local and regional earthquakes. We use the following criteria to further data screening:the number of P wave observation should be no less than3for one earthquake; The maximal travel-time residual (the difference between the observed and calculated travel time) is2.0s. After data selecting, the number of events involved in the inversion is10,403, and the total number is117,487, the P-wave observations is59,208, where the numbers of Pg and Pn phases are55,669and3,539, respectively, the total number of the S-wave observations is58,279. For the P wave, we research the influence of irregular discontinuity and level discontinuous for the inversion and the influence of adding follow-up phase for the inversion. We Simply analysis the S wave velocity structure、poisson ratio and BinChuan-zhefang profile, and select a seismic profile to do waveform forward simulation comparison. Overall we get the following conclusions: (1) Most of the earthquakes in the region occurred at the interface of high-velocity media and low-velocity media, i.e. areas with high velocity gradient, which further validates the previous results in other areas;(2) The Red River fault zone, which is the major fault systems in this region, cut through the entire crust, and in the lower crust the velocity contrasts between two sides are estimated higher than10%. While the velocity difference below Nujiang fault zone extends only in the upper crust;(3) The poisson ratio in the region showed obvious lateral abnormal and the poisson ratio between honghe fault changes obviously;(4) The waveform forward simulation of the selected seismic profile is similar with the original records, but has some difference local;(5) Irregular discontinuous interface can do better match the data and has better use of the existing data;(6) The use of a variety of waveform has a great helpful for the accuracy of the tomography model;(7) The quantity and the quality of seismic data are greatly improved after2010years.