Study On Direct Imaging Of Sichuan Basin And Adjacent Areas Based On Ambient Noise

The research area selected in this paper is located in the southwest of China(101~°E-109~°E,26~°N-35~°N,,the faults in this area are crisscross,located at the intersection of the uplifted Qinghai Tibet Plateau and the Yangtze platform,with frequent seismic activities and complex crustal structure.The construction of crustal velocity structure in this area is of great significance to the study of the evolution characteristics of geological structure and the ambient of earthquake genesis.Although some research results have been obtained in this area by using tomographic imaging methods such as teleseismic body wave and receiver function,most of them are not precise enough for inversion of crustal structure.In this paper,using the ambient noise imaging method which has been widely used in recent years to obtain a high-precision crustal shear wave velocity structure in the research area.In this paper,55 stations of China Seismic Network in Sichuan Basin and its adjacent areas are selected to record the continuous broadband ambient noise waveform for more than one year.First,a series of preprocessing is adopted for the original waveform record;then use cross-correlation operation and superposition to obtain the empirical grid function between the two stations;and then use a fast method of extracting phase velocity dispersion curve based on image analysis proposed by Yao et al,extracted the dispersion curve of Rayleigh wave phase velocity in 6-35s;and then use a direct inversion method of three-dimensional crustal shear wave velocity structure based on ray tracing proposed by Fang Hongjian and others;finally,obtained the three-dimensional crustal shear wave velocity structure of Sichuan Basin and its adjacent areas is obtained,and the imaging results are discussed.The results show that:(1,The inversion results show that the shallow structure is consistent with the surface structure.The basin is a low-speed area,and the Qinghai Tibet Plateau on the west side of the basin is a high-speed area.It is consistent with the previous research results that there is a thick sedimentary layer(about 10km,in the underground of Sichuan Basin.It is considered that the low-speed area is affected by the sedimentary layer.(2,The depth of 10-20km is a good reflection of the obvious horizontal and vertical inhomogeneous changes in the structure of the middle and upper crust,and the transformation of Sichuan Basin from low-speed area to high-speed area,and the transformation of Songpan-Ganzi block and Sichuan Yunnan area from high-speed area to low-speed area.(3,The Sichuan basin with a depth of 25-40km shows an obvious high-speed area,while the Songpan-Ganzi block shows a low-speed area.It is considered that the low-speed area is affected by the high heat flow value,and it is inferred that the east flow of materials in the eastern margin of the Qinghai Tibet Plateau is blocked by the hard middle and lower crust of the Sichuan Basin,resulting in the high accumulation of strain in the inner part of the brittle upper crust of the Longmenshan fault zone,which leads to the sudden occurrence of the Wenchuan earthquake.The research results obtained in this paper add new independent observational evidence for the study of the crust structure of Sichuan Basin and its adjacent areas,and also provide important reference materials for the following deep-seated research.