Rayleigh Wave Tomography In Fujian From Ambient Seismic Noise

Fujian is located among Pacific Plate, Philippine sea plate and Eurasian plate, which is also the transition zone between the Southeast borderland of Eurasian plate and the Ocean. Taiwan, the neighborhood of Fujian, is a region with the strongest seismic activity in our country, and there are large numbers of faults in Fujian region, where the tectonic and geologic setting are extremely complex. From the research of the shallow layer crust, we may deepen the understanding of the seismogenic mechanism and predict possible seismic activity in this area. So, it is very meaningful to research the shallow layer crust. Continuous vertical-component seismograms by broadband stations in Fujian, Guangdong, Jiangxi and Zhejiang are used in this study. Surface wave tomography is conducted to generate group velocity maps and 3-D shear wave velocity structure.Follows are the original points of this thesis:1) The cross correlation technique has been applied to analyze the ambient noise data of each station-pair in this study. It shows that the SNR of Green Function will become larger with longer observation time. In the mean time, the SNR of Green Function can be enhanced by using symmetric component. With the increasing distance between stations, the average SNR will become increasingly smaller, which is mainly because the coherent information of Green Function between the two stations would be attenuated in the dissemination process.2) Topographic maps with a grid spacing of 0.3°×0.3°of Fujian region in the Predominant periods are constructed. These maps display higher resolution than previous surface wave tomography maps. Tomography maps reveals the well within the 3-5km deep crustal velocity structure, which is closely related to the surface geological structure and the heat distribution. Geothermal area often shows a low velocity. Tomographic maps of Fuzhou basin and Zhangzhou basin, which both belong to the geothermal anomaly area, show a clearly low velocity.3) The pure path Rayleigh wave dispersion curves are extracted by using multiple filter analysis method. We can obtain more reliable dispersion measurement by introducing maximum cutoff period. The frequency-time analysis tomography has a larger interference when SNR of Green function is less than 10, so it is hard to extract higher quality dispersion curves.4) We construct the 3-D sheer wave crustal velocity model in Fujian region by using Analysis Software CPS 3.30. The results show that, the crustal thickness is thinning from inland to coastal, but thickening from south to north along the coast. The crust can be divided into three layers: the upper, the middle and the lower crust. S-wave velocity maps at different depths show that the S-wave velocity tomography in 0-6km depth is almost coincident with the topography and the Distribution of sedimentary. Fuzhou basin and Zhangzhou basin in 10km depth show a low velocity probably because of the very thick sedimentary layer. There is a well-defined low velocity zone in 16-20km depth. The southern coastal areas begin to show abnormal high-speed in 30km depth and the tomographic results become less reliable below 30km.