A Study Of Crust Anisotropy Using Azimuthal Pattern Of Receiver Functions

Since Hess found the Pn wave anisotropy in the Pacific region in the 1960s, more and more research shows that anisotropy exists in a broad region from the earth's crust, the mantel to the core. Anatomizing anisotropy information contained in the seismic wave can provide useful information about the earth's structure, deformation, dynamic processing and mantle convection.Because of its sensitivity to the velocity discontinuity of the earth, receiver function technique has become a routine procedure used to probe interior structure of the earth. Receiver functions contain anisotropic information of the earth's interior, however, traditional receiver function techniques such as migration imaging and waveform inversion method, which are based on isotropic media assumption, can not effectively extract the anisotropy information contained in the azimuth variation pattern. Only by using the anisotropic media, e.g. a model with symmetric axis of arbitrary orientation, computing the response, can we obtain the detailed anisotropy information hidden in the radial and transversal receiver function.Focusing on the receiver function variation pattern changing wtih different back azimuths, we introduced different kinds of symmetric systems of seismic anisotropy used often, and summarized some possible causes of anisotropy formation. We show details about how to calculate the response of a stratified anisotropy model with symmetric axis of arbitrary orientation. We also simulated receiver functions among different models and analyzed how the changing of anisotropic parameters influence the azimuth variation pattern of receiver functions.The anisotropy study by receiver function analysis was applied to Taihang Mountain Range (TMR) in North China in this thesis. The maximum entropy spectrum deconvolution technique was used to extract radial and transversal receiver functions from the waveforms of 20 portable seismic stations deployed in TMR. Considering the signal-to-noise ratio and the azimuth coverage, we got the variation pattern of receiver functions for 11 stations. After carefully analyzing the pattern of the receiver functions that we got, we obtained the reliable evidence on the existence of anisotropy in the shallow crust in TMR. Our results show that, although the thickness of the upper crustal layer is only about 1 km, the layer shows a strong anisotropy with magnitude of 8~15%; in the deeper of crust, the magnitudes of anisotropy is about 3%~5%, showing a pattern with fast-symmetric-axis. The crust anisotropy beneath TMR in North China obtained in this study also shows a significant difference in both the lateral and vertical scale, which might imply a regional anisotropy characteristic in the studied region.After years of efforts, the application of receiver function technique in the anistropic study seems to be effective. The method can be applied to study regions with complex anisotropic feature, from which we can constrain the lateral and depth distribution of anisotropy, as well as other important parameters, including the depth of crustal velocity discontinuity, the magnitudes of anisotropy and the orientation of symmetry axis. and all of these can be used to study the dynamic process in the earth deep interior. However, the requirement of high SNR and azimuth coverage limits the application of the method to more areas. There are also many other problems, such as trade-offs between the effects of different anisotropy parameters on the variation pattern of receive functions, resulting in the ambiguous detection among models during the inversion, which should be considered a lot in the future.