A Study On SKS Splitting Beneath The Yunnan Regions

The anisotropy exists in the interior of the earth with varying degree. By studying the seismic waves, we can find mass information from the anisotropy media. It is significant to understand the characteristics of seismic anisotropy. In this thesis, the seismic anisotropy are comprehensively discussed, from the anisotropy research of origin to its theory and method. Based on the characteristics of shear-wave splitting parameters with the seismic wave propagation in the anisotropic layers, several spitting shear-wave identification methods has been studied in the paper, in essence, such as the method of the minimal eigenvalue, the maximal eigenvalue, the ratio of both two eigenvalue, the minimal energy and the maximal energy, they are equivalent. Moreover, to check validity of identification results, method of F-testing is adopted by using 95% error region. Finally, the upper mantle anisotropy in the Yunnan region is studied by using the phase SKS.SKS shear waves are used to analyze the direction and extent of seismic anisotropy. The data are from the recordings at 47 stations located in the Yunnan province involved Yunnan digital seismic network, Yunnan temporary seismometer arrays, China National Digital Seismic Network (CNDSN), and China Digital Seismic Network (CDSN). Furthermore, the seismic anisotropy parameters and their implication are discussed. It is demonstrated that delay times vary from 0.58 to 1.88s and fast polarization directions are present at stations with nearly N-S in northern Yunnan and gradually rotate to W-E in the southern Yunnan. The shear wave splitting is mainly due to the anisotropic layer in upper mantle whose thickness are between 67 and 216km. It is possible that the collision of India and Eurasian resulted in not only crust thickening and subsequently the E-W extension beneath the Tibetan plateau, but southeastward flow. On the other hand, a tendency of rotation is showed beneath the Yunnan. Comparation with stress filed and the metrical result of GPS, it shows hat, while the movement modes of the shallow and the deep are different beneath Yunnan, synchronously there exists complex coupling effect between crustal block and upper mantle.