The Study Of Anisotropy Based On Shear-wave Splitting

Shear wave splitting can be regarded as one of the most common methods in studying the anisotropy of Earth.Seismic shear-wave will be split into two waves propagating at different speeds,which are polarized in two perpendicular orientations: fast S-wave and a slow one.Thus,we can infer the direction and strength of anisotropy taking advantage of the shear wave splitting.Here we have a detail research about anisotropy both in numerical simulation and practical application,based on the basic principle of anisotropy,source of anisotropy in the earth and common methods for observing anisotropy.In the aspect of numerical simulation,utilizing EDA medium to simulate the seismic records of anisotropic media with cracks,we use artificial fish-swarm algorithm combined with Pearson correlation coefficient to identify the fracture properties based on the correlation of fast and slow shear waves.The results show that this method can identify the crack properties quickly and accurately,compared with model space scanning,particle swarm algorithm and genetic algorithm which use waveform correlation to detect anisotropy of cracks,our method can effectively improve the convergence speed,and increase in stability.In practical application,we obtained 642 SKS shear-wave splitting results and 51 local S-wave splitting results to identify the anisotropy ofthe crust and upper mantlein Northeast China.In the north of Great Xing'an Range,the fast axis directions of anisotropy are mainly EW,which is consistent with the direction of continental migration in this area;The Yanshanian orogenic belt in the south of Great Xing'an Range has a EW direction of fast axis with a smaller delay time,we believe the anisotropy in this region may be mainly derived from the lithosphere.The direction of fast axis of the anisotropy structure in the middle part of Songliao Basin is mainly EW direction,which may be related to the structural tension of the Songliao Basin in the late Mesozoic.There is a regularity of anisotropy in the fast axis direction,which is mainly NW with a larger delay time.The results show that the Pacific plate in the mantle transition zone have strong anisotropy.From the comparison of the splitting results between local S and teleseismic SKS wave,we can infer that the high temperature and low velocity material around Changbai Mountain may also have strong anisotropy.The results of numerical simulation and practical application show that the shear wave splitting method can effectively identify the anisotropy of the cracks media and can be applied to the observed data of the actual stations,and then can be used to study the anisotropic structure of the crust and upper mantle.