The Crust Effects on the Analysis of the Upper Mantle Anisotropy

Abstract

Pseudo‐spectrum method is used to simulate the crust effect on the analysis of the upper mantle anisotropy. A model of two‐layer anisotropic media is used to simulate the crust and the upper mantle respectively and the elastic wave fields are calculated for four coupling conditions between the two layers. The characteristics of split shear waves at various depths are analyzed in detail. The results show when the crust is isotropic or its anisotropy orientations are parallel or perpendicular to that of the upper mantle, the fast or slow direction observed in the crust keeps the same as that in the upper mantle, the time delay increases when they are parallel to each other or decreases when they are perpendicular to each other. But in other conditions the motions of particles in the crust becomes complicated and the fast direction of the upper mantle is difficult to be directly picked out and the time delay is also changed due to the effects of the crust anisotropy. Furthermore, two popular approaches are used to inverse the anisotropy parameters from simulated results. It is found that when the anisotropy orientations of the crust and the upper mantle are neither parallel nor perpendicular, the inversed fast direction of the upper mantle may be incorrect. It is suggested that in such cases one can use several different inversion techniques together and refer to the contours of residual or cross‐correlation for further determination of anisotropy parameters of the upper mantle.