Constraining depth-dependent anisotropy: A new approach

Splitting of shear waves is commonly used to infer anisotropy of the Earth’s interior. However, most data, such as SKS splitting, provide depth-integrated measure of anisotropy along the ray path, and it is difficult to separate contributions from different layers within the Earth. There have been efforts to focus on specific depth range by analyzing differences in splitting between two ray paths, but these studies only report observed differences or rely upon forward modeling with limited parameter-space search. We have developed a new approach to examine the P-to-S converted phases that allows one to construct depth-dependent multi-layer anisotropy models through combination of inversion and grid search.

In addition to the conventional fast splitting direction and delay time, the technique allows one to investigate the tilt of the symmetry axis and dip of the discontinuity associated with the P-to-S conversion. Furthermore, the formulation is such that it naturally extends to include and examine multiple layers with different anisotropic properties. With these flexibilities, we can address anisotropic contributions from different layers in two separate procedures. The first scheme takes advantage of data with similar ray paths (e.g., SKS and SKKS waves recorded at the same station). The rays sample different structure when their ray paths differ (e.g., near the core-mantle boundary), but they sample the same structure when the paths are similar (e.g., in the upper part of the mantle and crust). Using our new approach, we can set up the problem as a two-layer anisotropy model where the layer with ray paths sampling different regions (e.g., lowermost mantle) is allowed to vary laterally. The second type of problem that can be addressed by the new approach is layer-by-layer investigation of anisotropy from top to bottom. This procedure combines the new method with receiver function analysis to obtain anisotropic properties of each layer using P-to-S converted waves from different discontinuities such as the Moho, 410-km and 660-km discontinuities. The application of the methods to real data shows significant lateral variations in anisotropy, both in deep Earth and near the surface.