Grid-search Algorithms to Estimate the Crustal Thickness and the Bulk Crustal Vp/Vs Ratio in the Presence of Anisotropy

Abstract:

To account for the presence of seismic anisotropy within the crust and to estimate the relevant parameters, we first discuss a robust technique for the analysis of shear-wave splitting in layered anisotropic media by using converted shear phases. In (weakly) anisotropic media, the P-to-S converted phases (Ps) exhibit a distinct variation in arrival time (cosine moveout) as function of back-azimuth. This variation can be exploited by time-shifting and stacking radial receiver functions to constrain a range of possible splitting parameters (i.e. the fast-polarization direction and the delay time) for an anisotropic layer. Then, the minimization of the transverse-component energy is used to select the pair of splitting parameters that best describes the anisotropic properties of the layer. This 2-step approach stabilizes the inversion process and significantly reduces the time for computing the best splitting parameters. In multi-layered anisotropic media, the splitting parameters for the individual layers can be inferred by a layer-stripping approach, where the splitting effect due shallower layers on converted phases from deeper discontinuities is successively corrected. The method represents a computationally-efficient extension to multiple anisotropic layers. We further investigate the influences of noise and of gaps in the azimuthal distribution of events.

The effect of anisotropy on the estimate of crustal thickness and average bulk Vp/Vs ratio can be significant. Here, we present a stacking scheme that extends the approach of Zhu & Kanamori (2000) to include P-to-S converted waves and their crustal reverberations generated in the anisotropic case. A ray-based algorithm is used to calculate amplitudes and arrival times of all phases generated in an anisotropic medium with hexagonal symmetry and a horizontal symmetry axis. The anisotropic parameters of the medium are first estimated using the splitting analysis of the Ps-phase as described above. Then, a grid-search is performed over layer thickness and Vp/Vs ratio, while accounting for all relevant arrivals in the anisotropic medium. Synthetic examples show the robustness of the stacking approach in the anisotropic case. Further applications of the stacking approach to real data illustrate the feasibility of the approach.