Inversion of shear-wave splitting parameters to retrieve three-dimensional orientation of anisotropy in continental lithosphere

A method for inversion of splitting parameters of shear waves was designed to retrieve the three-dimensional orientation of an inclined anisotropic medium. Data consist of the polarization vector of the fast shear wave and the time delay between the fast and slow shear waves obtained from three-component records of teleseismic shear waves. The model to be determined by the inversion of these splitting parameters is a homogeneous anisotropic layer where axes of hexagonal or orthorhombic symmetry generally plunge. The method determines the orientation of the symmetry axes and the thickness of the anisotropic layer beneath a region on the receiver side. Tests with synthetic data contaminated by random noise of various levels demonstrated a good performance of the method even for an irregular distribution of events. Inversions assuming a hexagonal model appeared to be fairly stable with respect to increasing noise variance, whereas an orthorhombic model yielded rather scattered solutions. The method applied to a selected distribution of events appeared to be unstable when deciding on the symmetry of the medium when noise is superimposed on the data to be inverted. If the anisotropic symmetry is fixed by adding supplementary information, e.g. from the P-residual pattern, the inversion of the splitting parameters of shear waves can recover the orientation of the symmetry axes of the hexagonal or orthorhombic model with a high confidence. An inverted real data set containing the shear-wave polarization vectors and time delays collected in south Sweden yields, in accordance with the distribution of the P-residuals, an inclined symmetry axis of the hexagonal anisotropy dipping to the SE.