Calculating splitting parameters for plume-type anisotropic structures of the upper mantle

Abstract

Summary We apply a forward-propagator method to calculate shear wave splitting parameters for general (3-D) weakly anisotropic upper mantle structures. The approach is valid under the assumption that the ray paths of the two quasi-shear (qS) waves can be approximated by a common reference ray within an isotropic background medium. Along the reference ray the incremental splitting is expressed in terms of qS polarizations and slownesses in the direction of propagation. Using a ray theory ansatz, we show that the resulting shear wave seismogram is equivalent to the coupling ray theory of Coates & Chapman (1990) under the assumption of smoothly varying anisotropy. Here, we use the forward-propagator method to calculate apparent shear wave splitting parameters that can be compared directly with observations of teleseismic shear wave splitting. A comparison with finite difference calculations is used to assess the validity of the method. As an application we consider shear wave splitting due to a plume-type anisotropic upper mantle structure. The anisotropy is assumed to result from the preferred alignment of orthorhombic olivine. We show examples of waveforms and splitting parameters as functions of backazimuth and angle of incidence. Anisotropic effects for SKS are weak at stations near the central upwelling due to the combined effects of initial polarization and olivine a-axis orientation. Here, the splitting parameters are irregular and fast-axis directions may vary by up to 70° depending on the vertical incidence angle. At larger distances splitting parameters slowly converge towards values expected for homogeneous media. The results suggest that direct evidence for mantle plumes from shear wave splitting is more likely to come from (OBS) stations at some distance away from the central upwelling. At shallower incidence, the apparent splitting parameters for S exhibit a 90° periodicity as functions of the initial polarization of the incident shear wave, provided that the ray path is kept fixed. Our results show that this is a general characteristic of shear wave splitting in inhomogeneous anisotropic media. The effect can be used to identify complex anisotropic regions within the Earth's mantle.