Seismic anisotropy is found in various parts of the Earth's crust and mantle. This manifests itself in elastic wave propagation velocities that depend on their polarization and/or direction of propagation. The presence of anisotropy in the crust has been known since the 19thcentury , and is due in part to the preferential alignment of anisotropic crystals in rocks, and in part to the non-uniform distribution of fractures and pores. In the upper mantle, this anisotropy is mostly attributed to the preferential alignment of intrinsically anisotropic crystals of olivine, the major constituent in this region of the Earth, during the large-scale deformation associated with the slow convective flow that takes place there. While the lower mantle is predominantly isotropic, the D" region, which represents the last 200-300 km at the base of the mantle, exhibits strong anisotropy, detected in the late 1980s by the observation of birefringence in the shear waves passing through it.
The aim of this lecture was to review our current knowledge of the intrinsic (microscopic) anisotropy of rock crystals at different depths in the mantle, and the macroscopic seismic anisotropy observed in recordings at the Earth's surface, as well as the relationship between these two phenomena, at very different spatial scales.