Seismic anisotropy is the dependence of seismic wave velocities on the direction of propagation or polarization of the waves.
Seismic anisotropy has been found in several regions and at different depths inside the Earth. The theory of seismic anisotropy is well established. Seismic anisotropy is due to the preferred alignment of intrinsically anisotropic minerals, or the layering of isotropic materials with contrasting elastic properties.
Elastic anisotropy in the Earth’s mantle can manifest itself in the azimuthal dependence of body and surface wave propagation speed, a discrepancy between propagation speeds of Love and
Rayleigh waves, and shear wave splitting or birefringence.
Seismic anisotropy is often assumed to have hexagonal symmetry (i.e., transverse isotropy) with an axis of symmetry that is either horizontal (azimuthal anisotropy) or vertical (radial anisotropy), but other orientations may, of course, occur.
Seismic anisotropy in the upper mantle is a consequence of the strain-induced crystallographic or lattice preferred orientation of intrinsically anisotropic mantle minerals, principally olivine.
Radial anisotropy is required in the uppermost mantle to reconcile Love and Rayleigh wave dispersion, but its depth extent and how it varies in different tectonic settings is still somewhat unclear. Radial anisotropy has also been reported for the transition zone, but the results are highly variable among the different studies.
Azimuthal anisotropy occurs when seismic wave velocity changes with the azimuth of propagation. It was first observed by Hess (1964) in the Pacific ocean through the azimuthal dependence of ropagation of Pn waves. Since then, it has been found at various places in the uppermost mantle and deeper using various data such as shear wave splitting and surface waves. It could also be present inside or just below the transition zone.
