- © The Mineralogical Society Of America
For many years, seismic anisotropy was often neglected, mostly because of the inherent heavy mathematical and computational tools needed to describe and model its effects on seismic waves. The usual basic knowledge about propagation in isotropic media cannot easily apply to anisotropic media, where new phenomena come up, such as birefringence (or shear-wave splitting), or difference between directions of propagation of phase velocity and of group velocity. Consequently, geophysicists often claimed that it was a second-order effect, and considered the Earth as isotropic.
This hypothesis was assumed to be a good approximation, because of the random orientation of crystals in most parts of the Earth, and of the random sampling of anisotropic regions by seismic rays. This assumption furthermore made easier the description of wave propagation, as well as the parameterization of media in inverse problems. An isotropic elastic medium can be described by two independent elastic parameters (λand μ Lamé parameters), but the simplest anisotropic medium (transverse isotropy with a vertical symmetry axis) requires 5 independent parameters (Love 1927, Anderson 1961). To date, seismic observations have been explained in terms of isotropic (and often thermal) lateral heterogeneities, ignoring manifestations of anisotropy. However, since the 1960s, it was recognized that most parts of the Earth are not only laterally heterogeneous but also anisotropic and that seismic anisotropy provides a simple explanation of different observational data:
azimuthal variation of Pn-velocities below oceans (discovered in the 1960s; Hess (1964) explained it by anisotropy),
Rayleigh-Love wave discrepancy: It is impossible to simultaneously explain Rayleigh and Love wave dispersion by an isotropic model (Anderson 1961, Aki and Kaminuma 1963, Mc Evilly 1964),
Shear-wave splitting (or birefringence), the most unambiguous observation of anisotropy, particularly for SKS waves (Vinnik et al. 1984).
Seismic anisotropy, contrary to isotropy, is reflecting some inherent …