Mantle seismic velocity tomography is a powerful tool for exploring the Earth’s deep structure. Interpreting the velocity anomalies as thermal anomalies allows us a snapshot glimpse of the dynamics of a convecting mantle.
Isotropic seismic velocities independent of direction are a simplifying assumption made in most tomography models. Velocity anisotropy, however, is often interpreted to exist in many areas. These range from the upper crust due to sedimentary layering, to the upper mantle as a result of viscous interaction between the underlying convecting mantle and the rigid lithospheric slabs, down to the core-mantle boundary, where chemical interactions with the molten outer core or a chemically distinct layer of old slabs may introduce anisotropy.
Anisotropy can have many causes. Often, it is a result of alignment of crystal orientation, or possibly alignment of fractures or pockets of melt within a strain field. In general, while isotropic velocities may only give us snapshots of the current thermal and chemical conditions of the mantle, anisotropy can give us a more dynamic picture by giving us additional information about the stress and strain present in the mantle.
With the increasing reliability of the details in elastic global models, it may be possible, in the near future, to accurately predict elastic effects on the amplitudes of globally traveling long period surface and body waves, thereby gaining access to more accurate estimates of the lateral variations of Q in the earth.