What is the Influence of Anisotropy on Tomographic Images of the Mantle

The primary source of information on heterogeneity within the Earth comes from seismic tomography. A powerful tool for examining the character of heterogeneity comes from the comparison of images of bulk-sound and shear wavespeed extracted in a single inversion, since this isolates the dependencies on the elastic moduli. The relative behaviour of bulk-sound and shear wavespeed has proved a useful tool in the definition of heterogeneity regimes. However, a basic assumption in this approach is that the Earth is isotropic. There is abundant evidence for anisotropy in the lithosphere and upper mantle, near the core-mantle boundary, and hints of complications near the 660 km discontinuity. What then are the likely influences of anisotropy on our interpretation of tomographic structure? The passage time of seismic body waves through the potentially anisotropic regions will generally be a small enough part of the total that the effects should not be large in the bulk of the mantle. It is possible though that the narrow structures in the lower mantle interpreted as remnant subducted slabs may carry some anisotropic signature. In the upper mantle, for younger subducted slabs, the large apparent signature of bulk-sound speed may be influenced by anisotropic effects. Similarly, the anti-correlation of bulk-sound and shear wavespeed variations near the base of the mantle needs to be assessed against the likelihood of contamination by anisotropy.