Earth models in which seismic wave speeds vary only with depth are sufficiently well constrained to accurately locate earthquakes and calculate the paths followed by seismic rays [Engdahl et al., 1998]. The differences between observations and theoretical predictions of seismograms in such onedimensional Earth models can be used to reconstruct the three-dimensional (3-D) wave speed distribution in the regions sampled by the seismic waves by a procedure known as seismic tomography, a technique akin to medical CAT scanning.

Caused by thermal, compositional, and textural variations, wave speed anomalies remain the premier data source to fully understand the structure and evolution of our planet [Romanowicz, 2003], from the scale of mantle convection and the mechanisms of heat transfer from core to surface to the interaction between the deep Earth and surface processes such as plate motion and crustal deformation.