Three-dimensional (3D) tomographic models describe lateral variations in the physical properties of a heterogeneous Earth as positive (blue) or negative (red) perturbations to the spherically-averaged (1D) values at each depth. Here are the isotropic shear-velocity variations from S362ANI+M around the equator looking down from the North Pole (left), globally at a depth of 150 km (middle), and for a cross-section across the Japan subduction zone (right).
