Recent observations of up to 6 secs of shear-wave splitting in deep focus earthquakes from the Tonga-Kermadec subduction zone provide insights into mantle dynamics near the 660 km discontinuity. Modeling of subduction body force stresses predicts large deviatoric stress (~40 MPa) in the topmost lower mantle below a viscosity increase at the 660 km discontinuity. Finite strain calculations produce significant amounts of natural strains (~7) in the lower mantle, and coherently aligned strain ellipses. For a viscosity model with a viscosity increase at 660 km, 5--10 secs of shear-wave splitting is predicted, compared to only 0--5 secs for uniform mantle viscosity. For a viscosity increase model, we predict that shear-wave splitting reduces from 7--10 secs to 4--6 secs when deformation above 410 km is ignored, and further reduces to 2--3 secs when deformation above 660 km is zeroed. Various conversions from finite strain to anisotropy have been explored. Predicted shear-wave splitting magnitudes are comparable to those observed from the Tonga-Kermadec subduction zone.