Of the five earthquakes since 1900 that had moment magnitudes (Mw) ≥ 9, the March 2011 Tohoku-oki earthquake had the smallest rupture area by 50%, but had the largest co-seismic displacements by 75%. We have considered physical explanations for these anomalous characteristics by applying a novel method of analyzing topography and gravity anomalies in subduction zones. In Japan, this technique reveals the offshore continuation of a prominent fault line, known as the Median Tectonic Line (MTL). Onshore, this fault sharply juxtaposes geological terranes composed of granite-batholiths and accretionary complexes. Offshore the contrast in rock type is strongly correlated with changes in earthquake behavior. I will briefly describe how this contrast in rock type may explain along-strike changes in the slip behavior of the Japan megathrust. I will then describe an ongoing 3-D active source tomographic study of the Japan subduction zones supported by the Green Foundation, which will produce the highest resolution regional 3-D images of any subduction zone on Earth.