Subduction zones are the sites of Earth’s most powerful earthquakes and many of the world’s active volcanoes. Despite their importance, there are many aspects of subduction zone composition and dynamics that are poorly understood. This is especially important for those properties and processes that have implications for subduction-related hazard. Here, I show results from using anisotropic receiver function analysis to interrogate the mantle wedges beneath Japan. In addition to elucidating dynamic processes in the mantle wedge, we also are able to constrain the location of hydrous phases above the subducting slab in the Ryukyu arc. Finally, I show recent results from modeling large megathrust earthquakes and their implications for seismic hazard. Modeling results emphasize the importance of constraining the up- and down-dip limits of the seismogenic zone, the location of high stress drop asperities, rupture “roughness”, and the 3-D velocity structure of the subduction system. Overall, these studies highlight the need for detailed geophysical studies to interrogate the mantle wedge and plate interface in subduction zones, with a particular focus on those properties and processes with implications for subduction-related hazard.