Separate ocean bottom seismography and multi-channel seismic studies provide insight into the structure of the northern end of the Ryukyu trench region. Features of note include a thick (7–12 km) low velocity (4–5 km/s) zone on the landward side of the trench, the existence of subducting paleo-arc crust near the top of the trench in contrast to simple oceanic crust located at the middle of the trench, and a zone in which the Philippine Plate subducts beneath low P-wave velocity material (Vp = 5 km/s) that coincides with the location of the 7.5 1968 Hyuganada earthquake. It has been hypothesized that the above structural heterogeneity, in particular the subducting paleo-arc crust and its associated bathymetric highs, is one reason why earthquakes in this region are not larger i.e. exceeding 8.0. The exact mechanism by which the subduction of paleo-arc crust prevents sufficient stress build up for a larger earthquake is unknown.