We present the results of an onshore--offshore wide-angle refraction and reflection experiment off Miyagi, in the central Japan Trench forearc region. There are two rupture zones of large interplate earthquakes here: the landward rupture zone and the trenchward rupture zone. To examine the influence of plate boundary geometry on the distributions of the rupture zones, we determined reflector geometries from reflections. The subducting oceanic plate increases its dip from about 5¿ to 13¿ around 143.2¿E. This bending point in the oceanic plate corresponds to the eastern edge of the trenchward rupture zone. Moreover, another bending point may be present at approximately 142.3¿E, which corresponds to the eastern edge of the landward rupture zone. The coincidence between bending points in the oceanic plate and the edges of rupture zones suggests that changes in the plate boundary geometry may affect rupture propagation of interplate earthquakes in the direction of plate convergence.