Relationship between interplate coupling and the velocity structure within the overriding plate. The area of strong interplate coupling (Nishimura et al.2000) is colored pink. The light blue areas show the low-V and high- VP/VS domains above the interplate megathrust (Zhao et al.2011). Brown lines show the distribution of the low-velocity sedimentary units observed by Tsuru et al. (2002). The blue line indicates the low-velocity part of the mantle wedge (Miura et al.2003). Other symbols are explained in Figure1.