P-wave velocity in the subducting crust beneath the eastern part of Hokkaido. (a) Colored lines show path-averaged P-wave velocity. Hypocenters are plotted as stars. The other symbols are the same as those in Figure 1a. (b) Depth profile of P-wave velocity in the crust. Gray dots are plotted for velocity (horizontal axis) and averaged depth (vertical axis) for pairs of earthquakes. Green diamonds represent the averaged velocity at every 10-km depth slice. Pink diamonds indicate P-wave velocity in the crust beneath northeastern Japan (Shiina et al. 2013). Blue and orange lines show isotropic P-wave velocity expected for a metamorphosed mid-ocean ridge basalt (MORB) model (Hacker et al. 2003) in northeastern Japan and those experimentally derived from lawsonite blueschist (Fujimoto et al. 2010), respectively.