Location map of wide-angle surveys in the northern Japan subduction zone. The inset map shows the location of the study area. The thick red lines labeled NS1 and NS2 are our wide-angle seismic survey transects, and the white circles on these lines represent OBSs. The thick gray lines labeled EW2000, EW1999, EW1997, and NS1999 are past wide-angle seismic survey profiles (Ito et al., 2004, 2005; Takahashi et al., 2004; Miura et al., 2005; Fujie et al., 2006). Gray contours show coseismic slip distributions of large interplate earthquakes (more than M 7) since 1930; the contour interval is 0.5 m, and the areas with values within half the maximum slip are shaded in yellow (Yamanaka and Kikuchi, 2004). Thin dashed black curves indicate regions of low microseismicity, where a thin low-velocity layer was identified between the overlying and subducting plates (Mochizuki et al., 2005). This region has been divided into three segments, A, B, and C, based on the distribution of interplate earthquakes; thick black dashed lines mark the segment boundaries. The green stars and green dashed line show the updip (eastern) limit of the forearc mantle wedge, and red diamonds show the bending line of the subducting Pacific plate (Ito et al., 2004, 2005; Fujie et al., 2006).