Fig. 2

a Distribution of earthquakes at depths of 30–100 km from October 1997 to May 2017. Colors and sizes of circles are proportional to focal depths and earthquake magnitudes, respectively. Dashed lines denote the western and northeastern boundaries of the slab contact zone. The isolated cluster of seismicity analyzed in this study is bounded by the blue rectangle. Line A–B is a profile along which a vertical cross section is shown in b. Other symbols are the same as in Fig. 1. b Vertical color-shaded cross section of P-wave velocity perturbations (Nakajima et al. 2009) along line A–B and seismicity (gray circles) within 10 km of the profile. Pink and black lines represent the upper surfaces of the PHS and Pacific Plates, respectively. Pink dashed line shows the bottom of the PHS Plate inferred from the distribution of high-velocity anomalies. c Magnitude versus time plot of 42 earthquakes that occurred in the area indicated by blue rectangles in a and b. The origin time of the 2011 Tohoku-Oki earthquake is indicated by a vertical blue line. Shaded zones at both ends of the panel denote unanalyzed periods