Fig. 1

a Seismic survey lines in the northwestern part of the Pacific plate. Red lines show seismic survey lines along which we shot the large-volume tuned airgun arrays of R/V Kairei and R/V Kaimei while towing a 6-km-long multi-channel hydrophone streamer cable to obtain multi-channel seismic (MCS) reflection data. White circles represent Ocean Bottom Seismometers (OBSs) aligned at a spacing of 6 km. The green contours show the slip distribution of the 2011 Tohoku earthquake derived from both terrestrial GPS and seafloor geodetic data (Iinuma et al. 2012); the contour interval is 10 m, and the areas with values \(>30\) m are shaded in green. Dashed black curves outline regions of low microseismicity, where a thin low-velocity layer was identified between the overlying and subducting plates (Mochizuki et al. 2005). Yellow dashed lines represent discontinuities of the magnetic anomaly lineations (Nakanishi 2011). The thick black arrow shows the plate convergence direction. b Slope of the seafloor bathymetry of the oceanic plate, showing the horst and graben structure along the Japan Trench and the Kuril Trench. Red dashed lines are magnetic anomaly lineations (Nakanishi 2011). The blue dashed curve shows the Joban seamount chain