Fig. 1

Map of the study area and seismic lines. Yellow lines indicate seismic survey lines used in this study. Seismic profiles along the red lines are shown in Figs. 2, 3, 4, 5. Orange dashed lines with arrowheads indicate the three segments inferred in this study. The red star denotes the epicenter of the 2011 Tohoku earthquake. White contours show the slip distribution during the Tohoku earthquake (from Iinuma et al. 2012): Dashed, thin solid, and bold solid lines denote 10 m, 30 m, and 50 m slips, respectively. Green crosses and open circles indicate tremors and very low-frequency earthquakes, respectively (from Nishikawa et al. 2019). The green shaded rectangle denotes the slow slip event in 2011 (Ito et al. 2013). The magenta shaded rectangles indicate the area where chert layers are apparently missing (according to Fujie et al. 2020). Cyan dashed rectangles are the source location of 1933 Showa Sanriku earthquake (Uchida et al. 2016). Gray thick dashed lines show the possible pseudofaults (Nakanishi 2011). Black contours show the bathymetry (interval, 1000 m). Abbreviation: smt, seamount