Fig. 1

Location map and bathymetry (a), and time series as a function of latitude of location (b) of sampling stations in this study. (c) Location map of our sampling stations (Table 1) encircled by the square in panel (a). The hypocenter of the 2011 Tohoku Earthquake is shown by the red star, and the hypocenters of large aftershocks (M_w ≥ 7) until 2019 in the northern and southern limits of the aftershock region and the earthquakes for the Ev1 and Ev2 are shown by yellow and purple stars, respectively. Ship-based hydrographic observations were performed onboard research vessels [Mirai (aqua-blue circles), Shinsei-Maru (red asterisks), and Hakuho-Maru (green circles) of JAMSTEC] and Japanese training ships [Oshoro-Maru (pink circles) of Hokkaido University and Bosei-Maru (blue circles) of Tokai University], and by the JMA before (yellow squares) and after (pink squares) the 2011 Tohoku Earthquake. The open orange triangle denotes the deep-sea station (~ 1000 m depth) used for long-term monitoring of the seafloor on the continental slope (Oguri et al. 2016, 2018). Bathymetric contours show 200 m depth (solid gray line), 1000 m depth (solid black line), 2000 m depth (dashed black line), and 3000 m depth (dotted black line). The color bar shows the depth of bathymetry