Fig. 1

Seafloor bathymetry and locations of multi-channel seismic (MCS) reflection lines (gray thin) in Nankai Trough margin off southwest Japan. Inset: Regional map showing the location of the study area (red box). The Nankai subduction zone may be divided into five discrete segments (A through E) (Ando, 1975). Heavy black parts on lines 1, 2, and 4 mark the MCS profiles shown in Figs. 2, 3, and 4. The areas with slip > 1 m for the 1944 Tonankai and 1946 Nankai earthquakes (Baba and Cummins, 2005) are shown in black and blue dotted lines, respectively. White and yellow stars show epicenters of the 1944 and 1946 events, respectively. Red circles are very low-frequency earthquakes (Obara and Ito, 2005). Green circles are cold seeps (Ashi et al., 2009). A seafloor lineament (heavy dotted line shaded in red) at the seaward edge of the Kumano basin appears to extend westward across the B–C boundary off Cape Shiono, Kii Peninsula.