Fig. 1

Study area from offshore Tokai to Hyuga in the Nankai subduction margin. Gray lines represent the MCS reflection surveys studied in this work. Yellow dots represent the locations of BSRs identified in Figs. 2, 6b, c. Red dots denote the locations used to calculate the thermal structure via thermal modeling, which adopts the temperatures at the BSR depths as a constraint and, therefore, requires continuous BSRs (see Additional file 1: Figures S2 and S3 for more detail on the modeling section, and Figures S4–S23 for modeling results). Black stars indicate the epicenters of the Tonankai earthquake in 1944 and Nankai earthquake in 1946 (Kanamori 1972). Site C0002 of the IODP Expeditions (Expedition 314 Scientists 2009) is indicated by a white star. Note that there are many other IODP wells where heat flows have been determined using borehole measurements (e.g., Harris et al. 2011; Marcaillou et al. 2012; Harris et al. 2013) (see Fig. 8a). The seismic profile of the A–B section is illustrated in Fig. 7, and the thermal structure modeled over the section is shown in Fig. 9. The parameters used in the thermal modeling of the A–B section are shown in Table 2. A total of 142 profiles are used in this study (see main text for properties of the studied MCS data)