Fig. 1

Bouguer anomaly map around the Togi-gawa Nangan fault (TNF). Blue and purple, as a profile for the analysis of 2D density structure, circles represent the gravity measurements points of this study; red ones denote those on the seafloor reported by Ishida et al. (2018). Black dots are the gravity measurements points reported from previous studies (Geographical Survey Institute 2006; Yamamoto et al. 2011; Honda et al. 2012; Geological Survey of Japan 2013). White triangles represent the locations of deep drilling holes (Sutou et al. 2004). Red lines show active faults (Imaizumi et al. 2018). A yellow line is the line on which we estimated the 2D density structure using the 2D Talwani’s method as depicted in Fig. 3. A white star shows the epicenter of the 2007 Noto Hanto earthquake. A white rectangle shows the range of Fig. 4