Fig. 1

a Tectonic setting in and around Japan. Broken lines indicate plate boundaries. Four major plates are converging in Japan. b Enlarged map of the Boso peninsula indicated by the rectangle in a. Broken lines indicate plate boundaries. The broken red line indicates the source area of the 1703 Genroku earthquake (Sato et al. 2016), while the solid red line indicates the source area of the 1923 Taisho Kanto earthquake (Matsu’ura et al. 2007). The blue line indicates the coupled area estimated by Noda et al. (2013) on the basis of GNSS data. The blue broken lines indicate the three asperities assumed by Sato et al. (2016). The black broken rectangle shows the fault patch used to estimate the aseismic slip on the plate interface between the Philippine Sea plate and the continental plate (see text). The green broken line schematically shows the Boso slow slip area, which lies in the transition zone between a locked area and an aseismic creeping area. The central Japan block is explained in the text