Fig. 9

Crustal structure models. a The blue region is the area where granite is present in the lower crust, as noted by Ishikawa (2017). The crustal structure of this region is Model B. The crustal structure of the other regions is Model A. The continuous dotted lines denote depth (km) contours to the top of the subducted PAC slab (Iwasaki et al. 2015; Lindquist et al. 2004). b The crustal structure of Model A is shown. The upper crust is granite, and the lower crust is gabbro. The thickness of the upper crust is 18 km, and the depth of the Moho discontinuity is 30 km. Five cases are assumed, depending on the type of rock exposed at the surface and the thickness of the sedimentary layers. The depth of the basement rocks is based on the Shallow and deep layers combined model (SDLCM) (NIED 2019). Rock thermal conductivity and crustal heating values are as noted. c The crustal structure of Model B is shown. The upper crust and lower crust are granite. As in Model A, five cases are assumed, depending on the type of rock exposed at the surface and the thickness of the sedimentary layer. d The crustal structures of Model C and Model D are shown. The crustal structure of Model C is based on Tanaka (2009). The thermal conductivity of the rock is assumed to be constant. The crustal structure model D is based on Okubo et al. (1998). The upper crust is granite, and the lower crust is gabbro. The thickness of the upper crust is 18 km, and the depth of the Moho discontinuity is 30 km. Rock thermal conductivity and crustal heating values are as noted