Fig. 1

Tectonic setting of the study area. a Map of southwestern Japan. The Philippine Sea Plate (PHS) is being subducted to the northwest beneath the Eurasian Plate (EUR) along the Nankai Trough. The box shows the study area. MTL Median Tectonic Line, OKT Okinawa Trough. b Geological map of the study area after the Geological Survey of Japan (1995). OKTL Oita–Kumamoto Tectonic Line, UYTL Usuki–Yatsushiro Tectonic Line, BSG Beppu–Shimabara Graben. The fault segments delineated by the Headquarters for Earthquake Research Promotion (HERP) (2013) are marked in red. FT Futagawa segment, UT Uto segment, UN north coast of Uto Peninsula segment, TS Takano–Shirahata segment, HN Hinagu segment, YT Yatsushiro segment, FZ fault zone. The solid rectangle indicates the target area of the tomographic analysis in Fig. 5. The dotted rectangles represent the surface projection of the source fault planes for the mainshock in Asano and Iwata (2016). The solid and dashed red lines denote the active faults determined by RGAF (1991) and HERP (2013), respectively