Fig. 1

a Tectonic map of Kyushu Island. Blue dashed lines are the Beppu-Shimabara graben “BSG” (Matsumoto 1979) and the Kagoshima graben “KG” (Hayasaka 1987). Red solid lines show five major calderas, namely, Aso, Kakuto, Aira, Ata and Kikai, in Kyushu. A red arrow shows Philippine Sea plate–Amurian plate relative motion (72 mm/years) (Wallace et al. 2009). b Enlarged geological map in and around the Aira caldera. An outline of the geological distribution of major rocks in and around the calderas is described (modified from Uto et al. 1997 and Uchimura et al. 2014). Black broken line indicates the projection line of the present seismic profile