Fig. 1

Major active faults and seismic activity in southwest Japan between 679 and 2018: a distribution of the major active faults (Earthquake Research Committee 2018) and epicenters of the crustal earthquakes between 679 and 2018. The catalog of earthquakes from 679 to 1921 is based on Utsu (1990, 2002, 2004), whereas that from 1922 to 2018 is based on the Japan Meteorological Agency (JMA). The epicenters are plotted north of the black solid line in the range of a. The light and dark red circles indicate the epicenters of crustal earthquakes that occurred within 50 years before and 10 years after a megathrust earthquake. The light and dark blue circles indicate the epicenters in the other periods. The light and dark colored circles indicate the epicenters during 697–1921 and 1922–2018, respectively. The white circles indicate the epicenters after 1968. The black arrow denotes relative plate motion (DeMets et al. 2010). The red-shaded areas are the source areas of the megathrust earthquake along the Nankai trough (Earthquake Research Committee 2001). EUR, AMR, PHS, ATF, SYF, and IYS are abbreviations for the Eurasia Plate, Amurian Plate, Philippine Sea Plate, Arima–Takatsuki Fault, Shikano–Yoshioka Fault, and the Iyonada segment of MTL, respectively), and b magnitude–time diagram of the crustal earthquakes shown in a. The yellow stars are the megathrust earthquakes. The red-shaded areas indicate the periods 50 years before and 10 years after the megathrust earthquakes along the Nankai trough