Fig. 1

Tectonic framework of western Japan. Blue solid denote the iso-depth contours of the Philippine Sea (PHS) slab at a 20-km interval (Hua et al. 2018, and references therein). Red triangles and orange stars represent active volcanoes and M ≥ 6.5 crustal earthquakes that occurred during the 2003–2020 period, respectively. Dots indicate the M ≥ 2 earthquakes that occurred at 0–20 km depth. The Median Tectonic Line (MTL), which divides southwestern Japan into the inner and outer arcs, is shown by the dashed red line. Blue dashed lines denote the location of the Nankai trough