Study area and S-wave velocity image in the Philippine Sea slab. (a) The tectonic setting of central Japan. Red triangles denote active volcanoes. Iso-depth contours of the Pacific and Philippine Sea slabs (Nakajima and Hasegawa 2007; Hirose et al. 2008; Nakajima et al. 2009) are shown by blue and pink curves, respectively. Green shading represents an area where the upper surface of the Pacific slab is in contact with the lower surface of the Philippine Sea slab, termed the slab contact zone (Nakajima et al. 2009; Uchida et al. 2009). Arrows represent the directions of motion of the Philippine Sea and Pacific plates. The extent of the studied area is marked by a dashed rectangle. (b) Map showing S-wave velocity distribution along a curved surface located 10 km above the Pacific slab (after Nakajima et al. 2009). Red stars denote two earthquakes that occurred in the Philippine Sea slab in 1921 (M7.0) and 1987 (M6.7). A dashed pink line represents the western boundary of the serpentinized mantle of the Philippine Sea slab.