Tectonic setting (a) and examples of seismograms in the study area (b, c). Seismograph stations in (a) shown as inverted triangles; solid inverted triangles indicate stations observing X phases. Black and orange broken lines denote the upper surface of the Pacific slab (Nakajima et al. 2009b) and a low-velocity (V p < 7.5 km/s) zone above the slab estimated by Kita et al. (2010), respectively. Red triangles show volcanoes. (b, c) Normalized vertical component seismograms with band-pass filtering (1 to 12 Hz). Stations and hypocenters are denoted as inverted triangles and stars, respectively, in inset maps. Colored solid inverted triangles denote stations where the X phase was observed. Red inverted triangles and black dots on seismograms indicate arrivals of the X phase and P wave, respectively. Lines denote theoretical times of P and S waves calculated from the JMA2001 1D velocity model (Ueno et al. 2002).