Fig. 1

(a) Tectonic setting around the study area. Depth contours of the Pacific (Nakajima et al., 2009) and Philippine Sea slabs (Hirose et al., 2007, 2008; Nakajima et al., 2009) are shown by dashed and solid lines, respectively. Black triangles denote active volcanoes. The plate motion of the Philippine Sea slab relative to the Eurasia plate is indicated by a black arrow (Seno et al., 1993). The Niigata-Kobe Tectonic Zone (Sagiya et al., 2000) is shown in gray shading. The black rectangle denotes the study area, which is magnified in (b). (b) Distribution of seismicity occurring from 2002 to 2007 at depth < 40 km (gray dots) and active faults (thin lines). The three major faults forming the Atotsugawa fault system, the Atotsugawa (AGF), Mozumi-Sukenobu (MSF), and Ushikubi (UKF) faults, are indicated by thick black lines. Earthquakes in the gray area (A–A′) are also shown in (c). (c) Cross-sectional view of seismicity around the Atotsugawa fault system. The dashed line denotes the approximate depth limit of earthquakes. An area of low seismicity in the central section is shown in gray. The Atotsugawa fault is divided into eastern, central, and western sections on the basis of seismicity distribution.