Fig. 1

Tectonic setting and hypocenter distribution in and around the study area. (Left) Hypocenter distribution in NE Japan. The red rectangle indicates the location of the study area shown in the right-hand panel. Thin broken black and orange lines show the position of the Japan trench and depth contours (in km) for the upper surface of the Pacific slab, respectively (Nakajima and Hasegawa, 2006; Kita et al. 2010). The arrow shows the direction of plate motion of the Pacific plate relative to the Okhotsk plate (Wei and Seno 1998). Red triangles denote active volcanoes. Circles denote hypocenters determined by JMA (3 June 2002 to 31 December 2017). (Right) Hypocenter distribution in the study area, with hypocenter depth shown by the color scale. The northern and southern parts of the study area are referred to as “region N” (southern Hokkaido) and “region S” (eastern Aomori), respectively. The region combining the southern half of region N and the northern half of region S is referred to as “region C” (Tsugaru Strait)