Fig. 1

a Map of studied area. Rectangle in lower-right inset indicates study area. Blue and red stars denote epicenters of foreshock and mainshock, respectively. Blue and red rectangles represent surface projection of assumed source fault planes of foreshock and mainshock, respectively. Triangles indicate strong motion stations used. Stations used for source inversion analysis of foreshock are gray-filled, and stations used for source inversion analysis of mainshock are bordered by thick lines. Brown lines represent surface traces of active faults compiled by Nakata and Imaizumi (2002). b Moment tensor solutions of foreshock and mainshock determined by F-net of NIED (Fukuyama et al. 1998) and Global CMT Project (Ekström et al. 2012)