Figure 1. Tectonic map of Japan. Thin black lines denote active faults. Gray lines denote plate boundaries and major tectonic lines. Dashed gray line denotes a possible plate boundary between the Eurasian(or Amurian) and the Okhotsk(or North American) plates. Thick gray line indicates location of the Niigata-Kobe Tectonic Zone (NKTZ). PAC: Pacific plate, PHS; Philippine Sea plate, EUR: Eurasian plate, AMR; Amurian plate, OHK: Okhotsk plate, NAM: North American plate, ISTL: Itoigawa-Shizuoka Tectonic Line, MTL: Median Tectonic Line .