Itoigawa-Shizuoka Tectonic Line (ISTL) , also Ito Shizu Sen (糸静線) is a major fault zone on Honshu island running from Itoigawa, Niigata Prefecture, through Lake Suwa, and on to Shizuoka in Shizuoka Prefecture. It is often confused with the Fossa Magna, a geological feature it forms the western boundary of.