Itoigawa-Shizuoka Tectonic Line
![The blue line to the left represents the fault, with the Fossa Magna shaded in orange.](/Images/godic/202501/14/Tectonic_map_of_southwest_Japan1530.png")
Itoigawa-Shizuoka Tectonic Line (ISTL)(糸魚川静岡構造線Itoigawa Shizuoka Kōzō Sen), also Ito Shizu Sen (糸静線) is a major fault on Honshu island that runs from the city of Itoigawa, Niigata Prefecture, through Lake Suwa to the city of Shizuoka in Shizuoka Prefecture. It is often confused with the Fossa Magna, a geological feature it forms the western boundary of.