The Itoigawa-Shizuoka Tectonic Line is one of Japan's most important geological boundaries, running roughly north-south across central Honshu from the city of Itoigawa on the Japan Sea coast to Shizuoka on the Pacific. It marks the western edge of the Fossa Magna, the great rift-like depression that separates northeastern from southwestern Japan.
The tectonic line is an active fault system with segments showing reverse and strike-slip motion, reflecting the intense compression where several tectonic plates converge beneath central Japan. It borders some of the country's highest mountains, the Japanese Alps, whose rapid uplift is driven by movement along this zone.
Its northern and central segments are considered among the most likely in inland Japan to produce a large earthquake, with paleoseismic evidence of past magnitude 8-class events and long recurrence intervals. Because it passes near densely populated valleys, the Itoigawa-Shizuoka Tectonic Line is a focus of national earthquake hazard planning.