The Nankai Trough is a subduction megathrust off the southern coast of southwest Japan, where the Philippine Sea Plate slides beneath the overriding Amurian (Eurasian) Plate. It runs for hundreds of kilometres offshore from Shikoku and the Kii Peninsula toward central Honshu, facing some of Japan's most densely populated and industrialised coasts.
The plate converges at about 40 to 55 millimetres per year, and the interface is divided into segments that can rupture separately or link together in a single giant event. Historical records show great Nankai earthquakes recurring roughly every 100 to 200 years, often in linked pairs.
Past ruptures include the 1707 Hoei earthquake, which may have broken multiple segments at once at magnitude around 8.6, and the twin 1944 Tonankai (magnitude 7.9) and 1946 Nankai (magnitude 8.0) events. Japanese authorities regard a future great Nankai quake and its tsunami as one of the nation's gravest disaster risks.