At 4:19 a.m. on 21 December 1946, a magnitude 8.1 earthquake ruptured a stretch of the Nankai Trough off Shikoku, southern Japan, where the Philippine Sea Plate dives beneath the Eurasian Plate. It struck a country still recovering from the Second World War, which had ended barely sixteen months earlier.
The quake and the tsunami that followed within minutes killed at least 1,330 people, according to Japan's disaster records and the NOAA National Centers for Environmental Information (NCEI) Significant Earthquake Database, with roughly 2,600 injured and around 100 missing. Tsunami waves of up to about 6 metres struck the coasts of Shikoku and the Kii Peninsula, washing away or destroying more than 1,400 homes in addition to some 2,600 flattened by the shaking itself.
The 1946 Nankai earthquake was the most recent in a long historical sequence of major ruptures along the Nankai Trough — a segment that has broken repeatedly roughly every 100 to 150 years, including destructive predecessors in 1854 and 1707. It was preceded two years earlier, in December 1944, by the neighbouring Tōnankai earthquake, illustrating how the trough tends to fail in a linked sequence of adjacent segments rather than a single rupture.
Because the entire Nankai Trough has not ruptured together since 1946, Japanese seismologists consider another Nankai megaquake, potentially exceeding magnitude 8.5 or higher if multiple segments break at once, one of the country's most significant long-term hazards — the Japanese government's Earthquake Research Committee has estimated roughly a 70–80% probability of a major Nankai Trough earthquake within the next 30 years.