The Meteorological Agency on Thursday issued its first-ever alert warning about a possible megaquake around the Nankai Trough, following a magnitude 7.1 quake that struck earlier in the day off the coast of Kyushu.

It is believed that the chance of a major earthquake occurring in the Nankai Trough is relatively higher than usual, according to the agency, but it added that this does not mean an earthquake will necessarily occur within a specific timeframe. The warning is expected to be in place for a week, but officials said residents should not let their guard down even after it is lifted.

M 7.1 - 20 km NE of Nichinan, Japan

• 2024-08-08 07:42:55 (UTC)
• 31.719°N 131.527°E
• 25.0 km depth

M 6.5 - 150 km E of Dolinsk, Russia

• 2024-08-10 03:28:32 (UTC)
• 47.178°N 144.763°E
• 406.5 km depth

The fault is divided into five segments in three zones, which rupture separately or in combination, and depending on location, the resulting earthquakes are subdivided by zone from west to east into Nankai earthquakes, Tōnankai earthquakes, and Tōkai earthquakes.

The earthquakes occur with a return period of about 90–200 years, and often occur in pairs, where a rupture along one part of the fault is followed by a rupture elsewhere on the fault, notably the 1854 Ansei-Tōkai earthquake and the 1854 Ansei-Nankai earthquake the next day, and the 1944 Tōnankai earthquake, followed by the 1946 Nankaidō earthquake. In one recorded case (the 1707 Hōei earthquake) the fault ruptured along its entire length. All of these great earthquakes have resulted in damaging tsunamis, which are particularly damaging due to the Japanese population being concentrated on the Taiheiyō Belt, especially the coastal cities of Tokyo, Yokohama and Osaka, the three most populous cities in Japan. The area remains seismically active, and future earthquakes are anticipated, with a high risk of a Nankai earthquake in the near future, which could be potentially very damaging.