The Meteorological Agency on Thursday issued its first-ever alert warning about a possible megaquake around the Nankai Trough, following  https://www.japantimes.co.jp/news/2024/08/08/japan/japan-kyushu-quake/ - 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  https://en.wikipedia.org/wiki/Nankai_earthquakes - Nankai earthquakes ,  https://en.wikipedia.org/wiki/T%C5%8Dnankai_earthquakes - Tōnankai earthquakes , and  https://en.wikipedia.org/wiki/T%C5%8Dkai_earthquakes - Tōkai earthquakes .

The earthquakes occur with a  https://en.wikipedia.org/wiki/Return_period - 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  https://en.wikipedia.org/wiki/1854_Ansei-T%C5%8Dkai_earthquake - 1854 Ansei-Tōkai earthquake  and the  https://en.wikipedia.org/wiki/1854_Ansei-Nankai_earthquake - 1854 Ansei-Nankai earthquake  the next day, and the  https://en.wikipedia.org/wiki/1944_T%C5%8Dnankai_earthquake - 1944 Tōnankai earthquake , followed by the  https://en.wikipedia.org/wiki/1946_Nankaid%C5%8D_earthquake - 1946 Nankaidō earthquake . In one recorded case (the  https://en.wikipedia.org/wiki/1707_H%C5%8Dei_earthquake - 1707 Hōei earthquake ) the fault ruptured along its entire length. All of these great earthquakes have resulted in damaging  https://en.wikipedia.org/wiki/Tsunami - tsunamis , which are particularly damaging due to the Japanese population being concentrated on the  https://en.wikipedia.org/wiki/Taiheiy%C5%8D_Belt - Taiheiyō Belt , especially the coastal cities of  https://en.wikipedia.org/wiki/Tokyo - Tokyo ,  https://en.wikipedia.org/wiki/Yokohama - Yokohama  and  https://en.wikipedia.org/wiki/Osaka - 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.

Japan has lifted it warning for a higher than usual risk of a major earthquake one week after a strong tremor on the edge of the Nankai Trough seabed zone caused the government to issue its first “megaquake” advisory.

Citizens can now return to normal life after no abnormalities were observed in seismic activity in the past week at the Nankai Trough, located along Japan’s Pacific coast, Yoshifumi Matsumura, the state minister for disaster management, said on Thursday.