Fear of Nankai Earthquake... Magnitude 6.9 Earthquake off Kyushu Coast, Japan

At around 9:19 PM on the 13th, a magnitude 6.9 earthquake was observed in the Hyuganada Sea area off the coast of Miyazaki Prefecture, Kyushu, Japan, raising local concerns about the potential occurrence of a Nankai Trough megaquake.

Following the earthquake, the Japan Meteorological Agency (JMA) issued a "Nankai Trough Earthquake Temporary Information (Under Investigation)" and held a late-night expert meeting to assess the connection to a Nankai Trough megaquake. They concluded that there was no phenomenon indicating an increased likelihood of a Nankai megaquake beyond normal levels and determined that no special disaster prevention measures were necessary, thus ending the investigation without issuing additional temporary information.

However, the JMA urged, "(The Nankai megaquake) could occur at any time. We hope people maintain thorough preparedness at all times." The Nankai Trough megaquake is a massive earthquake that occurs at intervals of 100 to 150 years along the Nankai Trough, which extends from the sea off Shizuoka Prefecture to the southern part of Shikoku and the eastern Kyushu sea area. It is predicted that if an earthquake of magnitude 8 to 9 occurs, it could result in approximately 230,000 deaths and missing persons, and damage to about 2.09 million buildings. The Japanese government estimates the probability of a Nankai Trough megaquake occurring within the next 30 years at 70 to 80%.

Earlier, after a magnitude 7.1 earthquake struck off the coast of Miyazaki Prefecture, Kyushu, on August 8 last year, the JMA held an expert meeting and judged that the likelihood of a massive earthquake along the Pacific coast had increased beyond normal levels, issuing the "Nankai Trough Earthquake Temporary Information (Megaquake Advisory)" for the first time. Although no megaquake occurred after the temporary information was issued and it was lifted about a week later, Japanese society was gripped by fear of a megaquake for some time, leading to hoarding of daily necessities and cancellations of travel reservations.

The Nankai Trough Earthquake Temporary Information is issued in two categories: "Megaquake Warning" and "Megaquake Advisory." The "Megaquake Warning" indicates a higher risk level, while the "Under Investigation" phrase is attached during the preliminary stage when the relationship is being analyzed. The temporary information system was first introduced in 2019 and can be issued when an earthquake of magnitude 6.8 or higher occurs near the Nankai Trough or when abnormal crustal movements are detected by special observation equipment.

Immediately after the earthquake on this day, the JMA also issued and then lifted a tsunami advisory for the coasts of Miyazaki Prefecture and Kochi Prefecture in Shikoku. Tsunami waves of up to approximately 1 meter were observed at some coastal areas. In the northern plain region of Miyazaki Prefecture, shaking of "seismic intensity 5 lower" was detected. The seismic intensity scale used by the JMA is a relative measure indicating the degree of shaking felt by people and objects around, unlike magnitude which measures absolute strength. It ranges from 0 to 7. Intensity 5 lower means most people feel fear and items such as dishes on shelves or books fall. Shaking of intensity 4 was also detected in large parts of other Kyushu regions including Fukuoka, Saga, and Kumamoto Prefectures. The JMA warned, "Seismic intensity 5 lower earthquakes may occur again in the next 2 to 3 days," urging caution.