TOKYO – An earthquake with a preliminary magnitude of 6.6 struck southwestern Japan on Monday. There were no immediate reports of injuries or damage, and a small tsunami was observed in parts of Miyazaki and Kochi prefectures, the weather agency said.

The Japan Meteorological Agency released its additional information on the Nankai Trough for the second time ever, having done so last August, and also launched an investigation into whether there was an increased risk of a megaquake. The agency later concluded that the quake did not increase the risk.

The 9:19 p.m. quake reached a lower magnitude of 5 on Japan’s 7 seismic scale in Miyazaki, Shintomi and Takanabe in Miyazaki Prefecture, the agency said. The quake struck in the Hyuga-Nada Sea off the coast of the prefecture at a depth of about 36 kilometers.

The magnitude was revised twice, first estimated at 6.4, then updated to 6.9 and later adjusted to 6.6.

Tsunamis about 20 centimeters high were observed in parts of Miyazaki Prefecture and 10 centimeters high in some areas of Kochi Prefecture. The agency has since lifted all tsunami warnings.

Some sections of the Kyushu Shinkansen bullet train line have been temporarily closed due to the earthquake, the train operator said.

According to the operators, no abnormalities were found at the Ikata nuclear power plant in Ehime Prefecture and at the Sendai nuclear power plant in Kagoshima Prefecture.

The quake was also felt across large parts of western Japan.

The agency said the quake occurred on the western edge of the hypothetical epicenter of an earthquake in the Nankai Trough.

Megaquakes in the Nankai Trough along the Pacific coast occur every 100 to 150 years. The most recent were the Tonankai quakes of 1944 and the Nankai quakes of 1946, which together shook a wide area from central to southwestern Japan.

A weather agency official speaks at a news conference in Tokyo on Jan. 13, 2025. (Kyodo)

According to a government estimate, there is a 70 to 80 percent chance that a magnitude 8 to 9 earthquake will occur within 30 years.

The central government issued a Nankai Trough megaquake warning on August 8 following an M7.1 quake in the Hyuga-Nada Sea, the first since the system was launched in 2017, urging caution over the increased risk of strong shaking over a large earthquake zone on area.

While the government called for increased disaster preparedness rather than evacuation in anticipation of a megaquake, some communities in the region set up evacuation centers and recommended that older residents in particular protect themselves.

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