Over 300 Earthquakes Rumble in Sea Off Japan’s Kagoshima Pref. Islands, Putting Residents on Alert

The Yomiuri Shimbun

17:59 JST, June 25, 2025

The sea off the Tokara Islands in Kagoshima Prefecture was hit with 336 earthquakes measuring at least a 1 on the Japanese seismic scale from Saturday to 5 p.m. Tuesday, according to a regional branch of the Japan Meteorological Agency.

Six of the earthquakes measured a 4 on the Japanese seismic scale, which goes as high as 7. Experts have suggested that even larger quakes could occur, and the agency’s Fukuoka regional headquarters is calling for people in the area to stay on the alert against earthquakes with strong shaking.

Tremors in the sea near the islands grew more frequent after 8 p.m. on Saturday, according to the regional headquarters. The largest quake over the period struck on Sunday afternoon and registered a magnitude of 5.1. There have been as many as 15 quakes per hour. On Tuesday, at 2:23 a.m. and again at 4:04 p.m., shaking measuring a 4 on the Japanese seismic scale was felt on Akusekijima Island in the prefecture’s village of Toshima.

The sea near the Tokara Islands has been the source of many prior earthquakes. In December 2021, tremors measuring 1 or higher on the Japanese seismic scale struck 308 times. On Dec. 9, 2021, a quake measuring an upper 5 on the seismic scale hit Akusekijima Island, forcing 30 residents to evacuate to Kagoshima City and elsewhere.

According to Hisayoshi Yokose, an associate professor at Kumamoto University specializing in marine volcanology, near the islands the Philippine Sea Plate is subducted beneath another tectonic plate connected to the Asian continent. This geology tends to cause a buildup of tension that results in earthquakes. Yokose thinks the recent quakes have occurred inside the continental plate.

“Past trends show that even bigger earthquakes occur after seismic activity subsides,” he said.

The area where the quakes have occurred is part of a volcanic cluster that stretches from the Kirishima mountain range, in Miyazaki and Kagoshima prefectures, to Io-Torishima Island in Okinawa Prefecture.

Junichi Nakajima, a Tokyo Institute of Technology professor in seismology, suggested that magma may have cooled when rising through the ground and water in that magma may have seeped out, making the fault prone to slippage.

On Sunday, Mt. Shinmoedake, part of the Kirishima mountains, erupted for the first time in seven years. However, the volcano, which rises to a height of 1,421 meters, lies at a distance from the Tokara Islands.

“I don’t think it had any effect” on the earthquakes, Nakajima said.

As of Sunday, 667 residents were living on seven remote islands in Toshima.

“You never know when a big quake will come. It reminds me of the one that we had four years ago that registered a 5 on the seismic scale,” said a 34-year-old man running a minshuku bed and breakfast on Akusekijima Island.

Since Monday, students have been commuting to a school on the island were wearing helmets. Some of the school’s 14 children have reportedly said they cannot sleep at night.

“We’ll try to relieve their concerns as much as we can,” said the school’s principal.