Undersea Magma, Water May Be Inducing Quakes Around Tokara Islands; Lack of Instruments Nearby Limits Data

The Yomiuri Shimbun

20:00 JST, July 10, 2025

The detailed mechanism for the succession of earthquakes occurring off the Tokara Islands in Kagoshima Prefecture since June 21 remains unclear.

Significant crustal movements have been confirmed in the surrounding area, and experts have said that these may be connected to the movements of undersea magma or water.

According to the Japan Meteorological Agency, there have been 1,725 earthquakes measuring 1 or stronger on the Japanese seismic intensity scale of 7 around the islands which include Akusekijima Island, by 5 p.m. Wednesday.

“Very large crustal movements have been observed,” said Naoshi Hirata, chairperson of the government’s Earthquake Research Committee and professor emeritus at the University of Tokyo, at a press conference Wednesday.

Takarajima Island shifted 1.8 centimeters northeast between early June and July 2, according to analysis by the Geospatial Information Authority of Japan. Just before an earthquake that measured lower 6 on the intensity scale hit Akusekijima Island on July 3, Takarajima Island moved another 4.2 centimeters, this time to the south.

“Knowing that the direction of crustal movements changed before the earthquake is very valuable information,” Hirata said.

The oceanic Philippine Sea Plate subducts beneath the continental Eurasian Plate on the eastern side of the Tokara Islands. Superheated water can be released from parts of the subducting plate that are under extreme pressures. This superheated water melts surrounding rock layers, creating a “magma reservoir.” It is hypothesized that an upflow of such magma is causing the earthquakes and crustal movements.

There is also a hypothesis that water is the direct cause of the earthquakes.

“The possibility is high that water has flowed into the fault plane, making the geologic fault more prone to slip and causes earthquakes. The change in the direction of crustal movements can be explained by the flow of water into another fault,” said Junichi Nakajima, a professor of seismology at the Institute of Science Tokyo.

Since there are no observation instruments on Akusekijima Island to determine the focus of the quakes, there is insufficient data for clarifying the mechanism. The Education, Culture, Sports, Science and Technology Ministry, which oversees the Earthquake Research Committee, is considering installing temporary observation instruments on the island to address this issue.