July 4, 2025
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
* Seismic activity in the area between Akusekijima and Takarajima Islands in the Tokara Islands has been ongoing since June 21. On July 2 at 15:26, a magnitude (M) 5.6 earthquake occurred at the western edge of this region, approximately 20 km northwest of Takarajima Island. This event reached a maximum seismic intensity 5 Lower at Kodakarajima Island. Its fault mechanism involved normal faulting with an NNE-SSW tension axis. Subsequently, at 16:13 on July 3, an M5.5 earthquake occurred at the eastern end, approximately 20 km southwest of Akusekijima Island, registering a maximum seismic intensity 6 Lower at Akusekijima Island. Both earthquakes occurred within the continental plate, and the fault mechanism involved strike-slip faulting with an NNW-SSE tension axis.
* By 16:00 on July 4, there had been 1,201 earthquakes with a seismic intensity 1 or greater. This total includes 4 earthquakes with a seismic intensity 5 Lower or greater, and 120 earthquakes with a seismic intensity 3 or greater, suggesting that seismic activity is still ongoing.
* GNSS observations indicate that during this series of seismic activities, the Takarajima observatory recorded crustal deformation of approximately 2 cm in the east-northeast direction from June 21 until just prior to the occurrence of the largest earthquake (M5.6 on July 2). Additionally, a crustal deformation of about 4 cm was recorded in the south direction before and after the occurrence of the largest event.
* The region surrounding the current seismic activity zone has a historical record of significant seismic events, including M5.5, M5.9, and M6.1 earthquakes in December 1995, October 2000, and December 2021, respectively. These seismic activities often alternate between periods of high and low activity, with some phases lasting several months or longer. The current series represents the highest number of earthquakes since 1995.
* The area between Akusekijima and Takarajima Islnads lies along a volcanic front. Past seismic activity in this region indicates that earthquakes of similar magnitude to the strongest quake in a series often occur repeatedly. Therefore, we should remain vigilant for earthquakes of the same magnitude (seismic intensity 6 Lower).