July 9, 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. This activity can roughly be divided into eastern and western areas.
In the eastern part, seismicity became active on June 21, declined around June 26, and then resumed. At 16:13 on July 3, an M5.5 earthquake (depth of approximately 10 km) occurred, the maximum seismic intensity 6 Lower recorded at Akusekijima Island. The fault mechanism involved strike-slip faulting with a tension axis oriented in the NNW-SSE direction. Seismic activity in this area remains ongoing.
In the western part, seismicity became active around July 2. The largest event in this series was an M5.6 at 15:26 on July 2, with a maximum seismic intensity 5 Lower recorded at Kodakarajima Island. The fault mechanism was a normal fault with a tension axis oriented in the NNE-SSW direction. Currently, seismic activity in this area is low.
This series of earthquakes are occurring within the continental plate.
* Between June 21 and 11:00 a.m. on July 9, a total of 1,720 earthquakes with a seismic intensity 1 or greater were recorded. This total included 8 events with a seismic intensity 5 Lower or greater and 168 events with a seismic intensity 3 or greater.
* GNSS observations indicate that crustal deformation and the direction of horizontal deformation changed before and after the largest earthquake (M5.6 on July 2) in this series of seismic activities. At the Takarajima observatory, horizontal deformation changed from E-NE (approximately 2 cm) to the south (approximately 4 cm). Similarly, at Kodakarajima Island, the direction shifted from NE to N-NW.
* 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. GNSS observations have also revealed crustal deformation linked to these historical seismic events. The current series represents the highest number of earthquakes since 1995.
* The area between Akusekijima and Takarajima Islands lies along a volcanic front of the Tokara Islands. 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).
Note: GNSS is a general name of satellite positioning system such as GPS.