Earthquake in the Southern Nagano Prefecture on June 25 (2)

In Area c, on September 14, 1984, there was an M6.8 earthquake ("the 1984 Western Nagano Prefecture Earthquake," hereafter called as the western Nagano prefecture earthquake) at about 5 km southwest of the recent event. After the western Nagano prefecture earthquake, the seismic activity became active such as the occurrence of an M6.2 aftershock. The recent event occurred within the active area of the western Nagano prefecture earthquake and its aftershocks.
In the neighborhood of the recent event, many earthquakes have focal mechanisms of a strike-slip or reverse fault type with a compression axis in an NW-SE direction. Also the distribution of the recent event and its aftershocks is generally consistent with the southeast dipping nodal plane of the CMT solution (reverse fault type).
For the seismic activity since January 1923, in the area around the recent event (Area d), there have sometimes been earthquakes of M5.0 or over, and there were 3 earthquakes of M6 over. Among them, the western Nagano prefecture earthquake occurring on September 14, 1984 caused damage with 29 fatalities and missing persons and so on (Comprehensive Bibliography of Destructive Earthquakes in Japan).

Japan Meteorological Agency
[Evaluation of Seismic Activities for June 2017 (July 11, 2017)]