Seismic Activity in the Northern Nagano Prefecture

From March 21, 2021 to April 18 in Area b in the northern Nagano prefecture, earthquakes with seismic intensity 1 or over occurred in March 5 times (maximum seismic intensity: number of earthquakes, 2: 3, 1: 2), and in April 15 times (maximum seismic intensity: number of earthquakes, 3: 1, 2: 5, 1: 9). All these earthquakes occurred within the crust. The largest event in this activity was an M3.6 earthquake (maximum seismic intensity 3) that occurred on April 13 at 22:53 (JST) at a depth of 2 km. The focal mechanism showed a strike-slip fault type with a compression axis in an NW-SE direction.
For the seismic activity since October 1997, in the vicinity of the recent event (Area a), there have sometimes been earthquakes of M2.0 or over. Note that on October 5, 2011, there was an M5.4 earthquake (maximum seismic intensity 3) at the western side of Area a.
For the seismic activity since 1919, in the region around the recent event (Area c), there have sometimes been earthquakes of M5.0 or over. On November 22, 2014, there was an M6.7 earthquake (maximum seismic intensity 6 Lower) which caused damage with 46 injured persons, 77 totally collapsed houses and so on (Fire and Disaster Management Agency).

Japan Meteorological Agency
[Evaluation of Seismic Activities for April 2021 (May 14, 2021)]