Earthquake in Northern Nagano Prefecture on December 1
On December 1, 2011 at 22:23 (JST), there was a M4.6 earthquake (maximum seismic intensity 3) at a depth of 1 km in northern Nagano prefecture. This event occurred within the crust. The focal mechanism had a compression axis in an NW-SE direction.
As to aftershocks of the recent event, as of December 31, there were two earthquakes with a maximum seismic intensity 1 or over (maximum seismic intensity 2:1, maximum seismic intensity 1:1) and the activity has been subsiding.
In the region around the recent event (Area b), there was a series of seismic activities with the largest M2.6 (maximum seismic intensity 1) on November 29, since late November. To the west of this region, there was a series of seismic activities in October.
For the seismic activity since October 1997, in the vicinity of the recent event (Area a), there has been almost no earthquake over M4.0.
For the seismic activity since August 1923, in the vicinity of the recent event, there were a few earthquakes of M5.0 or over. Among them, for example, a destructive event was a M5.9 earthquake (maximum seismic intensity 4) on December 30, 1986 which caused damage with 243 partially harmed houses.
Japan Meteorological Agency
[Evaluation of Seismic Activities for December 2011 (January 11, 2012)]