Earthquake in Southern Gunma Prefecture on May 2

On May 2, 2013 at 22:04 (JST), there was a M4.1 earthquake (maximum seismic intensity 4) at a depth of 6 km in southern Gunma prefecture. This event occurred within the crust. The focal mechanism showed a reverse fault with a compression axis in an NW-SE direction. In addition, on April 30 at 22:09 (JST), at a depth of 5 km and almost same place, there was a M4.2 earthquake (maximum seismic intensity 3).
For the seismic activity since October 1997, in the vicinity of the recent event (Area a), there have often been earthquakes of M4.0 or over, such as a M4.8 earthquake (maximum seismic intensity 4) on May 15, 2005. In addition, on February 25, 2013, there was a M6.3 earthquake (maximum seismic intensity 5 Upper) on the north side of Area a.
For the seismic activity since January 1923, in the region around the recent event (Area b), there was the Imaichi Earthquake (maximum magnitude 6.4) on December 26, 1949. This event caused damage with 10 fatalities, 163 injured persons and 290 total collapsed houses (Comprehensive Bibliography of Destructive Earthquakes in Japan, Latest Edition).

Japan Meteorological Agency
[Evaluation of Seismic Activities for April 2013 (May 13, 2013)]