Earthquake under Northwestern Chiba Prefecture on February 23

On February 23, 2015 at 17:00 (JST), there was a M4.5 earthquake (maximum seismic intensity 3) at a depth of 68 km under northwestern Chiba prefecture. The focal mechanism showed a reverse 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 b), seismic activity is high and there have sometimes been earthquakes of M5.0 or over. On July 23, 2005, there was a M6.0 earthquake (maximum seismic intensity 5 Upper), which caused damage with 38 injured persons and 12 partially collapsed houses (Fire and Disaster Management Agency). The seismicity has increased than before, since "the 2011 off the Pacific Coast of Tohoku Earthquake."
For the seismic activity since January 1923, in the area around the recent event (Area c), there have sometimes been earthquakes of M6.0 or so. On September 25, 1980, there was a M6.0 earthquake (maximum seismic intensity 4), which caused damage with 2 fatalities and 73 injured persons (Comprehensive Bibliography of Destructive Earthquakes in Japan, Latest Edition).

Japan Meteorological Agency
[Evaluation of Seismic Activities for February 2015 (March 10, 2015)]