Earthquake under the Southern Saitama Prefecture on September 14

On September 14, 2017 at 09:27 (JST), there was an M4.5 earthquake (maximum seismic intensity 3) at a depth of 50 km under the southern Saitama prefecture. This event occurred within the Philippine Sea plate. The focal mechanism showed a reverse fault type with a compression axis in an NNW-SSE direction.
For the seismic activity since October 1997, in the vicinity of the recent event (Area b), there have sometimes been earthquakes of M4 or so. On May 25, 2015, there was an M5.5 earthquake (maximum seismic intensity 5 Lower), which caused damage with 1 seriously-, 2 lightly-injured persons and so on (Fire and Disaster Management Agency).
For the seismic activity since January 1923, in the area around the recent event (Area c), there were 4 earthquakes of M6.0 or over. On September 21, 1931, there was an M6.9 earthquake (Western Saitama Earthquake), which caused damage with 16 fatalities, 207 totally collapsed hoses and so on (Materials for Comprehensive List of Destructive Earthquakes in Japan).

Japan Meteorological Agency
[Evaluation of Seismic Activities for September 2017 (October 11, 2017)]