Earthquake under the Tokyo Bay on September 12

On September 12, 2015 at 05:49 (JST), there was a M5.2 earthquake (maximum seismic intensity 5 Lower) at a depth of 57 km under the Tokyo Bay. The focal mechanism (CMT solution) had a tension axis in an NW-SE direction. This event occurred within the Philippine Sea plate. This event caused damage with 7 injured persons (Fire and Disaster Management Agency). The aftershock activity was low.
For the seismic activity since October 1997, in the vicinity of the recent event (Area b), there have sometimes been earthquakes of M4.0 or over, but no earthquakes of M5.0 or more until this event.
For the seismic activity since January 1885, in the area around the recent event (Area c), there has been no earthquake of M6.5 or over since 1923. On June 20, 1894, there was a M7.0 earthquake (Meiji Tokyo Earthquake) which caused damage with 31 fatalities and 130 totally or severely collapsed houses and so on mainly in the Tokyo Bay Coast, such as Tokyo and Yokohama (damage: Comprehensive Bibliography of Destructive Earthquakes in Japan). Also earlier on November 11, 1855, in the area
around the recent event, there was an earthquake (Ansei Yedo Earthquake) which caused damage with 7,468 fatalities and so on (damage: Comprehensive Bibliography of Destructive Earthquakes in Japan).

Japan Meteorological Agency
[Evaluation of Seismic Activities for September 2015 (October 9, 2015)]