Earthquakes under Tokyo Bay [under northwestern Chiba prefecture] on September 21 and under northwestern Chiba prefecture on October 8
On September 21, 2008 at 07:17 (JST) there was a M4.8 earthquake at a depth of 71km under Tokyo Bay [under northwestern Chiba prefecture] (maximum seismic intensity 3). This event occurred near the boundary between the Pacific and the Philippine Sea plates. The focal mechanism had a compression axis in a NNE-SSW direction.
On October 8, 2008 at 15:07, there was a M4.7 earthquake at a depth of 63km under northwestern Chiba prefecture (maximum seismic intensity 3). The focal mechanism showed a normal fault with a tension axis in a NW-SE direction. It is thought that this event occurred within the Philippine Sea plate.
The vicinity of the recent events (Area b) is a region of strong seismic activity where earthquakes equal to or larger than M5.0 sometimes occur, such as the recent M6.0 event on July 23, 2005 (maximum seismic intensity 5 Upper).
For the seismic activity since August 1923, in the area near the recent events (Area c), there were six earthquakes equal to or larger than M6.0. The largest event was the M6.3 earthquake on September 30, 1956 (maximum seismic intensity 4).
[ ] was the regional name of the epicenter used in the Japan Meteorological Agency announcement.
Japan Meteorological Agency
[Seismic Activity for September 2008 (October 10, 2008)]