Earthquakes Off-shore East of Chiba Prefecture in April

On April 12, 2011 at 08:08 (JST), there was a M6.4 earthquake (maximum seismic intensity 5 Lower) at a depth of 26 km, off-shore east of Chiba prefecture. The focal mechanism (CMT solution) showed a strike-slip fault with a compression axis in a N-S direction.
In addition, there was a M4.5 earthquake (maximum seismic intensity 3) at a depth of 35 km under northeastern Chiba prefecture on April 17, 2011 at 13:38, and there was a M4.6 earthquake (maximum seismic intensity 3) at a depth of 35 km under northeastern Chiba prefecture on the same day at 15:49.
In addition, on April 21, 2011 at 22:37, there was a M6.0 earthquake (maximum seismic intensity 5 Lower) at a depth of 46 km, off-shore east of Chiba prefecture. The focal mechanism (CMT solution) showed a reverse fault with a compression axis in a WNW-ESE direction.
The event of April 12 occurred within the Philippine Sea plate. The event of April 21 occurred on the boundary between the Pacific and the Philippine Sea plates. Furthermore, the aftershocks of these events have gradually been decreasing.

For the seismic activity since August 1923, in the vicinity of the recent event (Area e), there have sometimes been earthquake of M6.0 or so.

Japan Meteorological Agency
[Seismic Activity for April 2011 (May 11, 2011)]