Earthquake under the Ocean Area near Izu-Oshima on January 31
On January 31, 2011 at 17:21 (JST), there was a M4.2 earthquake (maximum seismic intensity 4) at a depth of 10km under the ocean area near Izu-Oshima. The focal mechanism showed a strike-slip fault with a compression axis in a NW-SE direction (This solution was obtained with insufficient precision). As of February 5, there have been two aftershocks with seismic intensity of 1.
For the seismic activity since October 1997, there have been a series of seismic activities off-shore east of the Izu Peninsula. In contrast with it, in the area near this event (Area a), the seismic activity has relatively been weak, and for earthquakes of M4.0 or over, there was only a M4.4 earthquake (maximum seismic intensity 3) that occurred on October 28, 2003.
For the seismic activity since August 1923, in the area near the recent event (Area b), there have been three earthquakes of M6.0 or over. The largest event is the 1978 Izu-Oshima-Kinkai Earthquake of M7.0 and it caused damage of 25 fatalities, 211 injured people and 96 completely collapsed houses (Comprehensive Bibliography of Destructive Earthquakes in Japan, Latest Edition). In addition, the tsunamis with heights of several tens of cm were observed in the Izu Peninsula and Izu-Oshima (Comprehensive Bibliography of Destructive Tsunamis in Japan).
Japan Meteorological Agency
[Seismic Activity for January 2011 (February 9, 2011)]