Seismic Activities Off-shore of Sanriku (Before the Main Shock)
From February 13, 2011, there was a series of earthquakes with the largest M5.5 earthquake (maximum seismic intensity 1) off-shore of Sanriku.
Adding to this, there was a M7.3 earthquake (maximum seismic intensity 5 Lower) at almost the same place. This event caused a tsunami with the largest height of 0.6 m in Ofunato.
Every focal mechanism showed a reverse fault with a compression axis in a WNW-ESE direction. These events occurred on the boundary between the Pacific and the continental plates.
The epicenter (starting point of initial rupture) of the M9.0 earthquake on March 11 was located at the southwestern end of the brisk activity area.
For the seismic activity since August 1923, in the vicinity of the epicenter of the recent event (Area b), there was a M6.9 earthquake (maximum seismic intensity 4) on October 11, 1939, and there was a M7.0 earthquake (maximum seismic intensity 4) on January 19, 1981.
Japan Meteorological Agency
[Seismic Activity for March 2011 (April 11, 2011)]