http://earthquake.usgs.gov/earthquakes/eqinthenews/2011/usb0001r57/#summary

Tectonic Summary

The 03/09/2011 earthquake near the east coast of Honshu, Japan, occurred as
a result of thrust faulting on or near the subduction zone interface plate
boundary between the Pacific and North America plates. At the latitude of
this earthquake, the Pacific plate moves approximately westwards with respect
to the North America plate at a velocity of 83 mm/yr. The Pacific plate
thrusts underneath Japan at the Japan Trench, and dips to the west beneath
Eurasia. The location, depth, and focal mechanism of the March 9 earthquake
are consistent with the event having occurred as thrust faulting associated
with subduction along this plate boundary. Note that some authors divide this
region into several microplates that together define the relative motions
between the larger Pacific, North America and Eurasia plates; these include
the Okhotsk and Amur microplates that are respectively part of North America
and Eurasia.

The Japan Trench subduction zone has hosted 9 events of magnitude 7 or
greater since 1973. The largest of these was an M 7.8 earthquake
approximately 230 km to the north of the March 9 event, in December 1994,
which caused 3 fatalities and almost 700 injuries. In June of 1978, an M 7.7
earthquake 75 km to the southwest caused 22 fatalities and over 400 injuries.
In December of 2008, a sequence of 4 moderate earthquakes (M 5.3-5.8)
occurred within 20 km of the March 9 event. In the first 12 hours following
the March 9 earthquake, the region has experienced over a dozen aftershocks
of M 5 or greater, the largest being M 5.7.


Earthquake Summary Poster
http://earthquake.usgs.gov/earthquakes/eqarchives/poster/2011/20110309.php