http://earthquake.usgs.gov/earthquakes/eqinthenews/2011/usc0001xgp/#summary

Tectonic Summary

The magnitude 9.0 Tohoku earthquake on March 11, 2011, which occurred near
the northeast coast of Honshu, Japan, resulted from thrust faulting on or
near the subduction zone 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 rate of
83 mm/yr, and begins its westward descent beneath Japan at the Japan Trench.
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 location, depth, and focal mechanism of the March 11 earthquake are
consistent with the event having occurred on the subduction zone plate
boundary. Modeling of the rupture of this earthquake indicate that the fault
moved upwards of 30-40 m, and slipped over an area approximately 300 km long
(along-strike) by 150 km wide (in the down-dip direction). The rupture zone
is roughly centered on the earthquake epicenter along-strike, while peak
slips were up-dip of the hypocenter, towards the Japan Trench axis. The March
11 earthquake was preceded by a series of large foreshocks over the previous
two days, beginning on March 9th with a M 7.2 event approximately 40 km from
the epicenter of the March 11 earthquake, and continuing with another three
earthquakes greater than M 6 on the same day.

The Japan Trench subduction zone has hosted nine events of magnitude 7 or
greater since 1973. The largest of these, a M 7.8 earthquake approximately
260 km to the north of the March 11 epicenter, caused 3 fatalities and almost
700 injuries in December 1994. In June of 1978, a M 7.7 earthquake 35 km to
the southwest of the March 11 epicenter caused 22 fatalities and over 400
injuries. Large offshore earthquakes have occurred in the same subduction
zone in 1611, 1896 and 1933 that each produced devastating tsunami waves on
the Sanriku coast of Pacific NE Japan. That coastline is particularly
vulnerable to tsunami waves because it has many deep coastal embayments that
amplify tsunami waves and cause great wave inundations. The M 7.6 subduction
earthquake of 1896 created tsunami waves as high 38 m and a reported death
toll of 27,000. The M 8.6 earthquake of March 2, 1933 produced tsunami waves
as high as 29 m on the Sanriku coast and caused more than 3000 fatalities.
Unlike the recent magnitude 9.0 earthquake, the 1933 earthquake did not occur
as the result of thrust faulting on the subduction-zone plate interface, but
rather within the Pacific plate just seaward of the Japan Trench.

The March 11, 2011 earthquake far surpassed other post-1900 plate-boundary
thrust-fault earthquakes in the southern Japan Trench, none of which attained
M8. A predecessor may have occurred on July 13, 869, when the Sendai area was
swept by a large tsunami that Japanese scientists have identified from
written records and a sand sheet.

Continuing readjustments of stress and associated aftershocks are expected in
the region of this earthquake. The exact location and timing of future
aftershocks cannot be specified. Numbers of aftershocks will continue to be
highest on and near to fault-segments on which rupture occurred at the time
of the main-shock. The frequency of aftershocks will tend to decrease with
elapsed time from the time of the main shock, but the general decrease of
activity may be punctuated by episodes of higher aftershock activity. Beyond
the ongoing aftershock sequence, the USGS does not believe that the
earthquakes in Japan have significantly raised the probability of future
major earthquakes. While the probability of future large earthquakes far from
northern Honshu has not increased, neither has it decreased and large
earthquakes will continue to occur just as we have observed in the past.


Earthquake Summary Poster
http://earthquake.usgs.gov/earthquakes/eqarchives/poster/2011/20110311.php