Tectonic Summary

The March 11, 2011, M 9.1 Tohoku earthquake, which occurred near the northeast coast of Honshu, Japan, resulted from shallow thrust faulting on the subduction zone plate boundary between the Pacific and North America plates. At the location of this earthquake, the Pacific plate moves roughly westward relative to the North America plate at a velocity of 83 mm/yr, and begins its westward descent beneath Japan at the Japan Trench, east of the March 11th earthquake. 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 part of North America and Eurasia, respectively.

The location, depth (about 25 km), and focal mechanism solutions of the March 11th earthquake are consistent with the event having occurred on the subduction zone plate boundary. Modeling of the rupture of this earthquake indicates that the fault moved as much as 50–60 m, and slipped over an area approximately 400 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 11th earthquake was preceded by a series of large foreshocks over the previous 2 days, beginning on March 9 with a M 7.4 event approximately 40 km from the epicenter of the March 11th 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 M 7+ since 1973. The largest of these, a M 7.8 earthquake approximately 260 km to the north of the March 11th epicenter, caused 3 fatalities and almost 700 injuries in December 1994. In June 1978, a M 7.7 earthquake 35 km to the southwest of the March 11th epicenter caused 22 fatalities and more than 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 northeast 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 caused 27,000 fatalities. 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 3,000 fatalities. Unlike the recent M 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 was much larger than other post-1900 plate boundary thrust fault earthquakes in the southern Japan Trench, none of which attained M 8. A similarly sized 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 related 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 mainshock. The frequency of aftershocks will tend to decrease with elapsed time from the time of the mainshock, but the general decrease of activity may be punctuated by episodes of higher aftershock activity. Beyond the ongoing aftershock sequence, the earthquakes in Japan have not 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; large global earthquakes will likely continue to occur just as we have observed in the past.

Hayes et al. (2016) Tectonic summaries of magnitude 7 and greater earthquakes from 2000 to 2015, USGS Open-File Report 2016-1192. (5.2 MB PDF)

Event Details

Event Map