M 7.1 - 29 km ESE of Ishinomaki, Japan

Tectonic Summary

The April 7, 2011, M 7.1 earthquake near the east coast of Honshu, Japan, occurred as a result of shallow reverse faulting on or near the subduction zone plate boundary between the Pacific and North America plates. Focal mechanism solutions indicate that rupture occurred on either a north-northeast- or south-southwest-striking, moderately dipping reverse fault. 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 April 7th 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 epicenter and focal depth of the April 7th earthquake are consistent with the event having occurred very close to the main interface thrust fault of the subduction zone plate boundary. Preliminary focal mechanism solutions, however, imply slip on a fault with steeper dip than that of the main interface thrust fault, which may imply that an intraplate source is most likely. In such an intraplate scenario, slip may have occurred on a fault aligned with either nodal plane of the focal mechanism solution.

The April 7th earthquake can be considered an aftershock of the March 11, 2011, M 9.1 Tohoku earthquake. The aftershock sequence of that event has been ongoing since March 11 and has included 58 earthquakes of M 6+ up until April 7, 2011; 2 of these were M 7+ (M 7.7 and M 7.9, both on March 11). Over the 2 days preceding the March 11th mainshock, a series of large foreshocks had occurred, beginning on March 9th with a M 7.3 event approximately 40 km from the epicenter of the March 11th M 9.1 earthquake, and continuing with another three earthquakes greater than M 6 on the same day. Prior to March 9, 2011, the Japan Trench subduction zone had hosted nine events of M 7+ since 1973.

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)