M 7.0 - 177 km ESE of Ishinomaki, Japan

Tectonic Summary

The July 10, 2011, M 7.0 earthquake off the east coast of Honshu, Japan, occurred as the result of shallow strike-slip faulting close to the boundary between the Pacific and North America plates, in the subduction zone region where the Pacific plate converges with and sinks beneath Japan and Eurasia to the west. Focal mechanism solutions indicate that rupture occurred on either a right-lateral northwest-striking fault or a left-lateral east-northeast-striking fault. At the location of the July 10th event, the Pacific plate moves west-northwestward with respect to North America and northern Japan at a rate of approximately 83 mm/yr. 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 mechanism solutions of this earthquake, together with a preliminary depth estimate of 23 km, suggest that the earthquake occurred within the subducting Pacific lithosphere, rather than on the overlying subduction plate interface itself or within the overriding North America plate. Slip on a fault aligned with either nodal plane of the focal mechanism solution is consistent with this intraplate setting. The July 10th event struck about 80 km to the east-southeast of the March 11, 2011, M 9.1 Tohoku earthquake, near the southern end of major rupture associated with that larger event. The July 10th earthquake can be considered an aftershock of the damaging March 11th event. Since March 11, more than 688 aftershocks with magnitudes greater than 5 have occurred, and 67 of these had M 6+, with aftershocks occurring both as interplate events on the subduction plate interface and as intraplate events within the overriding North America plate or the subducting Pacific plate. These aftershocks reflect the adjustment of stresses in the plate boundary region in response to the March 11th mainshock.

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)