M7.0 - OFF THE EAST COAST OF HONSHU, JAPAN

Tectonic Summary

The July 10, 2011 earthquake off the east coast of Honshu, Japan, occurred close to the boundary between the Pacific and North American plates, in the subduction zone region where the Pacific plate converges with and sinks beneath Japan and Eurasia to the west. At the location of the July 10 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 respectively part of North America and Eurasia.

The epicenter and focal mechanism 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 American plate. The July 10 event struck just over 80 km to the east-southeast of the March 11 2011 Mw9.0 Tohoku earthquake, near the southern end of major rupture during that larger event. The July 10 earthquake can be considered an aftershock of the March 11 event. Since March 11, over 688 aftershocks with magnitudes greater than 5 have occurred – 67 of M 6 or greater – with aftershocks occurring both as interplate events on the subduction plate interface and as intraplate events within the overriding North American plate or the subducting Pacific plate. These aftershocks reflect the adjustment of stresses in the plate boundary region in response to the March 11 mainshock.