The october 25
The October 25, 2013 M 7.1 earthquake offshore of Honshu, Japan
occurred as the result of normal faulting in the shallow oceanic
crust of the Pacific plate. The earthquake occurred outboard (east)
of the Japan Trench, which marks the seafloor expression of the
subduction zone plate boundary between the Pacific and North America
plates, and is immediately up-dip of the source region of the March
2011 M 9.0 Tohoku earthquake. At the latitude of this earthquake,
the Pacific plate moves westwards with respect to the North America
plate at a rate of 83 mm/yr before subducting beneath the island of Honshu.
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 October 25 2013 event
are consistent with normal faulting rupture near the outer-arc high
of the Japan Trench. In this region, normal faulting is encouraged by
both the bending of the Pacific plate as it enters the subduction
zone, and by stresses transferred from the locked subduction thrust
interface to the west.