Poster of the Offshore Honshu, Japan Earthquake of 25 October 2013 - Magnitude 7.1

Tectonic Summary

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. Since the March 2011 Tohoku earthquake, two large events of M 7.7 and M 7.3 have occurred in the vicinity of the October 25, 2013 earthquake. The M 7.7 event, on March 11, 2011, was also a normal faulting event near the outer-arc high and occurred 95 km north of the October 25 event. The M 7.3 event, on December 7, 2012, was a more complex earthquake resulting from thrust motion near the trench 100 km to the northwest of the October 25 earthquake. Since March 2011, 10 additional events, ranging in magnitude from M 6.1-6.4, have occurred in this region east of the Japan Trench.

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