Tsunami alert M7.1 – Off the east coast of Honshu, Japan


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.

Seismotectonics of Japan and Vicinity

Japan and the surrounding islands straddle four major tectonic plates: Pacific plate; North America plate; Eurasia plate; and Philippine Sea plate. The Pacific plate is subducted into the mantle, beneath Hokkaido and northern Honshu, along the eastern margin of the Okhotsk microplate, a proposed subdivision of the North America plate. Farther south, the Pacific plate is subducted beneath volcanic islands along the eastern margin of the Philippine Sea plate. This 2,200 km-long zone of subduction of the Pacific plate is responsible for the creation of the deep offshore Ogasawara and Japan trenches as well as parallel chains of islands and volcanoes, typical of Circumpacific island arcs. Similarly, the Philippine Sea plate is itself subducting under the Eurasia plate along a zone, extending from Taiwan to southern Honshu that comprises the Ryukyu Islands and the Nansei-Shoto trench.

Subduction zones at the Japanese island arcs are geologically complex and produce numerous earthquakes from multiple sources. Deformation of the overriding plates generates shallow crustal earthquakes, whereas slip at the interface of the plates generates interplate earthquakes that extend from near the base of the trench to depths of 40 to 60 km. At greater depths, Japanese arc earthquakes occur within the subducting Pacific and Philippine Sea plates and can reach depths of nearly 700 km. Since 1900, three great earthquakes occurred off Japan and three north of Hokkaido. They are the M8.4 1933 Sanriku-oki earthquake, the M8.3 2003 Tokachi-oki earthquake, the M9.0 2011 Tohoku earthquake, the M8.4 1958 Etorofu earthquake, the M8.5 1963 Kuril earthquake, and the M8.3 1994 Shikotan earthquake.



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