M 7.7 - 272 km ESE of Kamaishi, Japan

Tectonic Summary

The March 11, 2011, M 7.7 earthquake near the east coast of Honshu, Japan, occurred as a result of shallow normal faulting within the oceanic lithosphere of the Pacific plate, approximately 60 km east of the Japan Trench. Focal mechanism solutions indicate that rupture occurred on either a north- or south-striking, moderately dipping normal fault. Slip on a fault aligned with either nodal plane is consistent with the intraplate setting of this event. At the location of this earthquake, the Pacific plate moves roughly westward relative to the North America plate at a velocity of 70 mm/yr, and begins its westward descent beneath Japan at the Japan Trench. 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.

This earthquake occurred approximately 40 minutes after the devastating March 11, 2011, M 9.1 Tohoku earthquake. This 06:25 UTC earthquake can be considered an aftershock of the 05:46 M 9.1 event, despite occurring on a different fault structure (on a normal fault directly east of the subduction zone, rather than on the subduction zone interface). Over the 2 days preceding the March 11th mainshock, a series of large foreshocks had occurred, beginning on March 9 with a M 7.3 event approximately 40 km from the epicenter of the March 11th M 9.1 earthquake, and continuing with another three earthquakes greater than M 6 on the same day. Prior to March 9, the Japan Trench subduction zone had hosted nine events of M 7+ since 1973. While commonly plotted as points on maps, earthquakes of this size are more appropriately described as slip over a larger fault area. Normal-faulting events of the size of the March 11, 2011, M 7.7 earthquake are typically about 90x35 km (length x width); interference from the previous M 9.1 event makes modeling its source dimensions complicated, and as such a slip model is not available at this time.

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)