M 7.4 - Bonin Islands, Japan region

Tectonic Summary

The December 21, 2010, M 7.4 earthquake in the Bonin Islands near Japan occurred as the result of shallow normal faulting within the Pacific plate, in the region of the boundary between the Pacific plate and the Philippine Sea plate. Focal mechanism solutions indicate that rupture occurred on either a north-northwest- or an east-southeast-striking, moderately dipping normal fault. Of these two possible fault orientations, finite-fault modeling of globally recorded seismic data is more consistent with slip on the east-southeast-striking fault. In the epicentral region of the earthquake, the Pacific plate moves west relative to the Philippine Sea plate at a velocity of about 40 mm/yr. The Pacific plate subducts beneath the Philippine Sea plate at the Izu Trench just west of the December 21st earthquake and is seismically active to a depth of about 560 km. The stresses that generated the December 21st earthquake result from the bending of the Pacific plate as it begins to subduct beneath the Philippine Sea plate.

The Izu-Bonin and Mariana Island Arcs of the Pacific:Philippine Sea plate boundary region experience frequent moderate and strong earthquakes. In the past quarter century alone, the 1,000-km section of the plate boundary centered on the epicenter of the December 21st earthquake has produced 27 earthquakes of M 6+, two with magnitudes larger than 7.0. Roughly half of the moderate and large events in this region occur as intermediate-depth earthquakes (70–300 km below the Earth’s surface) or deep-focus earthquakes (greater than 300 km below the Earth’s surface). The December 21st earthquake, however, occurred within 20 km of the Earth’s surface and would be classified as a “shallow-focus” earthquake. The largest nearby event was a 400-km-deep M 7.4 earthquake in August 2000 roughly 250 km northwest of the December 21st event, with no recorded casualties or damage.

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)