M7.3 - FIJI REGION

Tectonic Summary

The Fiji region earthquake of September 15, 2011 occurred at a depth of approximately 630 km beneath the Earth's surface as a result of oblique-reverse faulting within the subducting Pacific slab. At the latitude of this earthquake, the Pacific plate moves approximately due west with respect to the interior of the Australia plate at a rate of 75 mm/yr. The Pacific plate is seismically active west of the September 15 epicenter to depths of over 650 km. The geometry of the Pacific plate at depth is known to be quite complex, and may involve interactions between the Pacific plate proper and one or more relict slab fragments. The stresses generating the September 15 earthquake likely result from the slow deformation within the interior of one of these slabs, near the base of the mantle transition zone.

Earthquakes that have focal-depths greater than 300 km are commonly termed "deep-focus" earthquakes, as distinguished from "shallow-focus" earthquakes, having depths less than 70 km, and "intermediate-depth" earthquakes, having depths between 70 and 300 km. Intermediate-depth and deep-focus earthquakes represent deformation within subducted plates, rather than deformation at plate boundaries. Intermediate-depth and deep-focus earthquakes typically cause less damage on the ground surface above their foci than is the case with similar magnitude shallow-focus earthquakes, but large intermediate-depth and deep-focus earthquakes may be felt at great distances from their epicenters.

The subducting Pacific plate beneath Fiji, which initiates its westward decent beneath the eastern edge of the Australia plate at the Tonga Trench, is one of the most seismically active regions in the world. Over the past 37 years, there have been well over 3000 deep-focus earthquakes within 200 km of the September 15 event. Of these, 14 had magnitudes of M 6.5 or greater. The largest was an M 7.7 earthquake 15 km to the southwest and 50 km shallower in August of 2002.