M7.0 - VANUATU

Tectonic Summary

The Vanuatu earthquake of September 3, 2011 occurred at a depth of approximately 135 km beneath the Earth's surface as a result of oblique-reverse faulting within the subducting Australia slab. The event occurred near the boundary between the Australia and Pacific plates, as the former sinks towards the east-northeast beneath the Vanuatu island chain. At the location of this earthquake, the Australia plate moves approximately east-northeast with respect to the Pacific at a rate of 79 mm/yr. The Australia plate is seismically active east of the September 3 epicenter to depths close to 300 km; the stresses generating this earthquake may result from the bending of the plate as it sinks deeper into the mantle.

This section of the subducting Australia plate has hosted numerous moderate and large earthquakes in the past, with 14 earthquakes of M 6.5 or greater within 100 km of the September 3rd event over the past quarter century. The largest was an M 7.2 earthquake 80 km to the south and 50 km shallower in 1979. The recent M 7.1 and M 7.0 Vanuatu earthquakes of August 20, 2011 occurred over 300 km to the northwest of the September 3 quake, along the shallow plate boundary interface, rather than within the subducting plate.

Earthquakes that have focal-depths between 70 and 300 km are commonly termed "intermediate-depth" earthquakes, as distinguished from "shallow-focus" earthquakes, having depths less than 70 km, and "deep-focus" earthquakes, having depths greater than 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.