M 7.2 - 56 km E of Luganville, Vanuatu

Tectonic Summary

The August 1, 2007, M 7.2 Vanuatu earthquake occurred as the result of reverse faulting at an intermediate depth, approximately 120 km beneath the Pacific Ocean and to the east of the New Hebrides Trench, within the lithosphere of the subducting Australia plate. Focal mechanism solutions indicate that rupture occurred on either a steep south-striking reverse fault or a shallowly dipping north-striking thrust fault. Slip on a fault of either orientation is consistent with the intraplate compressional tectonics implied by the faulting mechanism and earthquake depth. At the location of the earthquake, the Australia plate moves east-northeast relative to the Pacific plate, subducting beneath the New Hebrides Arc and North Fiji Basin at a velocity of about 100 mm/yr. The lithosphere of the New Hebrides Arc and North Fiji Basin consists of zones of deformation and small tectonic microplates that accommodate the convergence of the much larger Australia and Pacific plates. The subducted Australia plate is seismically active to a depth of about 300 km in the region of the August 1, 2007, earthquake. Earthquakes such as this one, with focal depths between 70 and 300 km, are commonly termed “intermediate-depth” earthquakes. Intermediate-depth earthquakes represent deformation within subducted slabs rather than at the shallow plate interface between subducting and overriding tectonic plates. They 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 earthquakes may be felt at great distance from their epicenters.

The Vanuatu region experiences a very high level of earthquake activity, and 22 earthquakes of M 7+ have occurred within 400 km of the August 1st event over the preceding 40 years. The largest was a M 7.7 earthquake in April 1997, near the Santa Cruz Islands to the north, with no recorded casualties and minimal 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)