M 7.1 - 64 km S of Port-Vila, Vanuatu

Tectonic Summary

The August 20, 2011 (18:19:24 UTC), M 7.1 Vanuatu earthquake was the second of two M 7+ events that occurred on the same day in this region, a result of shallow thrust faulting on or near the subduction zone plate boundary between the Australia and Pacific plates in the Coral Sea region of the southwest Pacific. Near the epicenters of these earthquakes, the Australia plate moves to the east-northeast relative to the Pacific plate at a velocity of about 83 mm/yr, and begins its eastward descent beneath the arc at the New Hebrides Trench to the west of the August 20th events. The subducting Australia plate is seismically active to depths of about 350 km beneath the islands.

The M 7.2 earthquake (August 20, 2011, 16:55 UTC) preceded the M 7.1 event (August 20, 2011, 18:19 UTC) by 84 minutes, and was located approximately 6 km away horizontally, and 12 km apart in depth—each offset within location uncertainties of typical global earthquakes. The two events also had approximately the same focal mechanism solution. The latter earthquake was almost certainly triggered by the earlier event. Seismologists sometimes refer to a pair of similarly sized earthquakes that occur at nearly the same time and location as an earthquake “doublet.”

These earthquakes are located approximately 80 km to the south of a M 7.3 earthquake that occurred in August 2010, and 600 km to the south of a sequence of large subduction thrust earthquakes that occurred in October 2009. The Vanuatu region experiences a very high level of earthquake activity, with almost 50 events of M 7 or larger recorded since 1973.

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)