M7.1 - VANUATU

Tectonic Summary

The Vanuatu earthquakes of August 20, 2011 (M 7.1 and M 7.0) resulted from 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 epicenter of the earthquakes, the Australia plate moves to the east-northeast with respect to the Pacific plate at a velocity of approximately 83 mm/year, and begins its eastward decent beneath the arc at the Vanuatu North New Hebrides trench.

The M 7.1 earthquake (August 20, 2011, 16:55 GMT) preceded the M 7.0 event (August 20, 2011, 18:19 GMT) by 84 minutes, and was located approximately 6 km away horizontally, and 12 km in depth - each within location uncertainties of typical global earthquakes. The two events also had approximately the same focal-mechanism. The latter earthquake was almost certainly triggered by the earlier event. Seismologists sometimes refer to a pair of similarly sized shocks that occur at nearly the same time and location as an earthquake "doublet."

These earthquakes are located approximately 80 km to the south of an M 7.3 earthquake that occurred in August of 2010, and 600 km to the south of a sequence of large subduction thrust earthquakes that occurred in October of 2009. The Vanuatu region experiences a very high level of earthquake activity, with almost 50 events of magnitude 7 and larger having been recorded since 1973. The subducting Australia plate is seismically active to depths of about 350 km beneath the islands.