M7.8 - SANTA CRUZ ISLANDS

Tectonic Summary

The Santa Cruz Islands earthquake of October 7, 2009, 22:18 UTC occurred on or near the plate boundary between the Australia and Pacific plates. In the region of the earthquake, the Australia plate moves to the east-northeast with respect to the Pacific plate at a velocity of about 91 mm/year. The Australia plate thrusts under the Pacific plate at the New Hebrides trench and dips to the east-northeast. The October 7 earthquake’s location, depth, and focal mechanism are consistent with the earthquake having occurred as thrust-faulting associated with subduction along the Australia-Pacific plate boundary.

The earthquake of October 7, 2009, 22:18 UTC occurred about 60 km north of, and 15 minutes after, a similarly sized earthquake of the same date, 22:03 UTC. The two events also share similar focal mechanisms. Seismologists sometimes refer to a a pair of similarly sized shocks that occur closely spaced in time and location as an earthquake "doublet."

Similarly sized earthquake pairs, or doublets, have previously been observed along this subduction zone. On April 21, 1997 an earthquake doublet occurred in the Vanuatu region with at least one event of M 7.7.

The Vanuatu region experiences a very high level of earthquake activity, with over a dozen events of magnitude 7 and larger having been recorded since the early decades of the twentieth century. The subducting Australia plate is seismically active to depths of about 350 km beneath the islands.

Recent large earthquakes near the October 7 event include the similarly sized event 15 minutes earlier, a M 7.2 earthquake in 2007 and a M 7.3 earthquake in 1999.