M 7.7 - 148 km NW of Sola, Vanuatu

Tectonic Summary

The Vanuatu region earthquakes of October 7, 2009: M 7.7, Vanuatu, 22:03:14; M 7.8 Santa Cruz Islands, 22:18:51 UTC; and M 7.4, Vanuatu, 23:13:48; all occurred as a result of shallow reverse faulting on or near the plate boundary interface between the Australia and Pacific plates. In the region of these earthquakes, the Australia plate moves to the east-northeast with respect to the Pacific plate at a velocity of about 91 mm/yr. The Australia plate thrusts under the Pacific plate at the New Hebrides Trench and dips to the east-northeast. The locations, depths, and focal mechanism solutions for the October 7th events are all consistent with them resulting from reverse faulting associated with subduction along the Australia-Pacific plate boundary.

While commonly plotted as points on maps, earthquakes of this size are more appropriately described as slip over a larger fault area. Reverse-faulting events of the size of the October 7, 2009, M 7.7 earthquake are typically about 100x50 km (length x width); modeling of this earthquake implies dimensions of about 70x50 km, predominantly to the north of the hypocenter.

The earthquake of October 7, 2009 (22:03 UTC), occurred about 60 km south of, and 15 minutes prior to, a slightly larger event of the same date (22:18 UTC). A third, smaller event occurred at 23:13 UTC, 15 km to the southeast of the first event. All three events are likely related; while the first two similarly sized events may be considered an earthquake doublet, preliminary analysis suggests that the 23:13 UTC earthquake is likely a large aftershock of the previous events.

The Vanuatu region experiences a very high level of earthquake activity, with more than a dozen events of M 7+ having been recorded since the early decades of the 20th century. Recent large earthquakes near the October 7th events include a M 7.2 earthquake in 2007 and a M 7.3 earthquake in 1999. The subducting Australia plate is seismically active to depths of about 350 km beneath the islands.

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)