M7.6 - KERMADEC ISLANDS REGION

Tectonic Summary

The July 6, 2011 Kermadec Islands region earthquake occurred near the Kermadec Trench where the Pacific Plate begins its descent into the mantle beneath the eastern edge of the Australian Plate. At the latitude of this earthquake, the Pacific Plate is converging with Australia in a westward direction at a velocity of approximately 61 mm/yr. The preliminary mechanism and depth of the event suggest it ruptured a normal fault within the shallow oceanic lithosphere of the Pacific Plate; the initial location suggests a source slightly to the west of the trench, within the down-going slab, rather than to the east and in the outer rise.

The July 6 earthquake struck in an oceanic region with few nearby populations, approximately 550 km south of Tonga and 650 km north of New Zealand. This region of the Tonga-Kermadec subduction zone experiences reasonably high levels of seismic activity, with nearly 50 events of M 6.5 and above over the past 38 years, and 4 greater than M 7.5. Two of these, M 7.8 and M 8.2 earthquakes on the same day in 1976, occurred just over 150 km west-northwest of today's event; an M 7.8 event in 1978 struck nearly 200 km to the southwest, and an M 8.3 event occurred in October 1986, about 130 km to the north. None of these events have recorded damage or casualties.