M 7.6 - Kermadec Islands region

Tectonic Summary

The July 6, 2011, M 7.6 Kermadec Islands region earthquake occurred as the result of shallow normal faulting near the Kermadec Trench where the Pacific plate begins its descent into the mantle beneath the eastern edge of the Australia plate. Focal mechanism solutions indicate that rupture occurred on either a north-northeast- or a south-southeast-striking, moderately dipping normal fault. Of these two possible fault orientations, finite-fault modeling of globally recorded seismic data is more consistent with slip on the south-southeast-striking fault. At the location of this earthquake, the Pacific plate is converging with Australia in a westward direction at a velocity of about 61 mm/yr. The focal mechanism solution and depth (about 20 km) of the event suggest that it ruptured a normal fault within the shallow oceanic lithosphere of the Pacific plate; the location suggests a source slightly to the west of the trench, within the downgoing slab, rather than to the east and in the outer rise region.

While commonly plotted as points on maps, earthquakes of this size are more appropriately described as slip over a larger fault area. Normal-faulting events of the size of the July 6, 2011, earthquake are typically about 85x35 km (length x width); modeling of this earthquake implies dimensions of about 110x40 km, predominantly surrounding the hypocenter and to the northwest.

The July 6th 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 high levels of seismic activity, with nearly 50 events of M 6.5+ over the past 38 years, and 4 of M 7.5+. Two of these, M 7.8 and M 8.2 earthquakes on the same day in 1976, occurred about 150 km west-northwest of the July 6th event. A M 7.8 event in 1978 struck nearly 200 km to the southwest, and a M 8.3 event occurred in October 1986 about 130 km to the north. None of these events have recorded damage or casualties.

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)