M 7.4 - Kermadec Islands region

Tectonic Summary

The October 21, 2011, M 7.4 Kermadec Islands region earthquake occurred as the result of shallow reverse 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 south- or northeast-striking, moderately dipping reverse fault. Of these two possible fault orientations, finite-fault modeling of globally recorded seismic data is more consistent with slip on the northeast-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 preliminary mechanism and depth of the event suggest it ruptured a reverse fault within the subducting oceanic lithosphere of the Pacific plate, rather than on the overlying thrust interface between the Pacific and Australia plates.

The October 21st earthquake struck in an oceanic region with few nearby populations, approximately 500 km south of Tonga and 700 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 5 of M 7.5+. Two of these, M 7.8 and M 8.2 earthquakes on the same day in 1976, occurred about 100 km west-northwest of the October 21st event; a M 7.8 event in 1978 struck nearly 250 km to the southwest, and a M 8.3 event occurred in October 1986 about 80 km to the north. None of these events have recorded damage or casualties.

In July of this year, a M 7.6 event occurred approximately 45 km to the south-southwest of the October 21st earthquake, breaking a normal fault within the subducting Pacific plate also very close to the Kermadec Trench. This event was notable because it triggered a large number of thrust faulting aftershocks to the west of the plate boundary, on or close to the interface between the Pacific and Australia plates. The October 21st earthquake may be related to that aftershock sequence.

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)