M7.4 - AUCKLAND ISLANDS, NEW ZEALAND REGION

Tectonic Summary

The magnitude 7.3 earthquake west of the Auckland Islands on September 30, 2007 indicates thrust faulting on the boundary between the Australian and Pacific Plates. The earthquake is located at the southeast end of the small Puysegur subduction zone where a segment of the Australian plate subducts to the NNE beneath the South Island of New Zealand (Pacific plate) at approximately 32 mm/yr. Its location is immediately north of the transition from dominantly strike-slip plate motions along the Macquarie Ridge to the south, to the subduction related thrusting to the north. This plate boundary transition is well delineated by numerous earthquakes of both thrust and strike-slip orientation, although this is the largest earthquake in the region since the magnitude 7.6 strike-slip event less than 100 km to the north in May 1981.

The Australia:Pacific plate boundary region west of the Auckland Islands has a history of large earthquakes both along the plate boundary proper and also west of the plate boundary internal to the Australian Plate. The September 30th event is approximately 200 km east of the magnitude 8.1 earthquake of December 2004, which occurred within the oceanic lithosphere of the Australian Plate, and about 450 km northeast of the magnitude 8.0 earthquake of May 1989 that occurred along the Macquarie Ridge plate boundary itself.