M 7.2 - 56 km NW of Te Anau, New Zealand

Tectonic Summary

The August 21, 2003, M 7.2 earthquake in the southwest corner of the South Island of New Zealand occurred as the result of shallow reverse faulting on or near the convergent Australia-Pacific plate boundary. Focal mechanism solutions indicate that rupture occurred on either a steeply dipping reverse fault or a shallowly southeastward-dipping thrust fault. Of these two possible fault orientations, finite-fault modeling of globally recorded seismic data is more consistent with slip on the steeply dipping reverse fault. At the location of the earthquake, the Australia plate moves to the northeast at a velocity of about 35–45 mm/yr relative to the Pacific plate. In central South Island, this plate motion results in predominantly strike-slip movement along the Alpine fault. In southwestern South Island, relative plate motion is accommodated by oblique subduction of the Australia plate along the Puysegur Trench and deformation of the overriding Pacific plate inland of the trench. The Southern Alps of New Zealand result from this oblique plate convergence.

The August 21st earthquake is located at the southwesternmost end of the Southern Alps mountain chain, just inland of the Australia-Pacific plate boundary. This southern tip of the South Island is a region known as Fiordland. The preliminary location, depth, and estimate of fault orientation are consistent with the earthquake having resulted from slip on the thrust interface between the Pacific and Australia plates. The deformed and subducted Australia plate beneath Fiordland and below the thrust interface is also highly active, and several surface strands of the Alpine fault are observed in the vicinity of the earthquake epicenter in the overriding Pacific plate above the thrust interface.

Over the past two decades, several large earthquakes have occurred in Fiordland. A M 7.0 event on August 10, 1993, caused power outages in the Te Anau area and was felt throughout South Island and as far away as Sydney, Australia. A M 6.4 earthquake struck on May 31, 1989, and was felt strongly in the southwestern part of South Island and a M 6.7 quake struck on June 3, 1988. However, neither of these latter two events resulted in 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)