Historic Earthquakes: Tectonic Summary

2003 August 21 12:12:50 UTC

Australian and Pacific plates New Zealand straddles the boundary of the Australian and Pacific plates. Along New Zealand, the Australian plate moves to the northeast at a rate of 35 to 45 mm/yr relative to the Pacific plate. In the 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 Australian 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 recent (03/08/21) thrust earthquake occurred near the southern tip of South Island in 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 Australian plates. The deformed and subducted Australian 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 magnitude 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 magnitude 6.4 earthquake struck on May 31, 1989 and was felt strongly in the southwestern part of South Island and a magnitude 6.7 quake struck on June 3, 1988.