M 7.8 - 97 km WSW of Te Anau, New Zealand

Tectonic Summary

The July 15, 2009, M 7.8 earthquake off the west coast of the South Island of New Zealand occurred as the result of shallow thrust faulting on or near the boundary between the Australia and Pacific plates. 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. The North and South Islands of New Zealand straddle the boundary between the Australia and Pacific plates. In southwestern South Island, this motion is accommodated by oblique convergence at the Puysegur Trench, where the Australia plate subducts beneath the Pacific plate. Farther north along South Island’s west coast, relative motion is accommodated via oblique strike-slip movement along the Alpine fault, driving the uplift of the Southern Alps.

This earthquake occurred near the southern tip of South Island in a region known as Fiordland, in a complex area of transition in plate boundary structure from Puysegur subduction to Alpine fault strike-slip motion. The preliminary location, depth, and estimate of fault orientation of this event are consistent with the earthquake having resulted from slip on the subduction thrust interface between the Pacific and Australia plates.

While commonly plotted as points on maps, earthquakes of this size are more appropriately described as slip over a larger fault area. Thrust-faulting events of the size of the July 15, 2009, earthquake are typically about 120x50 km (length x width); modeling of this earthquake implies dimensions of about 80x40 km, predominantly to the south of and up-dip from the hypocenter.

The deformed Australia plate beneath Fiordland is highly active both along its interface with the Pacific plate and internal to the subducted Australia plate. Over the past two decades, several large earthquakes have occurred in Fiordland, predominantly in a cluster to the northeast of the July 15th earthquake. Though the faulting mechanisms of those events are similar to that of the July 15th earthquake, their slip vectors are rotated clockwise with respect to plate motions, and thus may not represent exactly the same style of faulting.

The most recent of these previous large events occurred in August 2003 when a M 7.2 earthquake approximately 100 km to the northwest caused minor damage in Otago and Southland and numerous landslides across the Fiordland region. 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. A M 6.7 quake struck on June 3, 1988. 

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)