M6.8 - SOUTH ISLAND OF NEW ZEALAND

Tectonic Summary

New Zealand straddles the plate boundary between the Australian and Pacific plates in the Southwest Pacific Ocean. Along the length of New Zealands islands, this plate boundary consists of a transform fault system (the Alpine Fault) connecting two subduction zones of opposing convergence that lie to the north and south of the South Island, respectively. At the southern tip of New Zealand in a region known as Fiordland, the Australian Plate moves northeast with respect to the Pacific Plate at approximately 37 mm/year. Here, plate motion is accommodated via the subduction of a segment of the Australian plate beneath New Zealand (Pacific plate). Earthquake locations indicate that this subducted plate is highly deformed, and steeply dipping beneath Fiordland. Further to the north, this plate boundary transitions to strike-slip faulting along the Alpine Fault transform system.

The magnitude 6.8 earthquake in the South Island of New Zealand on October 15, 2007 occurred on or close to the Australian:Pacific subduction plate boundary, and either indicates subduction related thrusting towards the east on this interface, or thrusting on a more steeply-dipping, westward oriented structure. Slip vectors for this earthquake, which indicate the direction of motion along the ruptured fault, do not align with plate motions and thus indicate a complicated tectonic setting for this event. The exact nature of how plate motions are accommodated at this plate boundary is not clearly understood, and is presently being debated by the scientific community.

The October 15th event is approximately 50 km northeast of the magnitude 7.2 earthquake of August 2003, which indicated similarly complicated tectonics. Over the past two decades, this region has witnessed several large earthquakes, including the M7.2 event in 2003, a M7.0 event in August of 1993, and several other M6+ earthquakes.

Earthquake Information for New Zealand