Magnitude 7.1 OFF WEST COAST OF THE SOUTH ISLAND, N.Z.
Monday, November 22, 2004 at 20:26:23 UTC
Preliminary Earthquake Report
U.S. Geological Survey, National Earthquake Information Center
World Data Center for Seismology, Denver
New Zealand, Fiordland and Puysegur Trench Region
New Zealand is part of a largely submerged microcontinent in the South Pacific Ocean. It has two main islands, North Island and South Island. These islands straddle the boundary between the Pacific and Australian tectonic plates and are undergoing a gradual elongation and compression, due to the continuous 4 cm/year northeastwards motion of the Australian plate relative to the Pacific plate. Along New Zealand, the plate boundary consists of a transform fault system connecting two subduction zones of opposing convergent directions that lie to the north and south of South Island.
The recent earthquake occurred in the Fiordland and Puysegur Region. This region includes the southwest corner of South Island (Fiordland) and extends offshore to the southwest and covers the Puysegur Trench. Oblique subduction of the Australian plate underneath the Pacific plate occurs along the Puysegur trench. The subducted plate is moderately dipping in the south part of the trench and further to the north a gradual change in strike of 17 degrees accompanies a steepening in dip of the slab to near vertical below Fiordland. The exact age and mechanism of this young subduction zone is presently being debated in the scientific community.
Off the south coast of Fiordland active subduction at the Puysegur Trench produces earthquakes in several areas including: within the subducting Australian plate, in the overlying Pacific plate, and at the interface between the plates. The earthquake occurred to the west of the trench in the Australian plate. There are many fault systems in the region and the USGS is not certain of the particular fault responsible for this recent earthquake.
Although Fiordland is sparsely populated some earthquakes in this region are felt by people throughout the South Island, such as the magnitude 7.2 thrust earthquake of August 21st 2003. People felt this event as far north as Wellington on North Island and Sydney, Australia. Most of the historic major earthquakes have occurred offshore. For example, on October 12th, 1979 a magnitude 7.3 thrust earthquake occurred beneath the Puysegur trench.