Magnitude 8.1 NORTH OF MACQUARIE ISLAND
Thursday, December 23, 2004 at 14:59:03 UTC
Preliminary Earthquake Report
U.S. Geological Survey, National Earthquake Information Center
World Data Center for Seismology, Denver
Tectonic Summary: New Zealand, Macquarie Ridge Region
New Zealand is part of a largely submerged microcontinent in the South 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. In the New Zealand region, 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 Australian-Pacific plate boundary continues for 1500 km from the south of New Zealand to the triple junction with the Antarctic Plate.
The recent earthquake occurred in the Macquarie ridge region, in the South Pacific Ocean. The Macquarie ridge is a continuous bathymetric high with a north-south trend that extends from the Puysegar trench in the north to the Hjort trench in the south. The Macquarie ridge forms part of the Pacific-Australian plate boundary and is a predominantly right-lateral, strike-slip transform fault with a component of convergence that connects two subduction zones to the north and south. In this way, the Macquarie ridge is analogous to the better-known Alpine fault that cross cuts the south Island. It is a narrow active plate boundary less than 5 km wide, although deformation occurs to a lesser degree in a broad area either side of the ridge.
The magnitude 8.2 Macquarie Ridge Earthquake on May 23rd 1989 was a strike-slip earthquake. It was the largest earthquake recorded anywhere in the world during the 1980's, fortunately the epicenter was located 300 km from Macquarie Island and so the quake did not greatly affect people. However, a small tsunami struck the southern coast of Australia. Seismicity is high along the Macquarie ridge, and is evenly distributed along the ridge. The major earthquakes occur along the plate boundary, such as the 1989 earthquake and smaller earthquakes tend to occur on faults away from the boundary.
In common with other major strike slip faults the Macquarie ridge region includes many subsidiary faults.