Milford Sound, New Zealand Earthquake

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Posting Date: August 21, 2003, 12:30:00 PM

A major earthquake struck in the Milford Sound region of South Island in New Zealand today, August 21, 2003, at about 12:12 GMT (00:12 a.m. on August 22, locally). The USGS preliminary estimate of moment magnitude is 7.1 with focal depth of about 17 miles (28 km). The epicenter was located about 40 miles (70 km) northwest of the village of Te Anau, which is near a tourist attraction called Fiordland Sound. The Sound is currently cut off from contact by heavy snow. The quake was also about 75 miles (125 km) west of Queenstown, a mountain resort, and 300 miles (500 km) southwest of Christchurch, the main city on South Island. The quake was felt as far away as Sydney, Australia. This earthquake occurred at the northern end of the Puysegur subduction zone where the Australia plate subducts under the South Island of New Zealand, part of the Pacific plate. The South Island is a locus of collision between the Pacific and Australia plates. The two plates converge at a rate of about 40 mm/yr. The collision of the two plates resulted in the formation of the Southern Alps Mountains that run across the island from northeast to southwest. Most of the relative motion between the two plates in the South Island is accommodated along the Alpine fault bounding the northwest of the Alps Mountains. The Alpine fault extends southwestward and offshore, where the main deformation is gradually taken up by the Puysegur subduction thrust, located about 40 km northwest of the Alpine fault. Both the Alpine fault and Puysegur subduction thrust are very active in the epicentral area. The focal mechanism of the earthquake indicates that this event was generated along the Puysegur subduction thrust. The epicentral location of the current event is in one of the most seismically active areas in New Zealand over the past 100 years. There have been eight earthquakes of magnitude 6 and greater within 50 km of the current earthquake. Two of these had magnitudes similar to the current event. The largest event, with a magnitude of 7.5, occurred in 1826. There have been no reports of damage or injury as yet, but power was cut off in two small towns and the police received more than 100 emergency calls in 30 minutes. The New Zealand Institute of Geological and Nuclear Sciences has said the quake would have been felt strongly in the south of South Island. Because the earthquake occurred in a sparsely populated area, the AIR earthquake modeling team expects insured losses to be minimal.

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Posting Date: August 21, 2003, 12:30:00 PM