April 12 (Bloomberg) -- A magnitude 7.1 earthquake struck in the vicinity of Macquarie Island, which lies about halfway between Australia and Antarctica, the U.S. Geological Survey said. No tsunami was reported after an alert was issued.

The quake hit 120 kilometers (75 miles) south-southwest of Macquarie Island and 1,995 kilometers south-southwest of Wellington in New Zealand at 11:30 a.m. local time, the USGS said on its Web site. The USGS, which revised the magnitude three times from an initial 7.5, said it struck 10 kilometers below the seabed.

The Pacific Tsunami Warning Center, which put the magnitude at 7.2, issued a tsunami alert for areas within ``a few hundred kilometers'' of the quake zone. There was no threat to Pacific coastlines, it said. There were no reports of casualties or damage or of a tsunami nearly three hours after the quake hit.

Macquarie is a ``subantarctic'' island located in the Southern Ocean at 54 degrees, 30 minutes south and 158 degrees, 57 minutes east, according to the Australian government's Antarctic Division's Web site. The island, inhabited only by scientists at a research station, is a Tasmanian State Reserve.

Macquarie sits in a zone where the Pacific Plate, the Indo- Australian Plate and the Antarctic Plate meet. These tectonic plates constantly shift, sometimes causing earthquakes, some of them producing tsunamis. Quakes of magnitude 5 or greater can be destructive depending on their depth.

To contact the reporter on this story: Aaron Sheldrick in Tokyo at asheldrick@bloomberg.net.

Last Updated: April 11, 2008 23:20 EDT