23 November 2004

Large Offshore Quake Felt Widely In South Island

A large earthquake that occurred under the sea southwest of Invercargill this morning was felt throughout much of the lower half of the South Island.

The magnitude 7.2 earthquake struck at 9.26am about 100km off the southern Fiordland coast at a depth of 33km, Geological and Nuclear Sciences Ltd (GNS) said.

Many people in the lower half of the South Island reported feeling the quake as a long rolling motion, typical of large, distant quakes. Residents of a number of towns in Southland reporting goods falling off shelves.

People in high-rise buildings as far north as Palmerston North and New Plymouth reported feeling a gentle swaying motion.

The quake occurred in an area called the Puysegur Trench, which marks the boundary between the Australian and Pacific tectonic plates.

" This was a large earthquake in global terms. Had it occurred under or near a population centre, there would be major damage," GNS seismologist Warwick Smith said.

" The location of the earthquake, well offshore, meant that by the time the seismic energy reached land, it was relatively weak," Dr Smith said.

Puysegur Trench is known as a seismically active area, so the quake's location was no great surprise to seismologists. A quake of identical size (magnitude 7.2) occurred in similar location in October 1979.

Aftershocks would occur during the next few weeks, but because of their distance from land, it was unlikely they would be felt strongly, Dr Smith said.

Ground shaking from the earthquake was recorded throughout New Zealand on seismic instruments operated by the GeoNet project.

Worldwide, the earthquake was the eighth this year of magnitude 7 or more. Most of these were in the western Pacific or Indonesia.

An earthquake of this magnitude would be capable of generating a damaging tsunami.

" The absence of a tsunami at Bluff indicates that there was no substantial vertical movement of the ocean floor at the epicentre."

Immediately after the earthquake thousands of people visited the GeoNet website (www.geonet.org.nz), and many reported how they felt the quake.

Dr Smith said these first-hand reports were extremely useful and would enable earthquake scientists to determine preliminary estimates of damage if a quake of the same size occurred on land.

ENDS