Our Australian cousins just got closer

Last updated 05:00 22/07/2009

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The South Island has been twisted out of shape and shoved closer to Australia by last week's massive Fiordland earthquake.

Last Wednesday evening's 7.8 magnitude earthquake which was centred 12 kilometres under Resolution Island and was the country's largest shake for nearly 80 years wrought permanent changes in just seconds.

The Land Information New Zealand-GeoNet global positioning system (GPS) network shows Puysegur Point, at the southwest tip of the South Island, is now 300mm closer to Australia.

Te Anau has moved 100mm westwards. Bluff moved west by 30 millimetres, Alexandra, by 20mm, and Dunedin, by 10mm.

Christchurch may have shifted marginally closer to Australia, but only by a tiny amount.

GNS Science GeoNet project director Ken Gledhill said such changes were part of the continuing deformation of New Zealand, and showed the immensity of the forces involved.

"New Zealand has been very fortunate.

"This earthquake anywhere else would have caused huge damage.

"Basically, it's taken us closer to Australia. The country is deforming all the time because of being on the plate boundary, but this has done it in a few seconds, rather than waiting hundreds of years."

Earthquake scientists met yesterday afternoon to discuss how the earthquake had affected the Alpine Fault.

It lies about 50km offshore southern Fiordland.

"What we do know is it has caused an increase in stress in the southernmost offshore part of the Alpine Fault.

"Unfortunately, very little is known about that part," Gledhill said.

"The strain has to go somewhere. What the GPS has shown is most of the motion is in the horizontal direction, and that bit of Fiordland has gone towards Australia.

"What we think at the moment is that even if something happened on that segment, it is unlikely to rupture on to the onshore part further north.

"The problem with all of this work is you don't actually know where that bit of the Earth is in the earthquake cycle.

"What we would like to do is do a lot more modelling, and that will take months."

GNS Science geologist Simon Cox said New Zealanders should not be lulled into a false sense of security that the country had now had "the big one".

"There is no such thing as `the' big one," Cox said.

"New Zealand sits across the plate boundary between the Australian and Pacific plates, and there are a number of fault systems from the ones at Napier-Hastings that are different from the Wellington ones, which are different from those in Marlborough, which are different from the Alpine Fault, which are different from Fiordland.

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"All are capable of producing big earthquakes.

"We've just had a big one in Fiordland but that doesn't mean the big one has ripped through the whole lot.

"People shouldn't take too much comfort from this one having missed."

- PAUL GORMAN/The Press

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