6.4 quake struck in little-understood zone

Last night's quake struck 190km south-west of the subantarctic Snares Islands, at a depth of 35km. Photo / File

Last night's 6.4 quake came from a deep south seismic zone we hear little of - yet has produced some of the most powerful earthquakes in New Zealand's recent history.

The 7pm quake initially confused GeoNet's instruments, with the service initially reporting a 5.9 magnitude earthquake had struck near Invercargill and another near Blenheim with a magnitude of 5.0.

Felt by people around the South Island, the quake actually struck 190km southwest of the subantarctic Snares Islands, at a depth of 35km.

Department of Conservation senior biodiversity ranger Jo Hiscock said there was no one on either the Snares or Auckland Islands groups when the quake struck.

Otago University researchers were due to visit Enderby Island, in the Auckland Islands, in the next three weeks and they would be able to assess whether there was any damage to Enderby Hut, which was used by rangers and researchers in the summer months.

"There is also an emergency fuel depot on Enderby Island which will need to be assessed," Hiscock said.

Field huts on Auckland Island and Adams Island, at the southern end of Auckland Island, would be assessed during the summer.

GeoNet reported the quake's largest recorded ground motions reached about 0.5g, making it 600 times smaller than the largest motions recorded in the recent M7.8 Kaikoura earthquake - a difference largely due to the difference in sizes of the earthquakes and distance to the closest seismometers.

It happened near a plate boundary where the Australian plate dives below the Pacific plate - the opposite way to how the two plates subduct in the North Island.

Called the Puysegur Trench, this boundary stretches for more than 800km south from the South Island, to a point in the wild and windy Southern Ocean, around 400km west of the Auckland Islands.

"By the time you get that far south, it's a well established subduction system - so that means the Australian plate is diving down below the Pacific Plate in a relatively normal sort of way," GNS Science seismologist Bill Fry said.

"What I can't tell, because the quake was quite far away from our network, was whether the quake occurred on the plate interface, or on some other fault associated with it."

In 2009, the 7.8 Fiordland earthquake struck close to the northern end of the trench, releasing 25,000 times more energy than the atomic bomb dropped on Nagasaki in 1945 and twisted the South Island, moving Puysegur Pt on the southwestern tip of the island 30cm closer to Australia.

Its remote location, near Dusky Sound, meant there was relatively little damage.

A magnitude 7.2 quake hit the trench itself in November 2004.

Fry said there was much more scientists could learn about this quake zone, but its far-flung location had made it a difficult area to investigate.

"You can look at very large events over the past century or so, but it's quite difficult to get a good understanding about long term behaviour."

Could a major quake south of the country pose a tsunami threat?

"The potential impacts of a large megathrust earthquake that far south versus that of a large subduction quake in the North Island are quite different," Fry said.

The Hikurangi Subduction Zone, off the North Island's East Coast and now the focus of a major international project, ultimately posed the greater threat to people.

"So there are probably higher priorities we have for instrumentation, and certainly for impact to New Zealand."