KEY POINTS:
Scientists are planning to take a close look at aftershocks shaking Fiordland after a huge earthquake rocked Southland and woke lower North Island residents early yesterday morning.
Little damage has been reported after the quake measuring 6.7 that struck at 1.29am and was centred 60km west of Milford
Sound at a depth of 24km.
The quake was the biggest in New Zealand since a magnitude 7.2 tremor struck near Secretary Island off the Fiordland coast on August 22, 2003.
Milford Sound residents reported a strong shaking quake, rather than a rumbling or rolling tremor, that was felt widely in the South Island.
GNS Science recorded dozens of aftershocks - one with a magnitude of 6.2 - following the first quake. Although the first quake was strong enough to shake goods off shelves, authorities reported no major damage to buildings, roading or infrastructure.
Seismologists yesterday were planning to install portable instruments in Fiordland this week to better record the aftershocks.
GNS Science seismologist Bryan Field said the six battery-powered seismometers would enable pinpoint accuracy in recording the size, depth and location of the aftershocks.
Close study of aftershocks allowed seismologists to determine the size and orientation of the "fault-break" in the earth's crust, helping to indicate if the main shock had any effect on faults nearby.
Mr Field said aftershocks could continue for several weeks after a large earthquake.
Scientists recorded about 5000 aftershocks over several months after the 2003 quake.
"Aftershocks occur as the earth's crust adjusts to stresses caused by the main shock. No two aftershock sequences are exactly the same," he said.
"This looks to be a particularly rich aftershock sequence that should yield useful information about stresses in the earth's crust in the Fiordland region."
Of the dozens of aftershocks in the hours following yesterday morning's main shock, Mr Field said "half-a-dozen" were big enough to be felt widely in the southern South Island.
The portable instruments would record aftershocks continually for several weeks before scientists would recover them and analyse the data they had captured.
Transit New Zealand Central Otago area engineer John Jarvis said contractors out inspecting roads and bridges in the area for damage shortly after the quake had noticed "nothing significant".
Mr Jarvis said there may have been the "odd rockfall" and contractors would clear debris as it was reported. "We've got people out on the road doing regular inspections, but we haven't got any reports of any significant damage at all."
One Milford Sound resident, who would give her name only as Ann, claimed the quake was only a small one. "We've had 7s and 7.1s before today." Ann said she'd wakened just before the quake.
"You couldn't have missed that one. I don't think there'd be anyone still asleep after that."
Asked how it felt, she said: "It wasn't a rolly one, just a shaky one."
Milford Sound Development Authority operations manager Dave Inwood said he was at the Freshwater Bay visitor centre when the fourth 6.2 aftershock struck just before 10.30am.
The centre was busy with tourists waiting to take cruises on Milford Sound or sightseeing trips, but no one took much notice.
"I'm not sure if the tourists thought they were on a floating structure or not, but nobody showed any interest in moving," he said.
"We all just stood there and watched as the building shook. It was just very gentle.
"But there was no damage to buildings and no threat to life and limb."
Mr Inwood described the first quake as "just a good shake" throughout the Milford area.
"It's certainly rock and roll here today."
- NZPA
