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The magnitude 6.0 quake rattled West Coasters and Cantabrians awake just before 7am.
Dozens of aftershocks followed -- the largest reaching 4.8 in magnitude, which hit about 5pm, some 25km from Arthur's Pass.
The main tremor was initially measured at 6.4 in strength and centred north of Methven.
However, GNS Science later revised the magnitude down and updated its location to 30km west of Arthur's Pass -- in the West Coast region.
The quakes had caused no infrastructure damage, and the region escaped relatively unscathed because of the quake's isolated epicentre, GNS said.
More than 3000 people from Mandeville in Southland to the central North Island reported feeling the quake.
Seventeen insurance claims had so far been lodged with the Earthquake Commission, mostly from residents in the Christchurch and Canterbury areas, a spokeswoman said.
Among those shaken by the quakes was Kiwi actress and former partner of Sam Neill, Lisa Harrow, who said she was lying awake in their Port Levy home about 6.30am when she felt the quake hit.
"Just after that I felt the bed rocking as if I was on a boat in a big swell.
"I experienced the recent earthquakes in the Christchurch area so my immediate thought was, oh, that was another earthquake."
However, Ms Harrow said it wasn't violent enough to stir her sleeping husband, whale biologist Roger Payne.
"It was so gentle and smooth that my husband, who is very sensitised to earthquakes, slept right through it."
Sam Hay, who works at the Otira Hotel, not far from the quake's epicentre, said he was woken by a "pretty decent shake".
But having experienced the major Christchurch earthquakes, he was not rattled enough to get out of bed.
There was no damage to the hotel, he said.
"It was a decent shake, but it wasn't a violent shaking like we got in Christchurch -- it was a gentle 'I'm on a boat in the ocean' shaking."
KiwiRail had closed some of its South Island rail lines for a safety inspection following the quake.
Earlier this afternoon, clearance was given on the final two lines that remained closed, Stillwater and Midland, and they were now reopen.
"KiwiRail staff completed the final inspections on the Stillwater and Midland lines and safety clearance was given for the lines to reopen," a spokeswoman said.
"All freight services have now resumed and the TranzAlpine is scheduled to run as normal tomorrow."
GNS said it was still working out the probability of future tremors hitting the area.
It was very likely aftershocks would continue to decrease in frequency -- consistent with normal aftershock behaviour.
"We expect that aftershocks will occur in a slightly larger region than where aftershocks have already happened. Again, it is very early in the aftershock sequence and we will know more as this sequence develops."
GNS said it was unlikely a quake similar to this morning's in strength would occur again in the same area -- or that it the quake would trigger another, stronger quake greater than magnitude 7.0 on a separate fault line.
The central South Island is no stranger to strong earthquakes. The tectonics in the area are dominated by the Alpine Fault, where the Australian and Pacific tectonic plates meet, pushing together to form the mountain ranges of the Southern Alps.
"Surrounding the Alpine Fault there are numerous known and unidentified faults which, along with the Alpine Fault, take up stresses from the convergence of the Pacific and Australian plates," GNS said.
- Rebecca Quilliam and Nikki Papatsoumas of NZME. News Service