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A "severe'' magnitude 6.2 quake has damaged homes and closed roads in the lower North Island, toppling walls and chimneys and sending rockfalls across roads.
The quake, initially reported as a magnitude 6.3, struck 10km north of Castlepoint in Wairarapa, at a depth of 33km, at 3.52pm, GeoNet said.
Sara Page, GeoNet public information specialist at GNS Science, said GeoNet had received more than 6000 "felt" reports from the public by 4.30pm, with some reporting damage.
"As expected after a quake of this size, there have been multiple aftershocks, and these will continue for some time as the region settles," she said.
The Wairarapa is no stranger to large earthquakes, with two very damaging magnitude 7 quakes in 1942.
However, the region's last earthquake above magnitude 6 was in 1961.
Fifteen people will not be able to return home tonight after their building in the centre of Masterton was red-stickered as a result of today's earthquake.
Wairarapa area Civil Defence controller Kevin Tunnell said the three-storey Daniels Building on Queen St had sustained unspecified damage in the quake.
It was initially deemed unsafe by the Fire Service and then by Masterton District Council engineers.
Its 15 tenants would not be allowed home for at least tonight and engineers would carry out further inspections tomorrow.
Most of the tenants had managed to find alternative accommodation but two who had not would be housed by the council tonight.
Mr Tunnell said the building's locks had been changed to prevent the tenants trying to retrieve their belongings.
"The natural instinct is for people to get back into their residence but unfortunately that's against the law at the moment.''
Overall, a central ambulance communications spokesman said two people had suffered "very minor'' injuries due to falls during the quake. No further details were available.
There are still no reports of serious injury.
Wellington Region Emergency Management group controller Bruce Pepperell said very little damage had been reported in the Wellington region, but there was damage closer to the epicentre in Wairarapa.
"There's no serious injuries or deaths over there, but they got a very severe jolt. It has caused some isolated damage with stuff coming off shelves and things like that.''
Three 1920s-style buildings in Masterton had been evacuated as a precautionary measure.
"The engineers will check them out - safety is paramount.''
Coastal residents near Castlepoint reported the seas were quite calm, Mr Pepperell said.
There was no sign of the traffic chaos that gripped Wellington after the magnitude 6.6 earthquake in August last year.
"There is a bit of traffic with people coming home from a long weekend and extended holidays, things like that. But some people didn't even feel it,'' Mr Pepperell said.
GNS Science had told Mr Pepperell the forces at play in Wellington were "a hell of a lot less than we experienced last year''.
"It was one of those good reminders, but we got away relatively lightly down here.''
Emergency services have reported damage to roads and buildings in the lower North Island.
Most of the damage was reported in the Wairarapa and Palmerston North areas.
Fire Service central communications shift manager Mike Wanoa said there were no reports of major damage so far, but firefighters were "extremely busy''.
"The earthquake has been reasonably major in the Masterton-Eketahuna area, so we're getting multiple calls to all sorts of things at the moment, but we're right in the middle of it now.''
There were reports of fires, alarm activations and lines down. A lot of the damage was in the Wairarapa and Palmerston North areas, Mr Wanoa said.
Inspector Mike Coleman of police central communications said there were reports of damage to houses in Eketahuna, including broken windows, collapsed walls and fallen chimneys.
The number of reports of damage remained unknown.
"Obviously some houses have been damaged,'' Mr Coleman said.
"Windows have been smashed and crockery has been thrown around the place - the usual sort of movement with earthquakes.''
Mr Coleman said there were rocks and debris on roads between Woodville and Taihape due to various slips.
The Manawatu Gorge road was down to one lane, while the road between Pahiatua and Palmerston North was closed.
Bridges and roads around Eketahuna were being checked, Mr Coleman said.
New Zealand Transport Agency spokesman Ewart Barnsley said they were fairly confident the state highway network had come through the quake largely unscathed.
However, engineers were continuing to conduct checks and motorists were advised to exercise appropriate caution.
"We're pretty confident that our state highway network is okay and Manawatu Gorge is okay. Traffic's flowing okay,'' he said.
A Wellington Free Ambulance spokesman said: "Wellington Free Ambulance has had no callouts as a result of the earthquake. However, we have gone into emergency management mode just to be safe.''
Tranz Metro said all train services in the region had been suspended due to the quake.
The New Zealand Transport Agency said teams were busy checking the road network for damage but everything seemed to be okay.
Power is out in Linton, south of Palmerston North.
A spokeswoman for the Earthquake Commission (EQC) said the agency was still gathering information on the quake and the volume of calls received.
One of the two giant eagles hanging from the roof of Wellington airport to promote the Hobbit trilogy did fall down as a result of the shaking.
The Weta Workshop eagles each weigh 2 tonnes, have a wingspan of 15m, and were suspended from the roof by eight cables.
Greg Thomas from Wellington Airport said one of the eagles slowly became detached during the quake and had come to rest on the floor.
He said it was still partly suspended, and no one was injured when it came down.
The quake had not caused any other damage at the airport. A runway inspection had been carried out and the airport had been cleared to continue operating.
No flights had been disrupted, he said.
Damage at Pukaha Mt Bruce wildlife centre, near Eketahuna, was confined to items falling off shelves in the retail area, said general manager Helen Tickner.
"There's a bit of cleaning up to do tomorrow, but all the animals are fine and all the visitors were fine,'' she said.
"Everyone was a bit shaken up and a bit freaked out.''
She said there would be a careful check for any further damage, but it was expected the centre would open as usual tomorrow.
'This one came with a bang'
Karen Monk, who is on a farm in Mauriceville, just north of Masterton, said the quake was "really violent''.
"My baby daughter was in her cot asleep and I managed to leap across the hall and grab her and leap outside onto the lawn,'' she said.
They had stayed outside for about half an hour while aftershocks rolled through.
Ms Monk said the quake was sudden and violent, compared to the usual rolling shakes.
"It was certainly the biggest we've had since we've been here.
"It was really sudden. Usually the earthquakes we feel up here, whether they're from north or south, they're more rolling and you start start with a gentle shake.
"This one just came with a bang, with massive jolting.''
She said the contents of the pantry had spilled onto the floor, shelves tipped over. Almost every room in the wooden villa had cracks in the walls.
"We're on a farm here and the animals don't normally react to quakes but the horses were running around for a good 10 minutes afterwards. The sheep are all huddled together. That's really unusual.''
Hear a Radio trackside horse commentator interrupted by the earthquake live on air here:
An office worker in Masterton described the tremor as "a good quake - one of the best''.
"It was a roll rather than a jolt. It was not very long but it was long enough - it lasted about 20 seconds.''
Pam Lochore, wife of All Black great Sir Brian Lochore, said photographs had fallen off shelves in the couple's Masterton living room.
The shaking also caused water in the pool to "rock side to side'' and a "rugby ball went flying across the room''.
Raumati South resident Leigh Nichols was at her beachside bach when the quake hit.
"It was huge. The noise - it was like a train going along the track. It was so noisy, everything was just rattling.''
Mrs Nichols said a wine glass smashed and DVDs spilled to the floor. Her husband David clutched a wooden statue to prevent it tumbling over.
"It was the noise that got me, not the shaking. I just stood here. I don't get frightened, I'm fascinated.''
Asked how the quake compared to the big Seddon shakes in July last year, she said: "I think it was just as bad, at least.''
Mrs Nichols was about to head to her Raumati interior design store, Furnishing Affair, to check merchandise for damage.
"If it's only things, it's only things. But gosh it was big.''
In North Wairarapa crockery broke, fridge doors were flung open spilling food onto kitchen floors and disheartened homeowners described the aftermath as "a bloody mess''.
In Masterton initial reports showed there was little damage in shops although some crockery had been broken.
Anders Crofoot, owner of Castlepoint Station on the east coast of Wairarapa, said it was "the best shake we've had in 15 years''.
"Stuff off the shelves, stuff off walls, but nothing that we've come across that's too major,'' he said.
He said the shaking went on for about 40 seconds.
"I was up in the office and it was long enough to think about it and then get downstairs and outside and it was still going.''
He would now be checking the farm water supply for damage. "There's a high probability with some of these old pipes that there'll be a problem.''
Electricity retailer Powershop, which has its headquarters in Masterton, tweeted that it had evacuated its call centre following the earthquake.
The company said it would continue responding to email queries as best it could.
A DB Breweries spokeswoman said they had checked with staff at the Tui Brewery in Mangatainoka and there appeared to be no damage as a result of the quake.
Fonterra said all staff at the Paihiatua plant are safe but power is currently out and they are still checking the plant for damage.
A 6.2 just by Castlepoint. Hope those there are ok. 90 km depth which should reduce impact somewhat. #eqnz— David Farrar (@dpfdpf) January 20, 2014
My cistern water went crazy. And my antique birdcage is still swinging. And I'm scared but my kid isn't. #eqnz— Mïchaela Moët (@pinkdeedle) January 20, 2014
Was in car for #eqnz. Rocking and rolling. Thought the engine was going crazy.— Brenda Wallace (@BR3NDA) January 20, 2014