Major 6.0 earthquake west of Arthur's Pass
Seventeen claims for damage have been lodged with the Earthquake Commission (EQC) said by 3pm following this morning's magnitude 6.0 earthquake west of Arthur's Pass.
More than 30 aftershocks quakes were recorded by Geonet since the major magnitude-6.0 quake hit the South Island at 6.48am, today, centred about 30km west of Arthur's Pass.
The largest struck at 12.23pm and was recorded by Geonet as a magnitude-4.7 aftershock, also west of Arthur's Pass and 5 kilometres deep.
An EQC spokeswoman said most of the claims made today were from Christchurch. No further details were available but she said it was likely the claims were for minor damage or related to contents.
Today's early morning jolt was initially thought to be centred on the Canterbury Plains, near Lake Coleridge.
However, about 10.20am, seismologists issued a dramatic revision of the location and magnitude of this morning's earthquake which jolted thousands from their beds.
GNS Science spokesman John Callan said the latest information showed the quake was of magnitude 6.0, not the 6.4 reported earlier, and centred about 30km west of Arthurs Pass, close to where seismic equipment originally suggested, not 35 kilometres north of Methven, on the western edge of the Canterbury Plains.
It was 5km deep, not 10km deep as earlier reported.
Trains from Christchurch to the West Coast and to Oamaru were cancelled and dozens of aftershocks recorded, but the Oamaru and other links had since reopened.
However, there have been no reports of significant damage.
How did seismologists get it wrong?
Geonet duty seismologist John Ristau admitted there had been a dramatic change to the reported epicentre.
According to the Geonet website, a magnitude-2 foreshock five seconds before the main earthquake skewed the location further to the east, which also confused the magnitude estimation.
Ristau explained that more than 100 seismometers across the country would have determined the initial reading, but after such large quakes Geonet usually reviewed this.
They focused on 30 to 40 South Island-based seismometers closer to the epicentre, to generate a more accurate reading, he said.
"Because of the big difference in the structure of the earth between the North Island and South Island, if you include North Island stations for South Island earthquakes it can sometimes really throw off the system as far as the location and the magnitude," he said.
"Normally when we review, things don't change as dramatically as it did in this case, but unfortunately you can get some dramatic changes."
Earthquake not caused by Alpine Fault
Geonet scientist Caroline Little said it was likely the earthquake was caused by a previously unknown fault.
"It's still 20 kilometres away from the Alpine Fault, so it won't be that one," she said.
"There are lots of earthquakes around there every year and we don't attribute every earthquake to a fault, we just know that that's a zone prone to earthquakes."
Magnitude-6 earthquakes were just as likely to be caused by an unknown fault as one that had been previously identified, Little said.
The initial shake was followed by a swarm of more than 20 aftershocks in the area ranging in magnitude from 3.2 to 4.7, centred about 30km west of Arthurs Pass.
Ristau said the aftershocks would "probably go on for a while" and "it wouldn't be surprising" to have an aftershock above magnitude 5.
The Arthur's Pass area had had earthquakes of a similar size in the last 100 years or so, Ristau said.
Train services suspended for checks
KiwiRail suspended all services in the area, including the TranzAlpine train from Christchurch to Greymouth and all freight services, while the tracks were checked. Buses were provided for TranzAlpine passengers.
At 12.40pm, a spokeswoman said KiwiRail had reopened lines between Oaro in north Canterbury and Christchurch, Oamaru and Christchurch and the Hokitika Line.
The Stillwater and Midland lines are still being inspected.
TranzAlpine passenger Paul Blake said KiwiRail staff were helpful, and he had been offered a choice of a refund, rebooking, or a replacement bus journey.
Meanwhile, a New Zealand Transport Agency spokeswoman said inspections had found no damage on State Highway 77, near Methven, SH73 through Arthur's Pass, and SH6 on the West Coast.
St John Ambulance had no quake-related call-outs. And no reports of damage or injury were made to the West Coast or Darfield police, nor to the Methven fire station.
The 67km Rangitata Diversion Race, which provides irrigation water to about 500 farms, also appeared undamaged.
Geonet had about 3000 reports of people having felt the earthquake as far south as Mandeville, in Southland, to the central North Island.
West Coast cafes may suffer loss of business
The cancellation of the TranzAlpine train between Christchurch and Greymouth was likely to be the biggest impact of the earthquake on the West Coast.
Robert Harris cafe owner Julie McGeady said there was no damage or items falling from shelves but the axing of the TranzAlpine would "absolutely affect us".
As one of the closest cafes to the train station, "we can get 80 odd people off the train, if not more," McGeady said.
DP1 Cafe manager Sarah Burdon said she was woken by the shake but the loss of trade from the cancellation of the TranzAlpine could mean "a lot less stress".
Twitter lights up with quake reports
One Christchurch-based Twitter user said the quake "felt like the house was at sea".
That #eqnz just felt like the house was at sea (in Christchurch) — The Queen of Cobden (@QueenofCobden) January 5, 2015
Another Twitter user reported feeling the quake in Franz Josef.
Felt that wobble in Franz Josef #eqnz — Mark English (@MarkEnglish) January 5, 2015
Cheryl Bernstein said it was an "extraordinary swaying earthquake".
West of Arthur's Pass, apparently. I've never felt one quite like that before. #eqnz — CherylBernstein (@CherylBernstein) January 5, 2015
Karen Foreman-Brown said the quake was large enough to have set off alarms in her neighbourhood.
On the Stuff.co.nz Facebook page, Jackie Cooper said she felt the earthquake in Golden Bay at the top of the island, and Dale Wedlock said he felt it in Dunedin.
Don Gutsell said he had photos swinging at his house in Temuka.
That shake set off neighbourhood alarms #eqnz — Karen Foreman-Brown (@serenity22) January 5, 2015