Shallow quake
GNS Science said today's quake was centred at Lyttelton at a depth of 5km at 12.51pm.
GNS said the earthquake would have caused more damage than the original 7.1 earthquake on September 4 because of its shallow depth.
Its data centre manager Kevin Fenaughty said residents said the quake's epicentre was located in the "worst possible location" for the city.
"It's a nightmare. A lot of people were just getting back on their feet after the original quake."
Another earthquake of 4.5 struck at 1.21pm, 10 km east of Diamond Harbour.
Streets flooded
Herald reporter Jarrod Booker said the shake lasted approximately one minute and was extremely violent - rocking buildings back and forth.
He said people had left buildings and were out on the streets where tarmac had cracked and water mains had burst, causing extensive flooding.
Tuam Street had become a river as water poured from ruptures in the road and was impassable in places.
The whole central city was in grid lock as people tried to evacuate central businesses to check their homes, Jarrod Booker said.
Most traffic lights were out and cars were also having to negotiate around hordes of people on foot.
Jarrod Booker said that he could hear sirens but that it would be difficult for emergency services to access the city because of the gridlock.
"Even sitting in a car you can feel continual shaking on a smaller scale than the original quake," he said.
Some pedestrians were standing on the footpaths and staring into space, apparently in shock.
'Great confusion'
Mayor Bob Parker said he was "thrown quite a distance" by the earthquake.
"That was, in the city central anyway, as violent as the one that happened on the 4th of September," he told Radio New Zealand.
Mr Parker said there were scenes of "great confusion" on the streets, also saying the roads were jammed as vehicles sought to get out of the central city.
"I know of injuries in my building and there are unconfirmed reports of serious injuries in the city."
Mr Parker did not know the extent of damage to the city's infrastructure, but advised people not to drink the water supply.
"We've been through this before this once, we now need to think we did at that time."
Buildings collapsed
Jarrod Booker said Christchurch's historic cathedral of the Blessed Sacrament on Barbadoes Street had half collapsed, with the remaining part of the building filled with cracks.
There was huge damage to other older buildings with large amounts of debris falling to the ground, he said.
He said the carpark at the Christchurch Star had turned into a river with huge cracks and that the roads had risen in areas.
People were comforting people outside amid a general state of shock as they tried to absorb what had happened, he said.
Radio New Zealand reported widespread damage to the city centre, with a church on Durham St collapsed and concrete lifted by up to a metre.
TV3 reported the Provincial Chambers Building had collapsed and it was believed people were trapped inside.
A listener told Newstalk ZB that the Piko Wholefoods building on Kilmore Street near the city centre, which was hit in the September 4 earthquake, was now "practically non-existent".
The spire on the Christchurch Cathedral had also collapsed.
A man said he and 19 other colleagues are trapped in Christchurch's Forsyth Barr building on Colombo Street.
Gary Moore told NZPA workers were stuck on the 12th floor as the stairwell had collapsed. He was not sure if people were trapped on other floors.
People were in a state of shock but were not injured and he urged NZPA to let somebody know.
He described the first quake and the aftershocks as catastrophic.
"We watched the cathedral collapse out our window while we were holding onto the walls."
"Every aftershock sends us rushing under the desks. It's very unnerving but we can clearly see there are other priorities out the window. There has been a lot of damage and I guess people are attending to that before they come and get us," he said.
A Newstalk ZB reporter in Christchurch said liquefaction was spewing out of the ground at St Albans High School.
School children had to be removed from the fields with liquefaction also spewing from the tennis courts.
