"I was in the square right outside the cathedral -- the whole front has fallen down and there were people running from there. There were people inside as well," said John Gurr, a camera technician who was in the city centre when the quake hit.
Authorities ordered major hospitals up and down the country to make room for quake victims. There were reports of a shortage of ambulances.
"A lady grabbed hold of me to stop falling over...We just got blown apart. Colombo Street, the main street, is just a mess...There's lots of water everywhere, pouring out of the ground," Gurr said.
Emergency crews picked through the rubble, including a multi-storey office building whose floors appeared to have pancaked on top of each other.
SILT, SAND AND GRAVEL
Christchurch is built on silt, sand and gravel, with a water table beneath. In an earthquake, the water rises, mixing with the sand and turning the ground into a swamp and swallowing up sections of road and entire cars.
TV footage showed sections of road that had collapsed into a milky, sand-coloured lake right beneath the surface. One witness described the footpaths as like "walking on sand".
Unlike last year's even stronger tremor, which struck early in the morning when streets were virtually empty, people were walking or driving along streets when the shallow tremor struck, sending awnings and the entire faces of buildings crashing down.
Police said debris had rained down on two buses, crushing them, but there was no word whether anyone had been killed or injured.
The quake hit at 12:51 pm (2351 GMT Monday) at a depth of only 4 km (2.5 miles), according to the U.S. Geological Survey.
"It's huge, it's just huge," a priest told a TV reporter outside the remains of the city's stone cathedral, part of which had been reduced to a pile of large sandstone blocks.
"I just don't know whether there are people under this rubble," he said, before he appeared to add in a quiet voice: "I think so."
TALK OF POST-QUAKE RATE CUT
The quake helped knock the New Zealand dollar down to $0.75 , about 1.8 percent off late U.S. levels, on fears the damage could dent confidence in the already fragile economy.
Westpac Bank also raised the possibility that the central bank could cut interest rates over the next few weeks to shore up confidence after the quake, while other banks pushed out their expectations for the next rate hike. ANZ now expects the central bank to be on hold until the first quarter of 2012.
Shares in Australian banks and insurers, which typically have large operations in New Zealand, fell after the quake. But credit rating agency Fitch said the tremor would not itself trigger a downgrade of New Zealand sovereign rating.
The tremor was centred about 10 km (six miles) southwest of Christchurch, which had suffered widespread damage during last September's 7.1 magnitude quake but no deaths.
James Goff, of the University of New South Wales' Natural Hazards Research Laboratory in Australia, said some buildings that survived last year's quake had been weakened and unable to withstand the second one, especially such a shallow tremor.
"A lot of the infrastructure has still not fully recovered from the last earthquake so that would have still been feeling weak and susceptible to another big earthquake," he said.
The region has been struck by thousands of aftershocks since the original quake.
New Zealand, which sits between the Pacific and Indo-Australian tectonic plates, records on average more than 14,000 earthquakes a year, of which about 20 would normally top magnitude 5.0