Christchurch was hit Wednesday by the most damaging aftershock since a devastating 7.0 quake, forcing evacuations and cracking a major road tunnel in the New Zealand city.

The 5.0-magnitude aftershock struck at 7:49 am (1949 GMT Tuesday) sending frightened residents rushing into the streets, cutting already fragile power supplies and bringing down loose material from already damaged buildings.

The aftershock, the latest in a series, was a shallow six kilometres deep and much closer to the city centre than Saturday's quake, which caused billions of dollars of damage, seismologists said.

Christchurch Mayor Bob Parker said the intensity of the latest quake reduced many emergency workers to tears.

"It was a devastatingly, vicious sharp blow to the city," he told NewstalkZB radio.

"This was a terrifying moment. We have just had to evacuate our civil defence headquarters."

A 1.9-kilometre (1.2-mile) tunnel linking Christchurch to the nearby port of Lyttleton was closed after cracking appeared, the New Zealand Transport Authority (NZTA) said.

"There is some cracking that's apparent on the surface and there's apparently some damage to the canopy as well," NZTA spokesman Andy Knackstedt told national radio.

"We don't know what the extent of that is. They're in there right now making an assessment."

Parker said the force of the latest convulsion meant there would "inevitably" be more damage to already weakened buildings, adding that the ongoing aftershocks were taking their toll on the city's residents.

"We have got staff in tears, we have got fire engines going through the middle of the city, power is out and a lot of people are very, very churned up by that."

More than 100 aftershocks have rocked the area since Saturday.

"I'm bloody terrified all over again," resident Colleen Simpson told the Stuff website after Wednesday's tremor.

Christchurch City Council spokeswoman Diane Keenan said: "The jolt was absolutely huge. A really big, stiff jolt. And it was vertical, rather than side to side like the first one. If you were in a car the road moved up and down."

The latest quake came as staff were making their way to work at the few shops and companies in the inner city which have been able to open this week. They were immediately told to leave.

Parker said assessment teams were heading into the quake-scarred city -- New Zealand's second-largest -- to check the damage.

"We were restarting to think, maybe, just maybe, we are over the worst of this and now we have had this shocking event," he said.

Officials estimate up to 100,000 homes were damaged in Saturday's quake, which caused damage estimated at two billion dollars (1.45 billion US) but killed no one.

earlier related report
State of emergency extended in quake-hit New Zealand
Christchurch, New Zealand (AFP) Sept 8, 2010 - A state of emergency in New Zealand's earthquake-hit Christchurch has been extended for another week, officials said Wednesday after a shallow aftershock rocked the city.

"We've just been told the state of emergency in Christchurch will be in place for another week," a civil defence ministry official told AFP.

He was unable to confirm whether the extension was a direct result of a shallow 5.0-magnitude aftershock that hit the country's second largest city Wednesday morning, forcing evacuations and the closure of a major road tunnel.

The state of emergency in the wake of Saturday's 7.0-magnitude quake had been due to expire on Wednesday at midday (0000GMT).

The aftershock struck at 7:49 am (1949 GMT Tuesday), sending frightened residents rushing into the streets, cutting already fragile power supplies and bringing down loose material from damaged buildings.

Authorities closed a 1.9-kilometre (1.2-mile) tunnel linking Christchurch to the nearby port of Lyttleton after cracks appeared.

The aftershock, the latest of more than 100 that have rocked Christchurch since the main quake, was just six kilometres deep and much closer to the city centre than previous tremors.

Nobody was killed in Saturday's quake, although many residents reported close shaves. The Canterbury health service said some minor cuts and bruises were reported after Wednesday's aftershock but nothing serious.