Authorities in New Zealand's quake-ravaged Christchurch city extended a state of emergency for another seven days on Wednesday, as a strong aftershock cut power and closed a key highway, with the region told it can expect quakes for months to come.
The magnitude 5.1 aftershock at around 1949 GMT was not the largest since Saturday's 7.1 major quake, but scientists said it felt the strongest as it was shallower and located closer to the centre of the country's second-largest city.
"There will continue to be aftershocks for months, but the frequency and magnitude will diminish over time," Jennifer Coppola, a scientist at state-funded science agency GNS Science said.
There have been about 135 aftershocks above magnitude 3, including two at 5.4 since Saturday, and the largest may not have struck yet.
"The rule of thumb is that the largest aftershock should be expected to be about one order of magnitude below the main quake, so about six," Coppola told Reuters.
The latest shake closed the highway between the city and the adjoining port town of Lyttelton with reports of cracks in a tunnel.
There were reports of further debris falling from already damaged buildings, and people were evacuated from some offices in the central business district as checks were made.
Christchurch mayor Bob Parker said every aftershock was a setback to a city trying to get back on its feet.
"We're gradually making progress but we're in a very unpredictable situation," Parker told Radio New Zealand.
The main quake which struck at 4.35am local time on Saturday was New Zealand's most destructive since 1931, with an estimated 100,000 of the region's 160,000 homes believed to have been damaged. There were no deaths and only two serious injuries in the city of 350,000.
A state of emergency, which was due to expire later on Wednesday was extended to give authorities the necessary powers to control and direct recovery operations.
The latest aftershock temporarily cut power to about 30,000 customers but supplies were quickly restored.
Around 500 consumers, mainly in rural areas, are still without power, and water and sewage services have been restored to around 90 percent of the city. More than 300 people are in welfare centres because their houses are uninhabitable.
The initial estimate of repairing the damage has been put at NZ$2 billion ($1.4 billion) and prime minister John Key has pledged the government will step in to help with the cost of rebuilding.
The quake's epicentre was a previously unknown fault line about 20 km (12 miles) to the west of Christchurch. It opened up a 22 km-long (14-mile) rip in the earth, with fissures of up to 4 metres (12 feet).
The quake was among the 10 strongest recorded in New Zealand and the most damaging since the North Island city of Napier was devastated in 1931.
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.
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