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A 7.0-magnitude earthquake has struck off New Zealand's South Island, the US Geological Survey has said.

The epicentre was 55km (35 miles) north-west of Christchurch, at a depth of 12 km (7.5 miles), it added.

Police said there had been widespread damage to buildings and roads as well as power cuts. Two men were seriously injured by falling masonry and glass.

A state of emergency was later declared in Christchurch, New Zealand's second largest city with a 386,000 population.

New Zealand lies at the southern end of the so-called Pacific Ring of Fire, and above an area of the Earth's crust where the Pacific Plate converges with the Indo-Australian Plate.

The country experiences more than 14,000 earthquakes a year, of which only around 20 have a magnitude in excess of 5.0.

The last fatal earthquake was in 1968, when a 7.1-magnitude tremor killed three people on the South Island's western coast.

The earthquake struck at 0435 on Saturday local time (1635 GMT on Friday), the USGS said, when most people would have been asleep.

Radio New Zealand reported that the quake was felt as a long, rolling motion lasting up to 40 seconds, and that the area was continuing to feel aftershocks. The USGS said one aftershock had a magnitude of 5.7.

The local newspaper, The Press, said it was felt widely across the South Island, including Christchurch and the nearby port city of Timaru.

Police said damage and power outages had been reported as far afield as Dunedin, 360km (223miles) to the south-west.

Christchurch Mayor Bob Parker said the "sharp, vicious earthquake" had caused significant damage in parts of the city.

Chimneys and walls had fallen from older buildings, with roads blocked, traffic lights out and power, gas and water supplies disrupted, he added.

"There is considerable damage in the central city and we've also had reports of looting, just shop windows broken and easy picking of displays," police inspector Mike Coleman told Radio New Zealand. "It's very unsafe to be out and about."

Christchurch International Airport was closed after the earthquake as a precaution, as experts checked the runways and terminal buildings, a spokesman said.

Susan Birkbeck, who lives in the centre of Christchurch, told the ´óÏó´«Ã½: "It was absolutely shocking, we're all terrified and scared of what's going to happen next."

"I was asleep when suddenly the house started shaking and there was this smashing sound, I thought a large truck had just driven through the front window."

"I'm now sitting on my bed surrounded by broken glass and I've no idea what to do. The walls and roof are just hanging, it's terrifying," she added.

Colleen Simpson told The Press that there was a "row of shops completely demolished right in front of me", adding that many people in the city were out in the street in their pyjamas looking scared and worried.

Barry in Ashburton, 86km (53 miles) west of Christchurch, told the ´óÏó´«Ã½ it was the strongest earthquake that he had ever experienced.

"Very strong aftershocks are still going here," he said. "In fact, the moving has not really stopped since it started nearly an hour ago."