Its epicentre was only 5 km from the city and was at a depth of about 4 km below the surface with little ground to absorb the blow

New Zealand's Christchurch weathered a 7.0 earthquake, but a smaller 6.3 aftershock toppled buildings and killed scores largely because it was a ‘bullseye' direct hit, scientists said.

Very shallow

Tuesday's cataclysmic tremor, which left nearly 400 people dead or missing and the city centre in ruins, was so close to the city of 390,000 and so shallow that major damage was inevitable, they said.

“This quake was pretty much a bullseye,” said Professor John Wilson, deputy dean of engineering at Australia's Swinburne University of Technology.

“It was quite a large 6.3-magnitude event and so close to Christchurch that we weren't surprised to see significant damage. At that close range, the level of shaking is quite severe.”

The earthquake struck six months after the violent 7.0 tremor damaged 100,000 buildings and left a major repair bill, but caused no deaths, after striking overnight on September 4.

“Tuesday's quake is by far the largest earthquake to have occurred in the Christchurch region in historic time,” Melbourne University research fellow Gary Gibson said in a release from the Australian Science Media Centre.

“Earthquakes always cluster in time and space, with some large earthquakes having foreshocks and most large earthquakes have many aftershocks.” Gibson said authorities should not be faulted for not anticipating the quake.

Gibson, however, said Christchurch would always be prone to damaging quakes, saying “all earthquakes in the Christchurch region will be shallow, so the effect of a given earthquake will be worse than from a deeper plate boundary earthquake of the same magnitude.”

Newer office blocks such as the CTV and Pyne Gould buildings collapsed. “We expected the older buildings with unreinforced masonry to suffer — their masonry is heavy, brittle and vulnerable to earthquake shaking,” Wilson said.

“In general the contemporary buildings performed well, although a few contemporary buildings have collapsed, which did surprise us.”

David Rothery, of the Volcano Dynamics Group at Britain's Open University, said the soft ground on which the city is built would have magnified the shaking, making the 6.3 quake even more deadly.

Flat ground

“In much of Christchurch where the ground is flat and underlain by sand or silt, some structures have been shaken apart, causing upper stories to collapse onto the floors below,” he said.

“This is because soft ground magnifies how violently the surface shakes during an earthquake.” Australian Seismological Centre director Kevin McCue said the tremor could increase pressure on plate boundaries across New Zealand, increasing the likelihood of a tremor elsewhere, particularly in the capital Wellington.

“If you have one (quake) it ups the hazard,” he told the New Zealand Herald.

“This quake has the potential to load up the plate boundary, increasing the likelihood of a quake at Wellington.”

“Wellington has always been considered much more at risk because it straddles the plate boundary. New Zealand has been relatively quiet since the 1930s — maybe (it's) about to catch up.”

New Zealand sits on the ‘Pacific Ring of Fire', a vast zone of seismic and volcanic activity stretching from Chile on one side to Japan and Indonesia on the other.

But Gibson feels that “the September earthquake and this earthquake will have relieved the majority of stress in the regions in which they occurred so another large earthquake is unlikely.”

Tuesday's quake is the most deadly to hit New Zealand since a 7.8-magnitude tremor killed 256 people in the Hawke's Bay region in 1931.

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