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.

Tuesday’s cataclysmic tremor 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 John Wilson, professor and 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.”

Its epicentre was only five kilometres from the city at a depth of just four kilometres below the land’s surface, meaning there was little ground to absorb the blow.

Some of the worst-hit buildings, including Christchurch’s landmark cathedral – which lost its spire – and The Press Building housing the local newspaper, were historic structures in the city’s heart.

However, newer office blocks such as the CTV and Pyne Gould buildings collapsed, while the towering Grand Chancellor Hotel was tottering dangerously. New Zealand buildings have been designed to resist earthquakes since the 1970s.

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

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.

“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.

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

Meanwhile, parts of a 30-million tonne block of ice sheared off the Tasman glacier just minutes after the earthquake which devastated the city of Christchurch. The huge iceberg crashed into a lake shortly after the 6.3 magnitude tremor rocked the South Island and created waves up to three metres high for 30 minutes which moved two sightseeing boats on the lake at the time.

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.

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