New Zealand's Christchurch weathered a 7.1 earthquake on September 4, 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, which has left 75 dead and nearly 300 people 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 September quake damaged 100,000 buildings and left a major repair bill, but caused no deaths, after striking at 4.35am, when most people were safely in bed.
But this week's tremor hit at the worst possible time, at lunch on a weekday, when offices were open and streets were busy with shoppers who were vulnerable to falling masonry.
Its epicentre was only five kilometres (three miles) 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.
Newer office blocks such as the CTV and Pyne Gould buildings collapsed, while the towering Grand Chancellor Hotel was tottering dangerously.
Australian Seismological Centre director Kevin McCue also 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.
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.