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NEW ZEALAND is riddled with major active faults, but it seems the fatal 6.3-magnitude earthquake that hit Christchurch this week was caused by one that was not on the list.
鈥淐hristchurch has never been identified as a major earthquake zone, because no one knew this fault ran beneath,鈥� says , a seismologist at the British Geological Survey in Edinburgh.
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The that runs along the mountainous spine of South Island marks the boundary between the Australian and Pacific plates. It now appears likely that the Christchurch quake resulted from a previously unknown fault extending directly eastward from the Alpine fault.
It first came to light last September when a stronger but less calamitous quake shook Darfield, 40 kilometres west of Christchurch. Seismologists believe the latest quake resulted from an eastward continuation of activity on the same fault. 鈥淚t has probably not moved for tens of thousands of years, so lots of strain built up,鈥� says Musson.
Two factors made the damage worse, he says. The quake was just 5 kilometres down, limiting the amount of energy it dissipated before reaching Christchurch from its epicentre just 10 kilometres away. Also, the rock on either side of the fault accelerated almost three times as fast as in a typical quake, so the shaking was extra violent 鈥� and significantly greater than the levels Christchurch鈥檚 structures have been designed to withstand, according to local reports.