A 6.3-magnitude earthquake shook the New Zealand city of Christchurch on Monday. It was preceded about an hour earlier by a 5.6-magnitude foreshock.
The tremors are aftershocks from the 6.3-magnitude quake that unexpectedly hit the city on 22 February.
This time, several buildings collapsed and power was disrupted. The greatest force of the quake hit areas that were already damaged, however, leaving most of the city centre unaffected.
The recent earthquakes struck only 3 kilometres south of the epicentre of February's quake. But Bill Fry, a seismologist at the Institute of Geological and Nuclear Sciences (GNS Science) in Lower Hutt, New Zealand, says they occurred on "a completely different fault line".
Since February, GNS Science has been monitoring aftershocks in the New Zealand region of Canterbury, in which Christchurch lies. These observations revealed a "distinct cluster" of aftershocks in the new region, suggesting this is a new fault line, says Fry.
Moving and shaking
"The ground motions were also quite different in the two events," he says. Most of the movement in the February quake was caused by the tectonic plates thrusting over each other, but this time, the plates were moving "from side to side". At their fastest the plates accelerated at 20 metres per second2.
According to Fry this fault line was activated by February's quake, which triggered a lot of movement in the plates.
John Townend, a seismologist at Victoria University of Wellington, New Zealand, says the recent quake could "re-energise the system" and create a new wave of aftershocks in the region. Christchurch has been trembling since February, with minor quakes shaking the town almost every day, but this is the largest aftershock yet recorded.
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