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Monday's quake upgraded to 7.8

NZ Newswire logoNZ Newswire 16/11/2016

The monster earthquake that struck north Canterbury on Monday morning just got bigger - it's been upgraded from magnitude 7.5 to a 7.8.

Acting Civil Defence Minister Gerry Brownlee said government agency GNS Science revised the magnitude on Wednesday after reassessing data from across the country.

"Because it took over a minute for the fault to rupture during this event, the standard method normally used to calculate the energy released during an earthquake was insufficient," he said.

"It does not change what happened or how central government or local authorities responded. It simply provides us with more knowledge about how significant this earthquake was."

However, the larger magnitude meant "it is now forecast that aftershocks may be larger in magnitude for a longer period of time," Mr Brownlee said.

GNS Science spokesman Kelvin Berryman says so far the aftershocks are behaving as expected for a seven-plus earthquake.

"Going forward, the most likely scenario is that aftershocks will gradually diminish in size and frequency," he said.

"But we should also always be prepared for the low likelihood, the possibility, of another large earthquake in the aftershock region."

Dr Berryman says a good sign is that the aftershocks haven't extended beyond the area of the earthquake.

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