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‘High likelihood’ of another big Christchurch shake

Quake-weary Cantabrians may face another large aftershock after Government scientists said there was a “high likelihood” of another 5.4 magnitude shake.

GNS Science say there is an 88 per cent chance of a 5.0 to 5.4-mag aftershock – up 12 per cent from their September calculations.

The scientists say the 5.5-mag earthquake on October 9, which rattled Cantabrians watching the All Blacks Rugby World Cup semifinal against Argentina, has helped push the probabilities upwards.

GNS natural hazards manager Dr Kelvin Berryman also put the rise down to a new method in forecasting quakes.

“There is a high likelihood of a 5.0 to 5.4-mag aftershock,” he said.

“There is perhaps a 50/50 chance of something a bit bigger – a 5.5-mag or 5.9-mag and there was a 5.5-mag on October 9. So, it shakes people up a bit, though there was no damage from those sorts of magnitude.

“In September, there was a 10 per cent probability of a magnitude 6.0 to 6.4 quake, and that is now 15 per cent – that number, in my mind, is essentially the same number because of the 5.5-mag on October 9 which has spiked the numbers slightly.”

GNS also calculated a 4 per cent chance of another 6.5 to 6.9-mag event in the future, and a one in 100 chance of a 7.0-mag event or higher, like the one that struck last September.

Dr Berryman said the figures included the entire Canterbury aftershock zone, not just Christchurch city.

He added: “The message from this is that the numbers are very small.”

GNS has also adjusted its earthquake forecasting modelling technique for the Canterbury Plains region to reflect a longer-term outlook.

They say the updated figures released today are based on a combination of two different statistical forecasting models.

Until recently, the quake probabilities have been dominated by what is known as a short-term clustering model.

But last month, the medium-term clustering model started to exert a bigger influence on the calculations, GNS says.

They will now use a 50-year forecast model, which will be used to inform any updates to building codes, and for insurance companies.

Dr Berryman said: “We are now 13 months on and we have a better understanding to contemplate the longer term part of the earthquake sequence.

“Nothing has changed inside the earth to increase the risk of an earthquake – it’s just that there is a change in the way the probabilities are calculated.

“This is a very rich aftershock sequence. But the earthquake sequence is decaying away, as people of Christchurch will attest to.”



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