"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."
