The probability of another moderate earthquake in Canterbury has jumped 50 per cent, as GNS Science acknowledges the region may be subject to an extended aftershock ''cluster''.

GNS natural hazards research manager Kelvin Berryman today stressed the increased probability was a change in scientific approach and nothing had shifted beneath the ground.

''The aftershocks are decreasing but because of the way we have produced this combined model the numbers are slightly up,'' he said.

The new model shows a 15 per cent chance of Canterbury being struck by an earthquake between magnitude 6 and 6.4 - up from 10 per cent recorded under the old model in September.

There was also a 46 per cent chance of another tremor between 5.5 and 5.9, up from 37 per cent.

The change came after GNS moved from using a standard pattern for the decline of aftershocks to including the remote possibility that the region would not follow a standard pattern.

This more unusual ''cluster'' pattern, which would trigger new faults and aftershocks over a longer period, was unlikely but should not be discounted in Canterbury, he said.

''It has always been in our thinking because of the very rich aftershock sequence (in Canterbury),'' Berryman said.

It would be at least six months before it was clear which pattern the Canterbury quakes were following.