Canterbury was rocked by another large, long aftershock on Wednesday afternoon.
The magnitude 5 quake struck at 4.42pm.
It was centred 20km west of Christchurch, at a depth of 15km.
It is not yet known whether the aftershock caused any further damage, but GNS scientists say it was felt throughout Canterbury and minor damage is possible.
Radio New Zealand staff in Christchurch say the aftershock lasted about 20 seconds and shook buildings in the city centre.
It followed a 4.1 magnitude quake at 1.30pm on Wednesday.
A 7.1 quake on 4 September caused widespread damage in Canterbury.