The shaking in Christchurch city from aftershocks on Boxing Day was more severe than from the original earthquake in September.
The quakes damaged 115 buildings. Glass, bricks and masonry lay in the streets afterwards.
The strongest of the quakes, which damaged 115 buildings, was magnitude 4.9 at a depth of 12km.
GNS Science says although the shaking did not last as long in places, it was more violent than the original earthquake on 4 September.
Dr Ken Gledhill of GNS Science says the shaking was so severe because the aftershocks were within 5km, whereas the September quake was centred 40km away.
He says clusters of aftershocks will continue for months, but will happen less often.
Twenty-two quakes have been recorded since early on Sunday, most within 5km of Christchurch.
4 September the big one
A magnitude 7.1 earthquake occurred in Canterbury at 4.35am on 4 September.
A state of emergency remained in force until 16 September and the central business district in Christchurch was cordoned off for several days. Aftershocks have been occurring ever since.