Yesterday's magnitude 6 earthquake is likely to produce its own swarm of aftershocks, delivering a number of earthquakes between magnitude 4 and 5 in the coming days and weeks, scientists warn.

But although aftershock activity has been "rejuvenated" it still follows the pattern of aftershocks set off by September's 7.1 magnitude quake.

"They're following the same trend. The type of faulting that's happening is very similar to the February 22 earthquake," GNS seismologist Dr Martin Reyners said.

"All these earthquakes are responding to a very uniform regional stress field."

But Dr Reyners admitted the sheer number of aftershocks meant scientists were in "unknown territory".

"There have been way more aftershocks than a normal magnitude 7.1 [earthquake] would produce. So we've been revising our stats on what's going to happen as we go along.

"It's a very rich aftershock sequence, that's all we can say."

He said the earthquakes seemed to mark the boundary between "very strong" rocks underground and volcanic rocks formed six to 12 million years ago.

The epicentre of yesterday's magnitude 6 earthquake was 10km southeast of Christchurch, in the middle of the Port Hills above Sumner. It struck at 2.20pm at a depth of 9km.

The earthquakes were a combination of up-and-down ground movement - "reverse faulting" - and side-to-side movement known as "strike slip". The same movement featured in the September 4 and February 22 earthquakes.

Yesterday's magnitude 6 and 5.5 aftershocks were within the range of forecast aftershocks modelled by scientists.

GNS had predicted at least two earthquakes of magnitude 5 or larger around the middle of May to June, GNS seismologist John Ristau said.

"We also said not long after the February earthquake that there was up to a 25 per cent chance of there being another magnitude 6 earthquake within the year."

But while the swarm of earthquakes would make many Christchurch residents feel under siege, Mr Ristau said it didn't mean the chances of another big one had increased.

"The people who do their earthquake forecasts will have to go through and make their calculations again, but it's probably unlikely to change [the probability]."

The two aftershocks were centred on the coast and a short distance south of the fault that produced the magnitude 6.3 quake on February 22.

Smaller aftershocks began in the same area four days after September's earthquake.

It is not yet known in which direction yesterday's earthquakes "unzipped" from their faultlines.

"That can take quite a while for us to figure out, if it's even possible."

Shakes' strengths in numbers

The strongest earthquake to strike yesterday was rated one point below February's on a scale designed to measure an earthquake's effects on people and their environment.

The Modified Mercalli intensity scale runs to 12. The strongest of yesterday's quakes was rated 8, "heavily damaging", while February's earthquake was rated 9, "destructive".

"It's more of a descriptive scale, based on how people feel the earthquake," said GNS Science seismologist Dr John Ristau.

Magnitude measures the amount of energy released during an earthquake, but not all such energy will necessarily be felt at the surface.