The latest big quake comes after more than 15,000 others, most of them too weak to be felt, in an aftershock sequence triggered by the 7.1 Darfield earthquake on September 4, 2010.

That earthquake, which triggered the sequence that included the February 22, 2011, quake that killed 185 people, was caused by strike-slip faulting, where two blocks within the crust of the Pacific Plate violently tore past each other, near the eastern foothills of the Southern Alps, at the western edge of the Canterbury Plains.

It set in motion activity in a web of faults beneath Canterbury - most of which are still probably unknown to scientists - that has carried on with nearly 1000 recorded quakes in the past 12 months.

"You can trace it all back to that original Darfield quake," Geonet seismologist Dr John Ristau said.


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"Although it's been a while since then, the probability of having another aftershock of this magnitude had still been terribly high."

Today's quake was another case of "strike-slip" - a side-by-side motion rather than an up-and-down one - that typified Canterbury's recent big earthquakes.

Interestingly, though, its motion was unlike the reverse-faulting of most of the large aftershocks recorded in the same area in December 2011.

Dr Ristau said it was likely the quake happened on a previously unknown fault.

"Particularly because this quake was offshore in Pegasus Bay, the actual chances of us having mapped the specific fault in that area are much slimmer," he said.

"I guess the one fortunate thing you can take away from this is that because it happened a few kilometres offshore, no one was going to be sitting right on top of it, which is going to help minimise the damage."

By contrast, the 6.3 2011 Christchurch earthquake struck just 10km east of the city centre and 5km below ground - and with the force of 15,000 tonnes of TNT.

Christchurch has been shaken by a 5.7 magnitude magnitude quake at 1.13pm this afternoon. The severe quake was centred 15km east of the city and was felt as far north as Wellington.

The highest recorded peak ground acceleration - measuring the intensity of the shaking - of today's quake was 0.4g, while the February 2011 quake hit 2.2g at its most violent point.

However, the last time Christchurch experienced a quake this large was nearly four years ago, with the magnitude 5.2 quake 20km east of Christchurch.

Aftershock forecasts published before today's quake estimated the likelihood of a magnitude 5.0-5.9 quake as 49 per cent in the coming year for the Canterbury region.

The likelihood of a magnitude 6.0-6.9 quake is 5.7 per cent in the coming year, and there was less than a 1 per cent likelihood of an earthquake larger than magnitude 7.0.But those figures could change as a result of today's quake, GeoNet reported.

Trying to better understand the nature of earthquakes in Canterbury - and the potential for another huge quake in the region - has been a major ongoing focus of geologists and seismologists since 2010.

In a recent interview with the Herald, Victoria University earthquake scientist Associate Professor John Townend said what controlled the timing of the aftershocks that follow a large earthquake like the Darfield one was still not clear.

"A real challenge in understanding earthquakes is reconciling long-term estimates of seismic hazard with much shorter-term and rapidly changing hazards posed by aftershocks," he said.

"At the heart of this challenge is improving our understanding of earthquake physics - determining what processes control the stresses acting on faults, and how those stresses change in space and time."