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Saturday's earthquake took scientists by surprise, as it revealed a new faultline in a territory not known for seismic activity.

GNS Science staff yesterday found signs of the quake's epicentre, 14km south of Darfield, which lies 50km west of Christchurch.

Aerial surveys revealed a dark, 24km-long scar across paddocks and roads where the shallow tremor offset the land.

The faultline cut roads 4m apart, dug up mounds of earth, and in some places formed a waist-high step in the land.

GNS regional geologist Simon Cox said there were several known active faults under the Canterbury Plains and in the Canterbury foothills, but this earthquake came from a previously unknown, buried faultline.

"There is no evidence at this site for previous rupture. We don't think it has ruptured often, or at all, in the last 18,000 years."

While the tremor could awaken surrounding faults, it could also relieve pressure on the earth.

"We just released a lot of stress over this side. It's kind of like snapping an elastic band. Once you pull it out, and it snaps, hopefully most of the pull you've got in the band has gone," Dr Cox said.

"The earth has released a lot of energy. And you'd much rather have it happen at 5am than at a time when everyone was walking around downtown Christchurch. It's a good one from that perspective."

GNS reported that the 7.1-magnitude tremor was the eight-strongest on record in New Zealand and caused the most damage since the 1931 Napier/Hastings quake that claimed 256 lives.

The worst of the aftershocks, one of which was measured at 5.1, were expected to be over by today.

Smaller aftershocks were likely to continue for weeks, diminishing in size and frequency.

Seismologists had predicted that the South Island was due for a massive earthquake within the next 50 years. But that quake was expected on the 400km Alpine fault, which runs between Milford Sound and the Lewis Pass.

Because Saturday's earthquake occurred in new territory, that "big one" could still happen in the next 50 years.

GNS seismologist John Ristau said the large quake was a "rude wake-up call" for the rest of the country, especially Wellington, which lay on a major faultline.

"It had been 80 years since a highly damaging earthquake had occurred in the country. People tend to get a bit complacent about it."

Dr Ristau suspected that three earthquakes occurred nearly simultaneously on Saturday. A magnitude-5.4 jolt was followed five seconds later by two larger quakes 10 seconds apart. The larger, 10km-deep one caused the earth to shift horizontally, before being thrust up vertically.

Scientists will place 40 instruments near the faultline this week to determine whether stress was transferred to other faults.