Scientists fear a bigger earthquake could be in the wings.

They admit they were caught out by the location of yesterday's earthquake, expecting a serious jolt to occur further west.

Now they are warning the 4.36am 7.1 on the Richter Scale quake - centred 40km west of Christchurch and with its epicentre 10km south-east of Darfield - was not the big one they had been predicting.

GNS scientist John Ristau said while quakes of such magnitude were expected every so often in the South Island, the area of yesterday's quake was not known to be particularly seismically active.

The earthquake triggered a release of pressure caused by the collision and locking together of the Pacific and Australian plates, which are constantly moving in different directions.

''Stress builds up and then everything snaps,'' Ristau said.

Canterbury University geologist Mark Quigley said there was a network of blind faults under the Canterbury Plains that could not be detected on the earth's surface, but the location of yesterday's quake was unexpected and didn't match any known faults.

His students had located the scarp or stretch of displaced ground at the centre of the earthquake at Kirwee, 9km from Darfield. It was a long crack in the ground and about half a metre deep.

Other scientists said it was astonishing that for such a large, shallow quake so close to a main city, the damage in Christchurch was relatively light.

The quake was 10km deep, not surprisingly shallow, according to Quigley, who said aftershocks could continue for weeks but would gradually diminish in size and frequency.

But South Islanders can expect to be hit by a much bigger quake which would cause far more extensive damage, depending on where it was centred.

In an article in the Sunday Star-Times in February, Otago University geology professor Richard Norris said a huge earthquake, measuring about 8 on the Richter Scale, would hit the South Island within the next 50 years. He predicted it would be big enough to crush homes and cut electricity, water and sewerage.

Yesterday Norris said that would still happen, most probably along the Alpine Fault, a 400km stretch running from Milford Sound to the Lewis Pass.

''Yesterday's had nothing to do with the Alpine Fault and was not the big one,'' he said.

A magnitude 7.5 or 8 earthquake is expected to occur on the Alpine Fault about once every 300 years and it has been about 300 years since the last one.

The good news, according to Norris, was that the faults around Canterbury had a low rate of recurrence, producing earthquakes about once every 1000 years.

''But because there's a lot of them it's a lot more random and a lot more difficult to say which one will break.''

Norris said the Alpine Fault only released 70 percent of the seismic strain in the South Island. ''We've still got 30 percent displacement along myriad other faults. But why this one broke who knows.''

Teams of scientists have started arriving in Christchurch to capture seismic data from the continuing stream of aftershocks rocking Canterbury.

Up to 13 portable seismographs will be deployed from today by GNS Science, and Stanford University in the United States is sending more recorders.



Seismologists study aftershock sequences to find out more about the mechanics of the main shock and rupture, and to check whether if stress in the earth's crust has been transferred to other faults in the region.

'DECODING' THE QUAKE

The number of seismic recorders used may eventually number 40, and data from them will help to pin down the mechanics of yesterday's quake.



One theory is that if the "directionality" of the quake was away from the city, it may have been received a lesser impact, according to Professor Euan Smith of Victoria University.



Historically, seismologists have said the cities built on former river beds or even former swamps may have earthquake shocks effectively magnified as the soils act like jelly.



But data from Saturday's quake will also allow them to probe whether in some cases the water-filled soils may also be "dampening" some of the higher frequencies that would do the most damage to squat structures such as brick houses.

Such a shock-absorber mechanism would not reduce the impact on "longer" structures, such as chimneys or bridges.

- with NZPA