Tremors on path down country, say scientists

Aftershocks will continue, with reasonable chances of another big one, experts say.

A sequence of quakes rattling the centre of the country appears to be shifting down the South Island and away from Wellington.

But those in the capital are expected to feel aftershocks for some time yet and there is still a reasonable chance of a third big shake, with scientists putting the chance of another plus-6 magnitude quake this week at one in five, and 50/50 for the rest of the year.

And amid plenty of quakes large enough to be felt, they were likely to get at least one jolt registering between 5 and 5.9 - GNS Science puts the probability of this is at 90 per cent.

"The likelihood of another large quake is still really high," GNS duty seismologist Dr Caroline Holden said yesterday.

At least 15 small earthquakes rattled central New Zealand overnight following Friday's magnitude 6.6 shake near Seddon.

According to Geonet, there were 15 aftershocks which were likely to have been felt since midnight, the largest being a magnitude 3.7 at 2.02am 20km east of Seddon.

Dr Holden's colleagues had observed a progression of quakes that had shifted from Cook Strait, where the July 21 6.5 quake was centred, southward down the mainland.

New data showed how the handful of recent large quakes, including Friday's 6.6 quake, were gathered around inland Marlborough.

It was believed the latest activity was centred on a subsidiary fault southwest of the Awatere Fault, which runs through the small town of Seddon and produced a large quake in 1848.

"We are seeing the sequence progressing to the south rather than the north - that's something we can be quite comfortable with," Dr Holden said.

But indicators called focal mechanisms - which show what types of earth movement have occurred, based on high-quality seismic data - also showed that many of the quakes since late July had varied individual characteristics.

GPS stations have also revealed that Cape Campbell, on Marlborough's northeast coast, has moved 15cm west since Friday, while stations to the north had moved a few centimetres east.

"That's a pretty decent-sized displacement," Dr Holden said.

This movement was put down to the "strike-slip" nature of the quakes, where each side slides past the other without uplift and down-thrust.

In the case of this sequence, entirely horizontal shifts across the fault had sent southern parts west and northern parts east.

If a large city had been located above the epicentre of Friday's quake, near Lake Grassmere, there was no doubt it would have caused significant damage.

"The difference with Christchurch was there were people right above the epicentre, but in Seddon the closest populated areas from Lake Grassmere was Ward, which is 13km away," Dr Holden said.

"But then it was still large enough to make me get under my desk."

The chances

Cook Strait quake forecasts
*Magnitude 6+: This week: 21%. This year: 51%
*Magnitude 5.0-5.9: This week: 90%. This year: 99.9%
*Magnitude 4.0-4.9: This week: 99.9%. This year: 99.9%