A sequence of quakes rattling the centre of the country
appears to be shifting down the South Island and away from
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.
Her 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
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
"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
"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
In the case of this sequence, entirely horizontal shifts
across the fault had sent southern parts west and northern
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
"But then it was still large enough to make me get under my
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%
Data is subject to change.
- Jamie Morton of the New Zealand Herald