Little fault blamed for big jolt

A little fault that packs a big punch is behind Friday's magnitude-6.6 quake, seismologists say.

Under Lake Grassmere in Marlborough, it was thought to be between 12 and 14 kilometres long, shorter than the average fault capable of producing magnitude-6 earthquakes, GNS geoscientist Bill Fry said.

Some research implied strong quakes on short fault lines indicated high levels of stress in the ground, meaning seismic activity was likely to continue, though expert opinion was divided.

July's quakes were clustered further out in Cook Strait, while Friday's tremor was focused beneath Lake Grassmere, southeast of Seddon. It was thought they were on different faults, but scientists were investigating whether it was actually one fault that was kinked, or if that part of the earth's crust simply had a "continuous weakness" in it, he said.

With more than 3500 aftershocks since July 19's 5.7-magnitude quake, a clearer picture was emerging about the larger fault system, showing seismic activity moving away from the capital.

While GNS pinpointed the locations of large earthquakes on faults, it was more important to think in terms of a seismic network, Dr Fry said.

"There are a lot of faults in the upper South Island that could potentially give rise to earthquakes."

The Cook Strait quakes' sequence - a larger 6.6 quake following the 6.5 shake in July, known as a "doublet" - was also unusual, he said.

Seismologists were keeping a close eye on how the rumblings might affect the other fault systems, including the Wellington fault, thought to be capable of a devastating magnitude-8 shake.

"We're actively looking at these faults, trying to understand the implications," Dr Fry said. "The first evidence you have that something's getting closer and closer to failure is maybe you start getting small earthquakes on it."

So far, GNS had seen no higher-than-normal seismic activity on any fault.

A 4.6 shake at 5.37pm on Friday centred 15km north of Wellington was likely to be connected to the current slow-slip quake occurring near the Kapiti coast.

Complex calculations also allowed scientists to get an idea of how much stress might be transferred from one fault to another - not that such numbers would leave scientists much closer to a full understanding of when such faults might rupture, Dr Fry said.

"It's like dumping a cup of water into a swimming pool and asking how much water is in the pool. It's one cup more than it previously was, but I can't tell what it is in total."

FUTURE QUAKE PROBABILITIES

TODAY 3.5 per cent chance of a magnitude 6.0 or higher 30 per cent of a magnitude 5.0 to 5.9

THIS WEEK 14 per cent of a magnitude 6.0 or higher 76 per cent of a magnitude 5.0 to 5.9

EQC CLAIMS

More than 400 insurance claims for damage from Friday's earthquake had been received by the Earthquake Commission by yesterday afternoon.

About 5000 claims have been lodged from the July 19 and 21 earthquakes. EQC estimated the damage for both quakes would be up to $100 million.

Claims can be made through the EQC on 0800 DAMAGE (0800 326 243)

The Dominion Post