24 Jul 2013

Quake moved parts of Marlborough

10:08 pm on 24 July 2013

GNS Science says parts of Marlborough have shifted 5cm to the east after Sunday's 6.5-magnitude quake.

The research institute has been analysing data from 14 GPS stations to calculate the size and location of the fault rupture under southern Cook Strait that also hit Wellington.

GNS said on Wednesday the rupture was a 19km-long north-east to south-west slash in the Earth's crust, causing up to 8cm of horizontal movement on the seabed above its epicentre.

Parts of Marlborough moved up to 5cm to the east, with the greatest amount of surface displacement in a large area between Blenheim and the Awatere Valley.

However, seismologist John Ristau told Radio New Zealand's Checkpoint programme the level of movement is normal for a quake that size. He said during the 2010 Darfield quake in Canterbury, the ground moved more than 1 metre.

Mr Ristau said it's not yet clear whether the tremor was generated by the London Hill Fault, the Hog Swamp Fault or a previously unknown fault.

In Wellington, the city council says it is aware of damage to 28 buildings in the CBD. Sections of Featherston Street remain behind barriers and traffic is down to one lane while building repairs continue. The BNZ bank says its Harbour Quay building will be closed for the next six to eight weeks due to extensive water damage.

GNS has revised its aftershock probabilities for central New Zealand, with a 10% probability of a magnitude 6 or higher aftershock in the next seven days, down from 19% on Monday.

Meanwhile, a research vessel has completed mapping parts of the sea floor where the swarm of quakes that hit Wellington and Marlborough were generated.

The National Institute of Water and Atmospheric Research vessel Tangaroa carried out the work in Cook Strait on Wednesday.

NIWA marine geologist Scott Nodder said the studies would reveal where exactly the recent quakes have been occurring and from which fault line.

"We're trying to look to see if there's a surface rupture in the sediments underlying the sea floor there. We've also conducted a second survey up the axis of the Cook Strait canyon to try and see if any material has been dislodged from the canyon walls."

Mr Nodder said the results would be compared to previous surveys and the findings released in about a week.