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Quake pushes Blenheim to the east - 24/07/2013

GPS observations have shown that parts of Marlborough moved up to 5cm to the east during last Sunday's magnitude 6.5 earthquake.

The movement would have gone un-noticed to most people in Marlborough as their senses would have been fully taken up by the earthquake's strong ground-shaking at the time, GNS Science said.

The maximum amount of surface displacement occurred in a large area between Blenheim and the Awatere Valley, with smaller horizontal movements in other parts of Marlborough. Horizontal movement on the seabed in southern Cook Strait, above the epicentre, was up to 8cm.

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It's like grand central station down there. It's a challenge to understand the interactions among these faults

Dr Stephen Bannister

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By analysing data from 14 permanent GeoNet-operated GPS stations, between the outer Marlborough Sounds and Cape Campbell, researchers at GNS Science have been able to calculate the size and location of the fault rupture under southern Cook Strait.

It was a 19km-long northeast-southwest trending slash in the earth's crust. It's southern end came within a short distance of the coastline at Clifford Bay. The rupture plane was nearly vertical and came to within about 6km of the seabed.

The bottom of the rupture plane was about 18km below the seabed, indicating that the earthquake started at the bottom of the rupture and moved upward and outward, with its tearing action travelling at several kilometres-a-second.

"This solution for the geometry of the fault is consistent with the pattern of aftershocks," said seismologist Stephen Bannister of GNS Science.

However, it puts the rupture in between the London Hill Fault and the Hog Swamp Fault. Both are little-known on-land faults with no known offshore extensions.

"This new data is not a clean fit with either fault, so it raises the possibility that it could be a previously unknown fault," said Dr Bannister.

Little is known about these neighbouring faults other than the London Hill Fault is believed to rupture once every 3500 to 5000 years.

Previous mapping of the southern part of Cook Strait by NIWA had shown a complex network of faults on the seabed.

"It's like grand central station down there. It's a challenge to understand the interactions among these faults."

Owing to strong public interest in Marlborough, GNS Science seismologist Matt Gerstenberger is scheduled to give a public presentation on the quakes in Seddon on Thursday evening. Joining him will be Sarb Johal of the Joint Centre for Disaster Research, who will talk about the way communities respond to earthquakes.

Meanwhile, GNS Science has revised its aftershock probabilities for central New Zealand with most of the figures having declined sharply since Monday. They show for the next seven days there
is:

A 64% probability of a magnitude 5.0 to 5.9 aftershock – compared to 87% on Monday
A 10% probability of a magnitude 6.0 or higher aftershock – compared to 19% on Monday
And for the next 12 months, there is:

A 97% probability of a magnitude 5.0 to 5.9 aftershock – compared to 99% on Monday
A 30% probability of a magnitude 6.0 or higher aftershock – compared to 39% on Monday

The method for calculating these probabilities has been well tested on the Christchurch sequence of earthquakes over that past two years.

The zone covered by these calculations is a large box extending from Rai Valley in the west to Riversdale Beach on the Wairarapa coast in the east, and from Masterton in the north to Clarence on the Marlborough coast in the south..

Cook Strait Faults