GNS report says Nelson is sinking

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Tracy Neal

Radio New Zealand

Nelson, April 9, 2017

A GNS scientist says monitoring of land movement since 2004 shows the Nelson region has been sinking fractionally each year, but the Kaikura earthquake lowered it instantly by a further 20 millimetres.

A paper by GNS Science published in an international science journal shows that last Novembers magnitude 7.8 quake shunted parts of the South Island more than five metres closer to the North Island, and that some parts were raised by up to eight metres.

Noticeable drop

Geodetic scientist Sigrun Hreinsdotti said Nelson was relatively close to the epicentre, and even though the drop in the area was minimal, it was enough to be noticeable.

Nelson was fairly close to the earthquake and what we see is a little drop of the area, so the whole area is going down maybe 10 to 20 millimetres. It is not a huge amount but it is observable at our sites, she said.

She said that GNS used a mix of its own and private GPS recording stations positioned all over the country. She said they recorded continuously, and the effects of the quake could be seen from Fiordland up to Northland and in the Chatham Islands.

Auckland had moved 3mm in comparison to its position with Australia.

Ms Hreinsdotti is from Iceland and has been in New Zealand for just over three years. She said her field of work in geodetics (earth measurement science) meant she was often in seismically active locations.

I was in Alaska in 2002 when we had a 7.9 and the quakes were similar. I am in Wellington and it felt the same as it did in Alaska, she said.

The powerful earthquake ruptured the earths surface for 336 kms along a couple of major fault lines.

Enduring impact

Ms Hreinsdotti said that while the Kaikura quake alone did not have a huge impact on Nelson, the slow slip quakes off the Kapiti Coast directly north of Nelson will have the most enduring impact.

She said that it was part of living on a tectonic boundary, and could prompt local authorities to think more about their forward planning.

Everywhere on Planet Earth should be thinking about long-term changes. It is a reality that we have to think about sea level rise, and for New Zealand where we have a tectonic boundary with active deformation, we have to think about which regions are going up and which are going down in a 100-year time scale.

A Nelson City Council spokesperson said the Council had not seen the information and could not comment until it had a chance to properly consider the issue.

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Photo Caption:

GNS monitoring shows that the Kaikura earthquake lowered the Nelson region by 20 mm.Photo: RNZ/Tracy Neal

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