NZ on the move, changing shape

Scientists measuring how tectonic forces have stretched country

Twisting, warping, stretching — the dynamic tectonic forces responsible for New Zealand’s incredible landscape are changing our country every year.

Models have shown how the Wairarapa region is moving southwest at about 4cm each year relative to the Kapiti Coast, while also being gradually squashed downward.

Over the next few million years, it’s projected the South Island and the lower North Island will slim into a skinnier tract of land, while the North Island rotates and both islands grow closer together.

That can be put down to the perpetual scrum between the Pacific and Australian tectonic plates beneath the country, which have been responsible for hoisting modern New Zealand out of the ocean and driving ongoing uplift along the Alpine Fault.

Different regions were moving at different rates and in different directions — in some places this movement was increasing the stress in the earth’s crust, while in other places it is releasing stress.

But what happens to the picture when a major quake — or several of them in the space of a decade — strikes?

For six weeks over the summer, a team of researchers will hike across some of the South Island’s most remote spots to check on survey markers for the first time in eight years.

That involved setting up tripods with GPS instruments that could record locations down to millimetres, and leaving them to gather data for several days. They expect to find the usual background shifts, but also some lingering signs of the seismic upheaval New Zealand has experienced since a 7.8 quake hit Fiordland in 2009. — AAP

Two expeditions, visiting some 240 markers dotted across the central and western South Island, will come after scientists were able to see from a permanent array of 900 GPS sites how last year’s Kaikoura Earthquake had effectively shunted the country, instantly shifting Cape Campbell by 2m.

GNS Science geodetic surveyor Neville Palmer said the summer survey would fill in the gaps of that array, providing a denser, more detailed understanding of how much of the south had moved.

“For instance, there are still movements that are slightly non-uniform as a result of the Fiordland quakes that we had several years ago, and there will definitely be ongoing deformation for several years as a result of the Kaikoura earthquake, which will slowly decrease over time.”

“Obviously, they aren’t going to be as dramatic as they were immediately after the earthquake, but it’s giving us information on how those stresses and strains recover after an earthquake, and how quickly.”