Jamie Morton

Jamie Morton is science reporter at the NZ Herald.

Shaky Isles are also moving isles

GPS data from around 900 marker sites reveals regions are shifting about 4cm a year relative to each other.

New Zealand is moving by 4cm a year, new satellite surveys have revealed.

But rather than shifting as a whole, tectonic forces are deforming the land surface - stretching, slimming and sliding it southward.

GPS surveys taken over summer by a team of GNS Science geodesists show much of the Wellington region is marching southwest at a rate of about 4cm a year.

The survey, carried out last month at around 160 survey marks in the lower North Island, adds to an emerging picture which has shown much of the Wairarapa region is moving southwest at about 4cm each year relative to the Kapiti Coast.

There are also small changes in height as the region is gradually squashed horizontally by tectonic forces.

The project forms part of an annual campaign which moves around eight areas of New Zealand, ensuring the entire country is accurately mapped every eight years.

Different regions are moving at different rates and in different directions. In some places this movement is increasing the stress in the earth's crust, while in other places it is releasing stress.

A simulation, showing the country's distortion over a four-million-year period without allowing for earthquake impacts, projects much of the South Island and the lower North Island slimming into a skinnier tract of land, while the North Island rotates around and both islands grow closer together.

"The Bay of Plenty is stretching apart by around 4cm each year, while Wellington is kind of getting squeezed together by a similar amount," said GNS geodesist Neville Palmer, who recently completed a survey of Northland.

Nearly two decades of GPS technology have revolutionised the way scientists can model our ever-changing landscape, something that was impossible with older survey techniques.

They draw data from around 900 marker sites across the country, with 150 markers also streaming GPS locations to GeoNet monitors.

"Because we are straddling the boundary between the Pacific and Australian plates, there is a lot of deformation happening within New Zealand, and this is also happening globally," said Mr Palmer.

"So when you look at the boundary areas - New Zealand, Japan, the west coast of the United States - these are all areas where you get a lot of movement and earthquakes."

GPS monitoring had also identified what are called slow-slip events - earthquakes that occur over days and weeks, rather than seconds or minutes, releasing energy over a protracted period, he said.

The technology showed scientists that the original Darfield earthquake in September 2010 - an opener for thousands of Canterbury quakes that would include the devastating 2011 shake that claimed 185 lives - occurred at three slightly different faults almost simultaneously.

The dramatic geological effects of the quakes meant geodesists had to increase their GPS monitoring in Canterbury to every few months, Mr Palmer said.

The Taupo Volcanic Zone was also measured more frequently as the geological processes were occurring faster and there was more variation than in other regions.

- NZ Herald

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