'Silent' quakes moving us closer to South America

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A newly-discovered type of quake below the east coast has heaved the North Island closer to Chile - but only by the length of a pineapple lump.

Data collected from GPS stations has revealed a powerful, slow-burning earthquake has been shifting the earth beneath the Gisborne area over the past two weeks.

This silent or "slow-slip" quake, which scientists believe is now winding down, has been large enough to shift the east coast about 3cm horizontally eastward.

GNS Science seismologist Dr Stephen Bannister said the last time the area experienced an earthquake of this type and size was four years ago. It's the latest case of a large slow-slip quake beneath the North Island, which is being warped at centimetres at a time thanks to these little-understood underground phenomena.

Discovered only after the advent of GPS technology, slow-slip earthquakes can pack the punch of quick-fire quakes such as that which shook Canterbury in 2010, yet happen at such a slow rate that they are not felt on the surface.

Deep underground, the Pacific Plate dives westward below the eastern North Island, which is pushed to the east when the plate interface is locked.

While violent quakes can push along earth in a matter of seconds, slow-slip events can do so harmlessly over weeks to months, and scientists believe they take care of about 40 per cent of our total quake movement.

"So that's an enormous amount that doesn't need to be taken up by large earthquakes."

Further, the silent quakes may be relieving tension built up by the plate movement, reducing the risk of big shakes.

"The more that we learn about slow-slip events, the more it changes the way we think about our hazards - especially along the east coast," he said. "It really has quite a large impact on the way we think about things for the future."

While they could change landscapes enough to affect property surveys now, it might take a little longer to haul our country to South America.

"We might get there in a few hundred million years or so."