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Magma is moving under Matata

NZ NewswireNZ Newswire 3/06/2016

Magma pushing the land up around the coastal Bay of Plenty town of Matata has caused thousands of earthquakes but that doesn't mean there'll be an eruption, scientists say.

The scientists used satellite images, GPS data and conventional survey information dating back to the 1950s to examine several thousand small earthquakes near Matata between 2004 and 2011.

They decided the quakes were not caused by the movement of tectonic plates, as previously thought.

As the magma moved, it caused the surrounding rock to deform and break, resulting in small earthquakes.

The magma body is about 9km below the surface, and an area of land around Matata roughly 20km by 20km, about half of which is offshore, has pushed up by about 40cm since 1950.

The research titled "Off-axis magmatism along a sub-aerial back-arc rift" has just been published in the science journal Science Advances.

Most of the quakes were between magnitude two and four and occurred at depths between 2km and 8km.

The new finding concludes the swarm was caused by the growth of a previously unrecognised magma chamber.

The molten or semi-molten rock has caused land around Matata to uplift by about 1cm a year.

The scientists said the presence of a magma body did not mean an eruption was imminent and it had not changed the volcanic hazard of the Bay of Plenty region.

"Our modelling points to the presence of a magma chamber in an area where there has been no active volcanism for about 400,000 years," says lead author Ian Hamling.

He said bodies of magma were reasonably common under large areas of the central North Island, and identifying another magma accumulation was not a huge surprise.

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