Deep pool of magma caused Bay earthquakes

A swarm of earthquakes in Matata between 2004 and 2011 was caused by the growth of a magma chamber under the coast, GNS Science has revealed.

Scientists used a combination of satellite images, GPS data and conventional survey information dating back to the 1950s to uncover the reason for the swarm of several thousand small earthquakes over that period around the coastal Bay of Plenty town.

The three streams of data showed that an area of land around Matata - roughly 20km by 20km, about half of which is offshore - had been pushed up by about 40cm since 1950.

Until now the cause of the swarm was thought to be tectonic.

The new finding concluded the swarm was caused by the growth of a previously unrecognised magma chamber under the Bay of Plenty coast.

During this period, molten or semi-molten rock was being pushed up from below and caused land around Matata to uplift by about 1cm a year. As the magma moved in the sub-surface, it caused the surrounding rock to deform and break, resulting in small earthquakes.

Using modelling, scientists concluded a magma body lies about 9km below the surface. Since 1950, it has grown in volume by 0.2 cubic kilometres, or equivalent in size to about 80,000 Olympic swimming pools.

Scientists say the presence of a magma body does not mean an eruption might be imminent and it has not changed the volcanic hazard of the Bay of Plenty region.

The research has just been published in the science journal Science Advances.

Lead author Ian Hamling, a satellite image specialist at GNS Science, said the finding highlighted that accumulation of magma was not always where we might expect, based on the location of volcanoes at the surface.

"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," Dr Hamling said.

He added that bodies of magma were reasonably common under large areas of the central North Island, and identifying another magma accumulation was not a huge surprise. "There is every possibility the magma body under the Bay of Plenty coast had been there for centuries, and possibly even longer."