Magma chamber behind quakes

Between 2004 and 2011, the town of Matata - population 642 - had mysteriously been the centre of several thousand earthquakes. Photo / iStock

Scientists have found a startling reason for the thousands of quakes that have swarmed around a tiny eastern Bay of Plenty settlement over recent years - a huge magma chamber lying just a few kilometres beneath it.

Between 2004 and 2011, the town of Matata - population 642 - had mysteriously been the centre of several thousand earthquakes.

Until now, scientists suspected that the cause of the quakes, most of them measuring only between magnitude 2 and 4, and at depths of between 2km and 8km, were due to the typical triggers of tectonic movement underground.

But new findings, published today, have pinpointed a more dramatic explanation for why a 20km-square area of land around the town has been pushed up by about 40cm since 1950.

Over this period, molten or semi-molten rock was being pushed up from below, causing land around Matata to uplift by about a centimetre each year.

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

Using a model based on modern GPS and satellite radar data, along with decades of survey records, scientists have concluded a magma body lies about 9km below the surface - and since 1950 its volume had grown by the equivalent of 80,000 Olympic swimming pools.

However, they say the presence of the magma did not mean an eruption could be imminent, nor had it changed the volcanic hazard of the Bay of Plenty region.

The lead author behind the new study, GNS Science satellite image specialist Dr Ian Hamling, said the findings highlighted that accumulations of magma were not always marked by volcanoes on 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.

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

"There is every possibility the magma body under the Bay of Plenty coast had been there for centuries, and possibly even longer."

Observations of inflating magma bodies at depth were a reasonably common feature of volcanic areas worldwide.

There were cases overseas where the surface inflation rate had been considerably faster than at the Bay of Plenty coast, but with no associated volcanic unrest, Dr Hamling said.

"While there is absolutely no evidence pointing to volcanic unrest in coastal Bay of Plenty, this finding underlines the fact that we live in a geologically active country where it pays to be prepared."