The growth of a magma chamber was responsible for a seven-year swarm of earthquakes in the Bay of Plenty, a GNS study has found.
The study found the previously unrecognised magma body caused several thousand small earthquakes between 2004 and 2011.
Maps indicating (left) the study area and (right) the modern Taupo Volcanic Zone (grey transparency), Okataina and Rotorua Caldera boundaries (dashed lines) and relocated 2005-2009 Matata earthquake swarm. Photo: GNS / Science Advances
The expansion of the molten rock chamber approximately 9km below Matata has pushed up 400 square kilometres of land by 40cm, the research published in Science Advances said.
Until now the cause of the swarm was thought to be tectonic.
GNS scientist and lead author Ian Hamling said he discovered the uplift when reviewing modern satellite images and GPS data, and conventional survey information dating back to the 1950s.
This suggested something was accumulating at a depth of around 9.5 to 10km beneath the Earth's surface.
Dr Hamling said the magma underneath Matata accumulated at a rate equivalent to 3600 hundred Olympic swimming pools each year between 2004 and 2011, pushing up the surface.
"As the magma is pumped into the crust ... it causes the rocks around to deform and bend. They can then break.
"That's why we think you get these swarms of earthquakes during these periods where the ground gets this increase in the amount of magma that's coming into the system."
The active White Island volcano in the Bay of Plenty Photo: AFP
He compared the change in the Earth's crust under pressure of the accumulating molten rock to the movement of a blanket on a slowly inflating mattress.
"If you had an inflatable mattress and you were to cover it with a duvet or something, as you pump that up the [duvet] on top warps as the pressure gets increased in the mattress.
"You're essentially putting an increasing pressure into the rock mass which causes a volume increase, and the only way that can be accommodated is then for the ground or the rocks around it to get pushed out of the way ... which we see at the surface with the uplift.
"In doing that, some of these rocks then break and trigger some of these earthquakes," he said.
Dr Hamling said that did not mean an eruption might be imminent, and the finding had not changed the volcanic hazard of the Bay of Plenty region. .
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.
The magma body could have been there for centuries or more, he said.
The area was a "rift zone" where over time the crust had been stretched and thinned - but it was not clear whether the crust was already thin, or the magma made it thin.
"It is probably a thinner crust than the average you'd find elsewhere," he said. "But what happens is as you stretch the crust and the hot rocks beneath come to a shallower depth, as they become shallower they get less pressure, which actually then enables them to melt and become magma."
"That magma, because it's less buoyant than the surrounding rock, it then wants to percolate up through [into the crust].
"That's what we're seeing here where [the magma has] come up and it's now sitting within the crust itself."