The culprit behind a recent swarm of earthquakes in New Zealand’s Bay of Plenty has been found: a growing bubble of magma less than 10 kilometres below ground.
Geophysicists from the New Zealand research institute GNS Science, led by Ian Hamling, tracked how the ground lifted and sank in the Taupo Volcanic Zone, a 30-kilometre zone that runs northeast from the centre of the North Island to the Bay of Plenty coast. They saw the northern section deformed in a way consistent with a ballooning reservoir of magma beneath.
“There is every possibility the magma body under the Bay of Plenty coast had been there for centuries, and possibly even longer,” Hamling says.
Volcanism and New Zealand go hand in hand – especially in the North Island where Rotorua, a town famous for its hot springs, and Lake Taupo sit atop the Taupo Volcanic Zone.
The zone was formed as the Pacific plate, on which New Zealand sits, slowly slides beneath the Australasian plate at the rate of 38 to 49 millimetres per year.
Across its northern segment, earthquakes have shaken coastal towns such as Matata, with several thousand reported between 2003 and 2011.
So Hamling and colleagues used a combination of survey data dating back to the 1950s, as well as recent GPS and satellite images, to measure how much the earth around the area has lifted or compressed.
While a 2015 study, also led by Hamling, showed central and southern sections of the zone have tended to sink a little, the most recent work saw some 400 square kilometres around Matata lifted by 40 centimetres since 1950 – with a burst between 2003 and 2011. Half of this area was off shore.
The pattern and amount of lift couldn’t have been produced by tectonic processes (or movements in the crust), so they modelled how a magma reservoir might affect the overlying earth.
The best fit was a blob of magma around 9.5 kilometres below ground, which inflated by around 0.2 cubic kilometres since 1950.
Such reservoirs of hot rock are common, Hamling says, and uncovering one does not mean a volcanic eruption is around the corner.
“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.”
The work was published in Scientific Advances.