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EMBARGOED TO 6am Saturday 4 June 2016
ANALYSIS OF DATA PINS DOWN REASON BEHIND QUAKE SWARM
Scientists using a combination of satellite images, GPS data and conventional survey information
dating back to the 1950s have uncovered the reason for a swarm of several thousand small
earthquakes between 2004 and 2011 around the coastal Bay of Plenty town of Matata.
The three streams of data show that an area of land around Matata roughly 20km by 20km,
about half of which is offshore, has been pushed up by about 40cm since 1950.
Until now the cause of the swarm was thought to be tectonic. Most of the quakes were between
magnitude 2.0 and 4.0 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 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 moves in the sub-surface, it causes
the surrounding rock to deform and break, resulting in small earthquakes.
Using modelling, scientists have 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. It is titled 'Offaxis magmatism along a subaerial back-arc rift.'
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."
To model the magma body at depth scientists used a combination of modern GPS data and
conventional survey data dating from the 1950s. They augmented this with satellite radar data of
the North Island, which has been collected from about 2003 onwards.
Observations of inflating magma bodies at depth were a reasonably common feature of volcanic
areas worldwide, especially since the advent of highly accurate GPS and satellite measurements
which can pick up small amounts of surface inflation and subsidence.
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.”
Dr Hamling hoped funding would be available to continue this research so scientists could more
accurately model the size and depth of the magma body, and keep a lookout for any changes
that might occur at depth.
The authors of the science paper would like to thank the European and Japanese Space
Agencies for access to satellite images and GeoNet for the GPS data. The research was
supported by public good research funding from the New Zealand Government, with additional
support from the Natural Hazards Research Platform, EQC, and Land Information New Zealand.
END
Contact: Dr Ian Hamling, GNS Science, P: 04-570-4568
At a glance
Using a combination of satellite images, GPS data, and conventional survey information dating
back to the 1950s, scientists have found the reason for a swarm of several thousand small
earthquakes at Matata in Bay of Plenty between 2004 and 2011.
They have concluded that surface swelling of land around Matata has been caused by the
growth of a previously unrecognised magma body about 9km under the Bay of Plenty Coast.
It does not mean an eruption is going to happen and the finding does not change the volcanic
hazard status of the Bay of Plenty region.
Surface uplift is reasonably common in volcanic areas and occurs when magma slowly intrudes
into the Earth’s crust from below and pushes upwards.
It is possible the magma under the Bay of Plenty coast has been there for a long time, perhaps
thousands of years. It is largely through highly accurate GPS and satellite measurements that
such phenomena can be recognised.
There are sub-surface magma bodies further south towards Taupo and northeast at White
Island and beyond, so the existence of another magma body in the region should not be a
complete surprise.
There should be no cause for alarm. However, it underlines the fact the New Zealand is a
geologically active country and it pays to be prepared.
Investigations such as this are an important part of improving the knowledge and understanding
of New Zealand’s geological hazards.
The Bay of Plenty, like the rest of New Zealand, is monitored by the EQC-funded GeoNet
project which operates about 600 instruments nationwide. We could expect to get weeks,
months, or even years of advance warning of any new volcanic activity.
John Callan I Communications Manager
GNS Science
1 Fairway Drive, Avalon, PO Box 30 368, Lower Hutt 5040, New Zealand
Ph (04) 570-4732 I Mob 027-440-2571 I Fax (04) 570-4600
http://www.gns.cri.nz/ I Email: [email protected]