Research by GNS Science has overturned the view that the swarms of earthquakes centred near Matata in recent years were caused by tectonic activity.
Saturday, 4th June 2016
The Bay of Plenty town of Matata was plagued by lengthy earthquake swarms between 2004 and 2011. Most of the thousands of earthquakes were small (between magnitude 2.0 and 4.0) and at depths of between 2 and 8 kilometres.
The 2004 earthquake swarm was short-lived with a burst of activity on May 2nd. Between February and August 2005, there were three distinct swarms of earthquake activity. An intense rainfall event in May of that year caused serious damage to the coastal community.
Another earthquake swarm plagued Matata between December 2006 and May 2007 By March, 43 earthquakes had been reported felt by residents. By May, scientists had recorded 700 earthquakes as part of the swarm. Activity declined in June, with smaller bursts of activity until October 2007.
Activity then moved off-shore in what was identified as a third phase of activity before declining during February 2008. Nevertheless the swarm continued at a low level through until September 2008 when activity again picked-up.
At the time, it was thought that the earthquake swarms were part of the tectonic activity that is causing East Cape to slowly move away from the Coromandel Peninsula.
However, a new study by GNS Science has now found that a shallow chamber of magma situated beneath Matata and extending out into the Bay of Plenty was responsible for the earthquakes. The chamber, referred to as a magma body, may have been present for hundreds or thousands of years, but an inflow of magma (molten rock) caused the chamber to expand between 2004 and 2011. As the chamber grew, ground level rose by about a centimetre per year and the stress of the inflation triggered the shallow earthquakes in the Earth’s crust above.
The study used a combination of satellite images, Ground Positioning System (GPS) data and conventional survey information to examine changes in the area around Matata since 1950. It concludes that an area of land around Matata roughly 20km by 20km has been pushed 40 centimetres upwards since 1950. About half of the uplifted area is located off-shore. Scientists conclude that the magma chamber responsible for the uplift lies at a depth of about 9 km.
In a media release issued today, Dr Ian Hamling notes, “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 modelling has detected the magma chamber in an area where there has been no active volcanism for about 400,000 years.
The media release notes, “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.”
It is hoped that further funding will be made available to continue this research so that scientists can more accurately model the size and depth of the magma body at Matata and monitor for any changes that might occur.
[Compiled from GNS Science media release 4th June 2016 and earlier reports on Wild Land.]