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Could hidden seismic hints be the key to predicting New Zealand's next big eruption?
In a just-published study, a University of Canterbury geophysicist used machine learning to reveal a specific seismic signal that occurred in the days leading up to 2019's Whakaari eruption, which left 22 people dead.
As well, Dr Alberto Ardid found similar signals shortly before previous blows at five other volcanoes in New Zealand and around the world: Ruapehu and Tongariro among them.
"We discovered a change in frequency that tells us an eruption is much more likely to occur over the next few days," said Ardid, a postdoctoral fellow at the university's Civil and Natural Resources department.
"These frequency transitions allow us to listen in on what's happening at different depths under the volcano. Watching closely for this pattern could provide an early warning of future eruptions."
New Zealand's current volcano monitoring, run by the GeoNet project, draws on a range of indicators, including geochemistry, remote sensing, geodesy and seismic data.
Elevated tremor, gas and temperature levels at Ruapehu, for instance, has prompted GNS Science to raise Ruapehu's volcanic alert level to two – indicating "moderate to heightened unrest" - although it's still most likely that no eruption will occur.
As with earthquakes, uncertainty and variability has made predicting the precise likelihood and timing of eruptions largely out of scientists' reach.
But Ardid was hopeful his work might point the way to a new warning system.
Using an algorithm to sift through thousands of pre-eruption seismic recordings, he discovered a peak in what's called the displacement seismic amplitude ratio (DSAR) - suggesting a blockage in the shallow part of a volcano.
This signalled that a seal or lid had formed, which trapped hot gas, built pressure - and sometimes triggered an explosion.
Thanks to GeoNet, our group is able to track Ruapehu in real-time. Here's the Displacement Seismic Amplitude Ratio (DSAR) from 30 mins ago. It's been very high for a while.
"We found the same DSAR signal would build and peak in the days before the last five eruptions at Whakaari, and many of the eruptions at Ruapehu and Tongariro."
He believed the discovery was significant because the warning signal from one volcano could, under the right circumstances, be transferred to other volcanoes that don't have long histories of recorded eruptions.
"What we found is that this precursor to an eruption is common among several volcanoes, particularly the New Zealand ones which have a hydrothermal system beneath the crater."
The next step was to see how well this signal worked as a warning system for volcanoes around the world.
"It's really exciting that we can potentially contribute to a warning system that could help save lives."
University of Canterbury senior lecturer and study co-author Dr David Dempsey thought the DSAR signal could become part of a precautionary evacuation alarm system, particularly in tourist areas.
"Active volcanoes, including Whakaari, Ruapehu, Tongariro, and others around the world where visitors and skiers are likely to be nearby, are unpredictable and sometimes hazardous," Dempsey said.
"This research was motivated by the tragedy on Whakaari, but we think it has wider application because there are several other New Zealand volcanoes that have a similar style of eruption.
"We would like to use this for real-time hazard information, perhaps to let tourists know when it's safer to visit volcanoes."
University of Canterbury postdoctoral fellow Dr Alberto Ardid, pictured on a field trip in Chile, has identified a seismic signal that could help forecast volcanic eruptions. Photo / Supplied
GNS Science natural hazards and risks research leader Dr Jill Jolly said she and colleagues were "excited by the results and the way that they are using GeoNet's seismic data".
"This new modelling can be considered together with analysis of the full range of multi-disciplinary data that helps us understand volcanic unrest," she said.
"There are a number of research projects underway looking at precursors to phreatic eruptions, which will all hopefully feed into our volcano monitoring toolbox in future."
In another major new Marsden Fund-supported programme, scientists are taking our current understanding of what the insides of the Whakaari and Mt Ruapehu volcanoes look like, and turning them into physics-based models of how heat and fluid are flowing.
By combining these with other data, programme leader and GNS Science geothermal geophysicist Dr Sophie Pearson-Grant expected to glean new insights into what geological processes drove phreatic eruptions - and how these interactions happened to expressed in volcano monitoring data.
"In an ideal world, we will identify what changes lead to a phreatic eruption and how we can detect them with enough warning to prevent harm," she earlier told the Herald.
"Forecasting phreatic eruptions is really difficult, but with the great range of data we have we can hopefully gain a lot of understanding and make it possible worldwide."
