The massive January 2022 eruption of Hunga volcano in the Kingdom of Tonga took the world by surprise.
Authors
- Shane Cronin
Professor of Earth Sciences, University of Auckland, Waipapa Taumata Rau
- Jose Carlos Borrero
Adjunct Assistant Research Professor, Department of Civil and Environmental Engineering, University of Southern California
It blasted a plume more than 50km into the atmosphere , sent pressure waves around the globe and generated a series of tsunamis , devastating parts of Tonga and killing at least three people.
But one of the most important things we've learned since this historic event is that these tsunamis did not all stem from the same cause .
The first were generated by enormous explosions during the opening stages of the eruption. On nearby Tongatapu, waves with run-ups of one to four metres began arriving within minutes .
Then, more than an hour later, something much more destructive happened. A tsunami with run-ups reaching 18 to 40 metres struck islands within 100km of Hunga, destroying resorts and villages across southern and central Tonga.
Our newly published research shows this largest tsunami was generated not by another explosion, but by the sudden collapse of the volcano's caldera - and that the collapse produced an underwater sound detectable thousands of kilometres away.
That finding could point to a new way of warning communities about some of the most unpredictable types of tsunamis on Earth.
Listening to an underwater volcano
Monitoring submarine volcanoes is notoriously difficult. There are hundreds scattered around the Pacific " Ring of Fire ", yet we know relatively little about their state of activity - and even less about how to respond when they erupt.
Satellites can detect changes in heat and gas emissions, along with eruption plumes when clouds don't obscure the view. These observations can provide timely warnings of eruptions and help protect aircraft.
But satellites cannot tell us whether a deadly tsunami is on its way. Nor are conventional seismometers necessarily much help.
The closest seismometer to Hunga during the 2022 eruption was in Fiji, about 750km away. At that distance, many of the seismic signals produced by volcanic processes are poorly transmitted through the Earth.
So instead, we listened to what Hunga was doing in the ocean.
Underwater sound travels extremely efficiently over long distances as hydro-acoustic signals known as tertiary waves, or T-waves . An isolated volcano rising from the ocean floor can effectively act like a bell, radiating the sounds of violent underwater processes through the surrounding ocean.
We re-analysed records from 14 seismic stations around the southwest Pacific, some as far as 2,600km from Hunga.
During the first hour of the eruption, we could "hear" submarine landslide flows racing down the volcano's flanks. These flows were powerful enough to destroy submarine communications cables and their acoustic signals could be detected hundreds of kilometres away.
But the loudest underwater signal was still to come.
The sound of a volcano collapsing
At about 6.28pm Tonga time, the centre of Hunga began collapsing in on itself.
The collapse ultimately produced a caldera about 4km wide and more than 850 metres deep. Moving such an enormous volume of rock and seawater generated the largest local tsunami of the eruption.
Yet remarkably, the collapse was only weakly detected through conventional seismic monitoring.
Underwater, it was a different story. The collapse generated an enormous T-wave that radiated across the Pacific. We detected it at 14 stations, including sites more than 2,000km away.
The strongest part of the signal lasted about five minutes, giving us an indication of just how rapidly the main collapse occurred.
But to establish that this collapse had generated the devastating tsunami, we needed to know precisely when that wave reached Tonga. That required a rather different kind of evidence.
The tower that stopped transmitting
At Kanokupolu, on the western side of Tongatapu, a telecommunications tower stood 180 metres inland and about 13 metres above sea level.
The earlier tsunami had already swept ashore and residents had evacuated, but the tower remained standing. A weather station attached to it sent its last scheduled data transmission at 6pm.
Eventually, the much larger tsunami arrived. It flattened the tower and tore it apart, with pieces later found hundreds of metres further inland.
But exactly when had that happened?
Our team worked with Tonga Communications Corporation to examine data traffic through the tower. The answer was remarkably precise: communications stopped at 6:45:24pm.
That timestamp provided another piece of the puzzle. The huge underwater acoustic signal indicated Hunga's caldera had begun collapsing at about 6.28pm.
The destruction of the tower roughly 17 minutes later was consistent with the time needed for the resulting tsunami to develop and travel across the roughly 60km separating Hunga from western Tongatapu.
Eyewitness accounts provided another check. People had experienced the earlier, smaller waves and had time to evacuate before the catastrophic tsunami arrived.
All of this evidence enabled us to reconstruct something that conventional seismic monitoring had largely missed: a sudden submarine caldera collapse that generated the eruption's most destructive local tsunami.
Sound waves travel through the ocean at around 1.5km per second - more than seven times faster than a tsunami.
If monitoring systems can automatically recognise and locate these signals, they could provide early warning of volcanic tsunamis - much as existing systems do for those triggered by earthquakes.
Shane Cronin has received research funding from New Zealand's Ministry for Business, Innovation and Employment.
Jose Carlos Borrero has received research funding from New Zealand's Ministry for Business, Innovation and Employment, the International Tsunami Information Centre (ITIC) and The Pacific Community (SPC).
