When the Hunga Tonga-Hunga Ha’apai volcano erupted in January 2022, it unleashed one of the most extraordinary volcanic events witnessed in modern times.
The eruption sent a plume soaring high into the atmosphere, generated pressure waves that travelled around the world and triggered tsunamis across the Pacific. Scientists have spent years studying the event because it revealed just how many different ways a volcanic eruption can disturb the ocean.
Now, researchers have uncovered another important clue.
The underwater collapse of the volcano produced a powerful acoustic signal that travelled thousands of kilometres through the ocean. Detecting such signals could eventually give scientists another tool for identifying dangerous volcanic tsunamis before they reach vulnerable coastlines.
The discovery is particularly significant because submarine volcanoes are difficult to monitor, and the mechanisms that generate volcanic tsunamis can be very different from those behind earthquake-driven waves.
Tonga’s eruption produced more than one tsunami The January 15, 2022 eruption was not a simple case of one explosion producing one tsunami.
Scientists have determined that the event generated waves through several different mechanisms, including the initial underwater explosion, the collapse of the water cavity created by the blast and atmospheric pressure waves travelling across the ocean. Research published in Nature showed that the eruption even generated a global tsunami through interactions between the atmosphere and ocean.
Close to Tonga, however, the sequence was even more complicated.
Some waves arrived within minutes of the eruption. Later, a much larger tsunami struck nearby islands, producing devastating run-up heights and destroying coastal infrastructure.
That difference in timing became an important clue for researchers trying to determine exactly what happened beneath the ocean.
The volcano effectively rang like a bell underwater Monitoring a volcano beneath the ocean is far more difficult than monitoring one on land.
Satellites can observe volcanic plumes, changes in the Earth’s surface and other signs of activity. Seismometers can also detect vibrations generated by volcanic processes. But neither system necessarily provides a complete picture of what is happening underwater.
This is where sound becomes interesting.
Sound can travel remarkably efficiently through seawater. Powerful underwater disturbances can generate hydro-acoustic signals capable of travelling vast distances.
During the Hunga eruption, researchers found evidence that the volcano produced these signals as different processes unfolded.
The eruption itself was detected through a remarkable range of atmospheric and seismic signals. Scientists previously found that the event generated infrasound and audible sound detectable thousands of kilometres away, while atmospheric waves travelled around the planet multiple times.
The latest analysis adds another piece to that extraordinary acoustic record.
The biggest signal came when the volcano collapsed The critical event occurred when the centre of Hunga began collapsing.
The collapse created a huge caldera, displacing an enormous volume of rock and seawater. That sudden movement generated the largest local tsunami associated with the eruption.
Researchers analysing seismic records from stations across the southwest Pacific found that the collapse was not especially obvious in conventional seismic data.
But underwater acoustic recordings told a different story.
A powerful T-wave travelled outward from the volcano and was detected at stations located more than 2,000 kilometres away.
The signal lasted for several minutes, providing researchers with an indication of how rapidly the main collapse unfolded.
That was an important breakthrough because it linked a detectable underwater sound signal with one of the eruption’s most destructive tsunami-generating processes.
A destroyed communications tower helped solve the timing mystery There was another unexpected source of evidence: a telecommunications tower on Tongatapu.
The tower, located roughly 180 metres inland and about 13 metres above sea level, had continued transmitting data after the first tsunami wave
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