In 2022, the Tonga eruption sent a powerful sound through the ocean — the strange signal before a deadly disaster hit may change how scientists warn about tsunamis

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 waves arrived.

Then the much larger wave struck.

The tower was destroyed, and communications suddenly stopped.

Researchers examined the telecommunications data and determined that the final transmission occurred at approximately 6:45 pm local time.

That timestamp helped connect the dots.

The underwater acoustic signal indicated that the major collapse began around 6:28 pm. The destruction of the tower occurred roughly 17 minutes later, consistent with the time required for the tsunami to develop and travel from Hunga to western Tongatapu.

Together with eyewitness reports and other observations, the evidence helped researchers reconstruct the sequence of events.

Why sound could be useful for tsunami warnings

There is a major advantage to detecting underwater sound: it travels much faster than a tsunami.

Sound moves through seawater at roughly 1.5 kilometres per second. A tsunami travels considerably more slowly across the ocean.

That difference creates a potential window for detection.

If future monitoring networks could recognise the acoustic signature associated with a submarine volcanic collapse and rapidly determine where it originated, scientists could potentially identify a tsunami-generating event before the resulting waves reach nearby coastlines.

This would not replace existing earthquake and tsunami monitoring systems.

Instead, it could add another layer of information, particularly in regions where submarine volcanoes pose a hazard.

Volcanoes are a complicated tsunami threat

Earthquake-generated tsunamis are relatively well understood. When the seafloor suddenly moves vertically, it can displace a huge volume of water and send waves across an ocean.

Volcanic tsunamis are more complicated.

An eruption can generate waves through explosions, underwater landslides, collapsing volcanic flanks, caldera collapse and even atmospheric pressure disturbances. The Hunga eruption demonstrated that several of these processes can occur during a single event.

The event also showed why relying on one type of monitoring data can be risky.

Scientists studying the eruption found that atmospheric waves from the explosion interacted with the ocean and contributed to tsunami generation far from Tonga.

That means future warning systems may need to combine information from the atmosphere, ocean, satellites and seismic and acoustic networks.

Hunga Tonga changed what scientists know about volcanic tsunamis

The January 2022 eruption was exceptional not only because of its enormous power, but also because modern instruments captured so many aspects of the event.

The eruption generated atmospheric waves that travelled around the globe, powerful infrasound, seismic signals and disturbances in the ionosphere. The event has since become an important natural laboratory for understanding how the atmosphere, solid Earth and ocean can interact during a major volcanic eruption.

It also exposed weaknesses in conventional approaches to monitoring submarine volcanic hazards.

A volcano hidden beneath the ocean can produce destructive waves without giving coastal communities the same kind of warning associated with a large earthquake.

The discovery of a long-range underwater acoustic signature could therefore become an important part of the solution.

The ocean may provide its own early warning system

There is still a long way to go before underwater sound can be turned into a reliable tsunami warning technology.

Scientists would first need to establish which acoustic signals are associated with genuinely dangerous events and distinguish them from the many other sounds produced naturally in the ocean.

Monitoring networks would also need to determine the location and size of the source quickly enough for the information to be useful.

But the Hunga eruption has demonstrated something important: a catastrophic volcanic process can leave an acoustic fingerprint that travels far beyond the volcano itself.

That means the ocean may not simply carry the tsunami.

It may also carry an early warning of what is coming.

For scientists searching for better ways to monitor some of Earth's most unpredictable natural hazards, that could be one of the most important lessons left behind by the 2022 Tonga eruption.