Underwater 'booms' could provide early warning of volcanic tsunamis

When Hunga volcano erupted in January 2022, it did what volcanoes do—it announced itself with fury. A column of ash and gas shot more than 50 kilometers into the sky. Pressure waves rippled across the planet. Tsunamis rolled toward the Kingdom of Tonga. At least three people died. Villages and resorts were obliterated. But the eruption's real lesson came later, buried in the data that scientists would spend months untangling: the deadliest waves did not come from the explosions everyone saw. They came from something the conventional instruments almost entirely missed.

The first tsunamis arrived quickly. Within minutes of the initial blasts, waves one to four meters high were washing ashore on nearby Tongatapu. People had time to notice, to react. Then, more than an hour later, something far worse happened. A second tsunami, this one with waves reaching 18 to 40 meters, struck islands within 100 kilometers of Hunga. It was not generated by another explosion. It was generated by the volcano's sudden collapse—the caldera, the crater at its heart, caving in on itself. That collapse moved an enormous volume of rock and seawater, and the displacement of all that mass created the eruption's most destructive local waves.

The problem was that conventional seismic monitoring had barely registered what happened. The closest seismometer to Hunga sat in Fiji, roughly 750 kilometers away. At that distance, the seismic signals produced by volcanic processes traveling through the Earth become weak and difficult to read. Satellites could track the eruption plume and measure heat and gas emissions, but they could not tell whether a tsunami was coming. The instruments designed to detect earthquakes and volcanic activity were, in this case, nearly blind to the event that mattered most.

But the ocean itself had recorded everything. Underwater sound travels with remarkable efficiency over vast distances, moving through the water as hydro-acoustic signals known as T-waves. A submarine volcano acts like a bell, radiating the sounds of violent underwater processes through the surrounding ocean. Researchers re-examined records from 14 seismic stations scattered across the southwest Pacific, some as far as 2,600 kilometers from Hunga. During the first hour of the eruption, they could detect the acoustic signature of submarine landslide flows racing down the volcano's flanks—flows powerful enough to destroy underwater communications cables. But the loudest signal came later.

At approximately 6:28 p.m. Tonga time, Hunga's center began collapsing. The collapse created a caldera roughly 4 kilometers wide and more than 850 meters deep. The acoustic wave generated by this collapse radiated across the Pacific and was detected at 14 stations, including sites more than 2,000 kilometers away. The strongest part of the signal lasted about five minutes, a window that revealed how rapidly the main collapse had occurred. Yet to prove that this collapse had actually generated the devastating tsunami, researchers needed precise timing—evidence of when the wave reached land.

On the western side of Tongatapu, at a place called Kanokupolu, a telecommunications tower stood 180 meters inland and 13 meters above sea level. The earlier, smaller tsunami had already swept past, and residents had evacuated. The tower remained standing. At 6 p.m., a weather station attached to it sent its last scheduled data transmission. Then the larger tsunami arrived. It flattened the tower and tore it apart, scattering pieces hundreds of meters further inland. By examining data traffic through the tower in coordination with Tonga Communications Corporation, researchers determined exactly when communications stopped: 6:45:24 p.m. That timestamp was the key. The underwater acoustic signal indicated the caldera had begun collapsing at 6:28 p.m. The tower's destruction roughly 17 minutes later aligned precisely with the time needed for the resulting tsunami to develop and travel the roughly 60 kilometers separating Hunga from western Tongatapu. Eyewitness accounts confirmed the sequence: people who had experienced the first waves had evacuated before the catastrophic tsunami arrived.

Sound waves move through the ocean at around 1.5 kilometers per second—more than seven times faster than a tsunami. If monitoring systems could be trained to automatically recognize and locate these underwater acoustic signals, they could provide early warning of volcanic tsunamis much as existing systems do for earthquakes. The 2022 eruption revealed a gap in how the world watches submarine volcanoes. Hundreds of them are 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. The acoustic data from Hunga suggests a path forward: listen to what the ocean is telling us, and listen in time.