How underwater ‘booms’ could provide earlier warning of deadly volcanic tsunamis

The massive January 2022 eruption of Hunga volcano in Tonga caught the world off guard. It sent shockwaves around the globe and triggered a series of devastating tsunamis that wreaked havoc on Tonga and claimed at least three lives. However, researchers have since learned that not all of these tsunamis were created equal. The first ones were caused by explosive blasts at the start of the eruption.

Within minutes, waves up to four meters high began hitting Tongatapu. But over an hour later, a far more catastrophic tsunami with run-ups of 18 to 40 meters slammed into islands just 100 kilometers from Hunga, flattening resorts and villages in the southern and central parts of the island nation.

The team's recent study reveals that this colossal tsunami was not the result of another explosion, but rather the sudden collapse of Hunga's caldera. This underwater sound was detectable thousands of kilometers away. This discovery may pave the way for a novel method to alert communities about some of the most unpredictable and deadly tsunamis on Earth.

Monitoring submarine volcanoes is notoriously challenging. Hundreds of them dot the Pacific Ring of Fire, but our knowledge of their activity is limited. Traditional satellites can track heat, gas emissions, and eruption plumes, offering timely warnings of eruptions and helping protect aircraft. However, they cannot forecast the occurrence of deadly tsunamis, nor can conventional seismometers help much.

The closest seismometer to Hunga during the 2022 eruption was located in Fiji, about 750 kilometers away. During this time, many seismic signals produced by volcanic processes are weakly transmitted through the Earth.

Instead, the researchers turned to the ocean. Underwater sound travels efficiently over vast distances as hydro-acoustic signals known as tertiary waves, or T-waves. An isolated volcano rising from the ocean floor can act like a bell, radiating the sounds of violent underwater processes through the surrounding ocean. By re-analyzing records from 14 seismic stations around the southwest Pacific, some as far as 2,600 kilometers from Hunga, the researchers were able to hear submarine landslide flows racing down the volcano's slopes.

These powerful flows were strong enough to destroy submarine communications cables and their acoustic signals were detectable hundreds of kilometers away.

But the loudest underwater signal came when the volcano finally collapsed. At about 6:28 pm Tonga time, the caldera of Hunga began collapsing inward. This collapse generated the largest local tsunami of the eruption, reaching as high as 40 meters. Remarkably, the collapse was only weakly detected through conventional seismic monitoring.

However, underwater, it sent an enormous T-wave that radiated across the Pacific, detected at 14 stations, including sites more than 2,000 kilometers away. The strongest part of the signal lasted about five minutes, providing an indication of the rapidity with which the main collapse occurred.

To confirm that this collapse indeed triggered the devastating tsunami, the researchers needed precise evidence of when the wave reached Tonga. They collaborated with Tonga Communications Corporation to examine data traffic through a telecommunications tower situated 180 meters inland and 13 meters above sea level on the western side of Tongatapu.

The tower's weather station sent its last scheduled data transmission at 6:00 pm. Eventually, the larger tsunami arrived, flattening the tower and tossing it inland. However, the exact time of the tower's destruction provided crucial evidence. Communications with the tower ceased at 6:45:24 pm. The time it took for the large underwater acoustic signal to indicate the collapse of the caldera - approximately 17 minutes - aligned with the time required for the resulting tsunami to travel the roughly 60 kilometers from Hunga to western Tongatapu.

Eyewitness accounts further corroborated this timeline. People had already experienced the smaller waves and had time to evacuate before the catastrophic tsunami struck.

By analyzing this evidence, the researchers were able to reconstruct a previously overlooked event: a sudden submarine caldera collapse that generated the eruption's most destructive local tsunami. If monitoring systems could automatically recognize and locate these underwater sound signals, they could provide early warnings of volcanic tsunamis, much like existing systems do for those triggered by earthquakes.