Tonga eruption was triggered by explosion equivalent to 'five nuclear bombs', research says

The eruption of the Hunga Tonga volcano on 15 January 2022, was one of the most powerful seismic events of our era. It produced a column of ash 58 kilometres high, generated devastating tsunamis and seismic waves that were felt all over the planet.

Such an event is still under the magnifying glass of scientists, who, with increasingly sophisticated tools, investigate the causes and consequences of the eruption and why it was so violent.

A colossal explosion

A study that was published a few days ago in Geophysical Research Letters concludes that the explosion was equivalent to that of five underwater nuclear bombs. "The volume of water that rose during the event was enormous. According to our estimates, there was enough water to fill about a million standard Olympic pools," said Dr. Phạm, co-author of the study.

Scientists at the Australian National University (ANU) sought to better understand the mechanisms that caused such a powerful eruption. For that, they combined two analysis models: on the one hand, the seismic moment tensor (MT), which is the movement and propagation of rocks on earth. On the other hand, the single force (SF), which describes that upward reaction that occurs due to the explosion.

The researchers used data from seismic waves that traveled across the Earth's surface and recorded at several regional stations. They used a one-dimensional model of the Earth, applying temporal changes to realign observed waveforms and predictions, to understand how seismic waves propagated.

They considered several parameters to adjust their analyses and make them more accurate. With all this, they managed to obtain a more complete picture of the eruptive phenomenon.

The forces that interacted to cause an underwater cataclysm

The results revealed that the event can be described as the combination of an underwater explosion and a significant rising force. The upward force is comparable to a spring that is released: the water over the volcano rises abruptly, causing a sudden drop in pressure on the seabed.

This causes the bottom to react with an upward rebound force, as if the water was trying to return to its place after being compressed.

"Through the use of seismic waveform models, we observed an important vertical force pointing upwards during the event. At first, we were confused, but then we realized that the solid earth bounced upwards after the water column rose," explained Professor Hrvoje Tkalčić, co-author of the study.

Eventually, they managed to identify that the first major eruption event probably involved both an explosion (MT) and an ascending force (SF) due to the rapid rise of water over the volcano.

"With Hunga Tonga, we have an explosive event of relatively short duration observed worldwide and, again, academically driven curiosity and forensic seismology at its best," Tkalčić said.

"This is one of the biggest events of our lives. Fortunately, we had multiple ways to record it, from satellite image data to seismic sensors that record sound waves and structure," said Jinyin Hu, co-author of the study and doctoral student at ANU.

News references:

Jinyin Hu, Thanh-Son Phạm, Hrvoje Tkalčić. A Composite Seismic Source Model for the First Major Event During the 2022 Hunga (Tonga) Volcanic Eruption. Geophysical Research Letters (2024).