Mysterious signal heard minutes before most powerful eruption ever recorded

Scientists have identified a mysterious signal that occurred just minutes before the most powerful explosion in modern times: the eruption of a volcano in the Pacific Ocean.

When the Hunga Tonga-Hunga Ha’apai underwater volcano exploded in 2022, it released the energetic equivalent of 61 megatons of TNT, or a magnitude 8.4 earthquake.

A team from Japan’s Earthquake Research Institute discovered a “seismic wave” created before the explosion, which was released when a weak section of the ocean floor near the volcano collapsed.

This caused tremors to travel through the seafloor, which were picked up by distant monitoring stations 15 minutes before the eruption.

The research explained that this crack in the walls of the Tonga volcano allowed seawater and magma to mix in an area between the seabed and the underground magma chamber, ultimately causing the massive ‘steam eruption’.

“Many eruptions are preceded by seismic activity,” said Takuro Horiuchi, a student volcanologist at the University of Tokyo who was the lead author of the study.

But Horiuchi noted that “such seismic signals are more often than not subtle and only detected within a few kilometers of the volcano.”

The team hopes that the lessons learned from their work could one day help deploy “this kind of analysis in real time” for emergency responders, ahead of future volcanic activity.

When Hunga Tonga-Hunga Ha’apai blew into the South Pacific on January 15, 2022, scientists were amazed that sound waves from the historic submerged explosion could be detected as far away as Alaska, 6,000 miles away.

About 58,000 Olympic-sized pools of water vapor were launched into the air, creating thunderstorms and a tsunami in the Tonga volcano’s water wake.

At the time, researchers noted that comparisons to the famous 1883 explosion of Indonesia’s Krakatoa volcano put the Tonga blast in the running for the largest explosion ever recorded by modern geophysical equipment.

Knowing that seismic activity causes volcanic eruptions, the team looked at data from two earthquake monitoring stations on the islands of Fiji and Futuna, more than 750 kilometers away.

They found evidence of Rayleigh waves, a vibration only detected by instruments.

Tonga erupted at 4:47 PM local time and the waves formed at 4:32 PM, exactly 15 minutes earlier.

Despite that enormous distance, the Rayleigh waves ‘dominated’ in the frequency range of 0.03 to 0.1 hertz (Hz), they wrote, giving measurements (in amplitude) comparable to a magnitude 4 earthquake, 9.

Hunga Tonga-Hunga Ha’apai, an underwater volcano in the South Pacific, spewed debris up to 40 kilometers into the atmosphere when it erupted on January 15, 2022

But the earthquake-like rumbling caused by the eruption of the volcano itself was even more powerful.

“The difference in arrival time of the Rayleigh wave between the two stations was consistent with that of the M5.8 earthquake during the eruption,” the team wrote in their study, published this month in Geophysical research lettersan open-access journal of American Geophysical Union.

Another important set of information that helped predict the explosive event of this submarine volcano came from satellite gravity data.

The data was collected from satellites that continuously measured their distance from each other, using microwave instruments, as a measure of the Earth’s gravity on their mass.

The Tonga event, they reasoned, might well be a “caldera-forming eruption,” emptying so much magma that areas above the magma reservoir collapse.

Tonga erupted at 4:47 PM local time and the waves formed at 4:32 PM, exactly 15 minutes earlier than

“Very few caldera-forming eruptions have been observed, and even fewer caldera-forming eruptions have been observed in the ocean,” said Dr. Mie Ichihara, a volcanologist at the University of Tokyo and co-author of the new study.

“This provides one scenario on the processes leading to caldera formation,” Dr. Ichihara noted a press statement“but I wouldn’t say that’s the only scenario.”

“Penetration of magma, magmatic gas and seawater” into a fracture in the Earth’s crust along the Tonga volcano’s “caldera” wall, they reported in their new study, could have led to a steam pressure event, a “magma-water” interaction that sparked the outburst.”

An earlier study of the The ‘plumbing’ of the Tonga volcano had used this gravity data to determine “a circular distribution of low-density material” around the volcano, which they interpreted as “ring faults” around the caldera walls, the team wrote.

Dr. Ichihara expressed hope that all these diverse sensors could one day be used to more easily anticipate and safely respond to potentially fatal activity from undersea volcanoes.

“Early warnings are very important for disaster mitigation,” he said. ‘Island volcanoes can produce tsunamis, which poses a significant hazard.’