Volcanic eruptions displace air, land, and water in their wake, and according to a new study, they also tickle the atmosphere. A group of Japanese researchers found that this high-altitude signal can warn people several hours in advance of approaching tsunamis.

On January 15, the Kingdom of Tonga’s Hunga Tonga–Hunga Haʻapai volcano in the Pacific Ocean unleashed a violent eruption as powerful as up to 18 megatons of TNT. This explosion triggered an acoustic shock wave that rippled through the Earth, producing a worrying tsunami that traveled faster than those created more directly by the eruption.

In a new study published in the journal Earth, planets and spaceatmospheric researcher Atsuki Shinbori explains how a vertical shock wave left an imprint on ions floating more than 50 miles above the Earth’s surface and sent an early warning to some instruments in Japan ahead of the first tsunami wave triggered by the largest atmospheric explosion in human history. .

Shinbori, who works at Nagoya University’s Space-Earth Environmental Research Institute in Japan, tells Reverse that, if their findings lead to a screening approach, “it may be possible to estimate the size of [a] tsunami, like the height… several hours before it arrives”.

The bewildering waves of the eruption.

“The Tonga tsunami was characterized by a uniformly small forward wave that arrived earlier than theoretically expected for a freely propagating tsunami wave from the volcano,” writes Matías Carvajal, associate professor at the Institute of Geography at the Pontifical Catholic University of Valparaiso in Chile. in a March 2022 study.

This first was then followed by the largest waves, reaching three meters in height. According to a model developed by the US Geological Survey (USGS) of how waves propagate north from the volcano, there are approximately three types of waves: the first two are generated by the explosion atmospheric and the last ones are the product of the vibrations of the eruption in the sea. The US National Oceanic and Atmospheric Administration (NOAA) admits that these types of airborne waves, called meteotsunami, are not well understood. That’s a problem when the catalyst for these sinister ridges is pressure exploding at the speed of sound.

But luckily, something is faster than sound: the speed of light.

Appeared from nowhere

“Volcanic tsunamis of this magnitude are very rare,” says Carvajal Reverse. “The last one that seems to be similar occurred in the 19th century, in 1883, in Indonesia.”

The catastrophic eruption of 1883 was Krakatoa, which killed 36,000 people, destroyed hundreds of villages, and produced a thick layer of ash and pumice that plunged the planet into darkness. NOAA says it took five years for things to get back to normal. Its descendant volcano, Anak Krakatau, also left a horrific wake when it erupted in December 2018, killing at least 430 people. The tsunami it caused left thousands of people injured or missing.

These catastrophes have been difficult to predict because they are rare. Carvajal admits that the power of the atmospheric shock wave on January 15 was a surprise.

“Until January, it was thought that the main sources or mechanisms of tsunami generation associated with a volcanic eruption had to do with processes that occurred right there in the volcano,” he says.

“For example, we thought that the sudden collapse of the volcano’s caldera or the explosion of the submarine were the main mechanisms of generation of volcanic tsunamis, since both processes are theoretically capable of displacing large volumes of water.” This is what happened in 2018: a flank of Anak Krakatau’s caldera fell into the sea to create an initial wave 43 meters high.

The danger of atmospheric waves from an eruption is not unknown, but Carvajal believes it has not received much attention. He says the 2022 Tonga eruption “demonstrated, once again, that our understanding of tsunami generation is far from complete.”

Aside from atmospheric mechanics, the eruption itself came out of nowhere. Even a 38-year veteran like Jim Garvin, chief scientist for Earth and planets at NASA, says Reverse that Hunga Tonga–Hunga Haʻapai was not expected to erupt for centuries.

What is faster than sound?

Shinbori made a find that could save lives.

When the Hunga Tonga–Hunga Haʻapai volcano erupted, it created sound waves. Some of these waves shot upwards, reached space and left an imprint on the sunlit charged particles of the ionosphere. This footprint then traveled along Earth’s magnetic field lines, from south to north. Shinbori describes them as the lead wires of an electrical generator, carrying electricity at approximately the speed of light.

Instruments in Japan noticed this ionospheric signature, an electric field, before their other instruments detected the air shock from the tsunami and before other instruments detected the water waves from the tsunami. The size of the electric field corresponds to the strength of the shock wave, so it can also give scientists an estimate of the size of the incoming tsunami wave.

What will come in the future

Scientists may not get that kind of early ionospheric signal during a nighttime eruption.

“Sunlight affects signals from ionospheric disturbances,” he says. Reverse, explaining that its amplitude decreases when the ions are in the shadow. At night, “there is a chance that we may not be able to distinguish the origin of a tsunami from other sources,” she says.

Shinbori wants to analyze more data in the future to see how this technique could be fine-tuned. He hopes to one day establish an ionospheric warning system that can reveal threatening volcanic tsunamis at any time of day, sooner than deep-sea buoys and satellites can currently transmit.