When a volcano on the South Pacific island of Tonga erupted Jan. 15 in the largest documented explosion in the modern geological record, a complex tapestry of sound waves rippled through the atmosphere and wrapped around the globe several times.
Geophysicists and infrasound researchers hadn’t seen anything like it. The closest comparison is the 1883 eruption of Krakatau. The explosion triggered low-frequency acousto-gravity waves with periods of more than five minutes, infrasound waves (frequencies below human hearing, lower than the subwoofer on your stereo), and even audible sound that people heard more than 6,000 miles away in Alaska. Fifty miles above the Earth’s surface, the blast perturbed the ionosphere. Down below, detectors registered seismic waves along with waves coupled between the air and the ground and between the air and the sea.
Despite the devastation left in its wake, the event was an all-you-can-eat buffet of novel data.
“It was a unique event, in terms of scale,” said Philip Blom, a geophysicist and infrasound scientist working at Los Alamos National Laboratory and coauthor of a recent paper in Science summarizing the findings bonanza from the Tonga eruption. As an unplanned global experiment, the eruption “produced a rich dataset that infrasound grad students will be studying for a couple decades.”
Decoding the acoustic waves
Making sense of this vast, complex phenomenon and unraveling the mysteries of its source, which wasn’t directly observed, required special tools. Among them, infrasound-modeling software — authored by Blom and his colleagues on the Seismoacoustics Team at Los Alamos — helped researchers follow the thread from observations back to initial cause.
“To understand the source, we have to connect and relate observations back to the eruption,” Blom said. “And to do that, we have to understand how the waves propagate through the atmosphere.”
Thousands of kilometers away from the eruption, the arriving acoustic waves have interacted with terrain and weather. “The signals far from the source have lots of structure because of propagation effects,” Blom said.
A highly dynamic atmosphere complicates things. Close to the source, the explosive blastwave is relatively easy to understand, but further away, researchers analyze the temperature, pressure and density structure of the atmosphere and what winds are doing at a given time and location. Then they consider how these factors influenced wave propagation.
In one big surprise, Blom said, data from sensors often indicated the waves were coming from a different location than the volcano.
“To get the right location of the source, you have to account for those propagation effects through modeling,” Blom said.
The infraGA/GeoAc software, an open-source code that performs geometric acoustic modeling for infrasound propagation, allows researchers to parse the particulars from the vast complex data set.
An animation captures the surging, rippling sound waves swarming across the globe.
In an iterative process, the modeling helps understand the event, and data from the event helps researchers fine-tune the model.
“We hadn’t done global-scale propagation before,” Blom said. “With the enormous energy generated by the eruption, that big scale helps us understand the limitations of our algorithms and at what point they break down.”
Paper: “Atmospheric waves and global seismoacoustic observations of the January 2022 Hunga eruption, Tonga,” in Science. DOI: science.org/doi/10.1126/science.abo7063
Funding: National Nuclear Security Administration Office of Defense Nuclear Nonproliferation