Scientists offer first look into massive Hunga Tonga volcanic eruption

(CN) — Researchers looking into the origin and effects of the January 2022 volcanic Hunga Tonga eruption say the phenomenon displayed an extraordinary level of energy comparable to historically destructive volcanic eruptions.

On Jan. 15, the undersea volcano Hunga Tonga-Hunga Ha‘apai near the South Pacific island nation of Tonga erupted. The eruption was one of the most powerful in recorded history, with audible sound detected over 6,200 miles away. The tsunami generated by the blast was, according to scientists, partially driven by an unexpected atmospheric wave.

Two studies published Thursday in the journal Science reflect a complex main event, generating as much energy as the 1883 Krakatau eruption in Indonesia — one of the most destructive volcanic events in recorded history.

Researchers headed by University of California, Santa Barbara, associate professor Robin Matoza present infrasound and seismic recordings alongside other geophysical observations from the event. 

According to the team, the Hunga eruption “presents an extraordinary opportunity to advance understanding of rarely captured physical phenomena, including global Lamb wave propagation.” During this eruption, a Lamb wave — atmospheric pressure waves which can be detected around the world by both ground and space-based geophysical instrumentation systems — circled the planet four times. The eruption also generated long-range infrasounds and ionospheric interactions. 

The researchers’ report reflects six days of global observation of at least four minor-arc Lamb wave passages and three major-arc passages. The researchers wrote that the number of Lamb wave passages observed for Hunga is approximately the same as observed for the 1883 Krakatau eruption. However, the report called the resolution of the evolving wavefield from the Hunga eruption “exceptional” in comparison to Krakatau, reflecting more than a century of advances in instrumentation technology and global sensor density.

Another study focused on the global tsunamis which resulted from the eruption. National Research Institute for Earth Science and Disaster Resilience research fellow Tatsuya Kubota and colleagues investigated generation and propagation mechanisms of the tsunami “forerunner" — the first waves, which arrived more than two hours earlier than expected.

Kubota's team found that for the Hunga Tonga eruption, Lamb waves contributed to resulting tsunami waves arriving much earlier than expected for typical tsunamis which result from water displacement during eruptions. According to the authors, future tsunami models should incorporate this phenomenon.

Asked about their study, Matoza said via email his team intended to provide initial interpretations of the atmospheric wave types generated and their propagation and impact around the globe. Most of the research was performed within about one month of the eruption.

Matoza said the team showed the Hunga pressure pulse was greater than that of the 1980 Mount St. Helens eruption in Washington state.

“The 1883 Krakatau event was recorded by over 50 weather barometers around the world,” Matoza said. “In contrast, we document the Hunga signals as captured by thousands of stations with much more extensive and dense global coverage. For the 2022 Hunga event, the current global instrumentation density fills in travel-time branch details and provides exceptional spatiotemporal resolution of the evolving wavefield compared to what was available from the historic 1883 Krakatau dataset.’’

For this reason, he said, “This atmospheric waves event was unprecedented in the modern geophysical record. All of these new observations provide opportunities to refine our understanding of a variety of physical phenomena related to large volcanic explosions, atmospheric waves and their impacts.”

Matoza added, “Further developing capabilities in geophysical monitoring of natural hazards is an important component of future resilient societies.”

Another researcher, Norwegian Seismic Array (NORSAR) geophysicist Quentin Brissaud, said in an email he thought one of the most interesting observations resulting from their study was the coupling of atmospheric pressure waves to the ocean that generated tsunami-like signals, called "meteo-tsunamis."

"Since meteo-tsunamis are generated by acoustic waves they were recorded from the Pacific Ocean to the Atlantic Ocean seemingly 'jumping' over continents," Brissaud said. He said audible sound waves were reported across Alaska as far as 10,000 kilometers from Hunga, the furthest documented accounts of audible sound, which he said is likely related to global population increases and advances in societal connectivity.

"Our current modeling tools are unable to capture source processes and propagation effects to explain observations of acoustic, seismic, and tsunami waves," he added. "This is particularly relevant to understanding the impact of tsunami waves on coastal populations which varies strongly from location to location."

Kubota said in an email that his team of researchers found the eruption displayed much longer-lasting waves compared with recorded earthquake tsunamis, in which the waves lasted more than three days.

He said he was surprised to see the magnitude of tsunamis recorded around the world from this eruption, and called the phenomenon “something distinct and enigmatic compared with past major earthquake-induced tsunamis.”

"Such volcanological knowledge will also be essential, in addition to tsunami scientific knowledge, in order to accurately estimate the possible tsunami height excited by future major volcanic eruptions," Kubota added.

University of California, Berkeley, professor Michael Manga, who specializes in earth and planet science, said via email after reviewing the two studies, "I think we still don’t understand the mechanisms for generating the tsunamis."

"The great recording of so many waves -- in the atmosphere, through the ocean, and through the solid Earth -- are going to provide great new ways to image what happens during large explosive eruptions," Manga said.

He added he does not agree with the scientists’ belief the Hunga eruption was comparable to the Krakatau blast.

“The atmospheric waves may be similar in magnitude,” he said. “But (the) Krakatoa eruption (measured) about 10 cubic kilometers (13.1 billion cubic yards) of magma. Tonga was probably closer to 1 cubic kilometers.”

Asked what could be learned when studying future eruptions, Manga said, “There have been much larger submarine eruptions. If we scale up this one, we can get a better picture of how larger eruptions impact Earth systems. We know so little about submarine eruptions because they are hard to see and most go undocumented.”

The Hunga eruption killed at least four people, including two people who drowned when a 6 1/2-foot wave struck Peru. Across the Tongan archipelago, hundreds of buildings were damaged or destroyed by tsunami waves up to 49 feet.

NASA estimates the eruption was "hundreds of times more powerful" than the atomic bomb dropped on Hiroshima.

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