2022 Tongan volcanic explosion among largest in over a century

(CN) — On January 15, 2022, an underwater volcano in the South Pacific Kingdom of Tonga erupted. It blasted a giant plume of ash and sulfur dioxide into the vast reaches of the atmosphere, sending out a powerful shockwave measured across the world for days to come.

Unbeknownst to many at the time, the eruption of Hunga Tonga-Hunga Ha'apai was the largest natural explosion in over a century, yielding 15 megatons of TNT to create an explosion more powerful than any U.S. nuclear detonation.

That was among the key findings of a new analysis published Friday in Science Advances. Researchers from the University of Miami’s Rosenstiel School of Marine, Atmospheric and Earth Science combined before-and-after satellite imagery, drone mapping and field observations to produce a tsunami simulation of the archipelago.

Another key finding: that the tsunami could have been far worse had it not been for the region’s Tongan Archipelago. Researchers hope the study will serve as a guide to the tsunami risks posed by future eruptions of the still-active Hunga Tonga-Hunga Ha'apai or other submarine volcanoes.

The explosion of Tonga-Hunga Ha'apai created one of the most significant tsunami events in the last 100 years, with waves peaking as high as 148 feet. But despite the event’s catastrophic tsunami — which killed six people in Tonga and two in Peru and almost killed several others in California — the waves reaching the shores of Tongatapu, Tonga’s most populated island, peaked at just 56 feet.

Located 40 miles north of Tongatapu, Hunga Tonga–Hunga Haʻapai started turning heads in 2015 when new land mass began rising above the ocean surface to join two existing islands: Hunga Tonga and Hunga Ha’apai.

It was an underwater volcano. Digital elevation maps from NASA in 2021 showed the uppermost part of the volcano rose 1.1 miles from the seafloor and stretched 12 miles across, topped by a submarine caldera three miles in diameter.

This type of young volcano is familiar to the area. Researchers note how the Tonga-Kermadec Arc — a region comprising 169 islands — is the Earth’s fastest-converging, most seismically active subduction boundary with the highest known density of submarine volcanoes.

After the 15-megaton eruption on January 15, all of the new land that emerged between the two islands disappeared.

Not just one explosion blew the volcano to bits and set off a massive tsunami. According to the researcher’s simulation, three blasts were responsible for most tsunamis formed that day, each varying in strength.

With the help of scientists from the University of Auckland, researchers found the archipelago’s shallow ocean floors acted as a low-velocity wave trap, capturing more than an hour’s worth of waves as high as 279 feet after the initial explosion.

“Looking at the explosion, it was very clear already on that day that this was an event beyond comparison ," Sam Purkis, professor and chair of the Department of Marine Geosciences at the Rosenstiel School and chief scientist at the Khaled bin Sultan Living Oceans Foundation, said in an interview with Courthouse News. All the way in Florida, for example, the sea level "started to wobble, to go up and down as the shock wave from the explosion made its way around the world."

Purkis assembled a research team the day after the explosion, determined to discover what happened. After concluding that there must have been a series of explosions to generate the largest tsunami, Purkis explained he was initially skeptical of the findings.

“Just one minute after the explosion, we predicted the waves to be nearly 85 meters high," Purkis said. "I mean, that's just sort of beyond imagination, really, isn't it? That's more than 300 feet."

"You imagine looking at a building that big, you just can't imagine the sea rising up to such a high level,” he added. Their model predicted the wave would run up the flanks of the island to more than 150 feet, "which also just seemed so large to be out of this world and very unlikely.”

Fortunately, co-author Shane Cronin of the University of Auckland visited Tonga shortly after the tsunami to observe what happened at over 100 different sites. He and his colleagues verified the simulation’s predictions via drone by measuring where the water level reached during the tsunami event.

“We were very close in our predictions,” Purkis said. “And with that we understood that we were dealing with what is called a mega-tsunami. I mean, this is an incredibly powerful explosion and a very large wave indeed, and pretty much unheard of in modern history.”

Purkis said the last time an event of such magnitude occurred was with Krakatoa over 100 years ago in Indonesia, which killed nearly 40,000 people.

“A huge humanitarian disaster [happened] when Krakatoa erupted, but [that was] not the case here in Tonga," Purkis said. "Any loss of life is awful, but it was very minor compared to the humanitarian crisis that you would predict."

Purkis credited Tongans with being mindful about tsunamis, particularly given past events dating back to the 15th century that have informed residents to prepare and provide drills for their schools and hospitals.

It also helped, Purkis said, that Tonga was effectively closed at the time of the eruption, given that the COVID-19 pandemic was still raging and all the tourist resorts on the coastline were closed.

However, it was also the location of Tonga’s main city on the coastline that saved it from the worst of the tsunami, Purkis said. The city is sheltered by a coral reef and a broad lagoon that refracted waves around the island to get into an even smaller lagoon.

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