The Hunga Tonga-Hunga Ha'apai undersea volcano eruption in 2022 was larger than any natural explosion in the past century or even any US nuclear explosion, according to a study released recently in Science Advances. It rivals the massive Krakatau volcanic explosion near Indonesia in 1883 that took more than 36,000 lives, though the Tongan volcanic explosion in the southwest Pacific caused four deaths.
"The only way you can make an explosion of this size is with a hydrogen bomb," said Sam Purkis, lead author of the study and marine geoscientist at the University of Miami. "This is way off the charts of anything" in human experience.
Hunga Tonga-Hunga Ha'apai undersea volcano eruption in 2022
Using satellite data, field observations and drone mapping, the team created a simulation of the eruption and resulting tsunami waves to provide a new detailed look of the explosive event. They found tsunami waves reached heights of 45 meters (148 feet) on Tonga's Tofua Island and up to 17 meters (56 feet) on Tongatapu, the country's most populated island and site of its capital Nuku'alofa. The study also showed why the tsunami was particularly damaging but how it could have been much worse.
Uncovering mindboggling facts
Since the January 15, 2022, eruption, researchers have uncovered several records reached during the event. It released the most water vapor into the atmosphere by a volcano on record, enough to fill 58,000 swimming pools, which may temporarily warm the climate in years to come. It set a world record for highest volcano plume in the satellite record, sending ash 36 miles high into the atmosphere, surpassing what many scientists had considered physically feasible. It triggered the fastest atmospheric waves ever observed at 720 mph, circling the planet at least six times.
Weeks after the eruption, co-author Shane Cronin from the University of Auckland and his colleagues visited the area to examine the damage on the ground. Climbing mountainsides and using drone imagery, they found downed trees and vegetation high along the coast.
"For the first time now, we can really confirm on the ground that this happened," said Purkis, chief scientist at the Khaled bin Sultan Living Oceans Foundation that helped gather data of the underwater terrain for the model as well. "It's an event which is unique in modern history."
On January 15, witnesses reported hearing two blasts shortly before 5 pm local in Nuku'alofa, about 40 miles away from the submarine volcano. Plumes began rising like an umbrella over the island within 30 minutes. Despite the sounds and visible plume, Purkis said these two blasts were probably small and did not trigger any meaningful tsunami. Then, two more much louder booms were reported. This time, the blasts triggered damaging tsunami waves on the western coast and city center of Nuku'alofa.
But then came the fifth - and most powerful - blast.
Strength of the blast
Running simulations and gauging pressure changes from broken windows on surrounding islands, the team estimated the strength of the last blast wave to be equivalent to 15 megatons (equivalent to 15 million tons of TNT). That's roughly equivalent to the largest nuclear test performed by the United States.
The last blast was so large that it physically displaced large amounts of seawater, creating the massive local tsunami. If you were theoretically at the center of the blast when it occurred, you would have then been standing on dry seafloor, Purkis said. The simulation suggests that one minute after the blast, the displaced wave was 85 meters high, "mind boggling and hard to believe," he said. By the time it reached Tonga's Tofua Island, the team's ground observations showed that the wave was 45 meters high, although Purkis said that might be an underestimate.
This seismic signature isn't common for volcanic eruptions, and has never been described before.
The team found that the shallower waters toward the shore played an important part in slowing down and dampening the waves. As the water moves toward the shore, it drags along the sea floor and slows down. It's the same reason big surfer waves further out in the ocean don't knock us off our feet on shore. One by one, the waves from each blast slowed down as they reached the coral reef platforms but remained circling in the shallow water. But Purkis said the waves from the different blasts caught up with one another, prolonging the tsunami risk.
Seawater infiltrating and steam-generated eruption
The most powerful blast, Purkis explained, was created by seawater infiltrating into the hot magma and exploding, created a steam-generated eruption. The first four blasts probably fractured the rocks, so that a huge quantity of water could infiltrate the magma chamber and create the incredibly large fifth blast.
While not addressed in this new study, other researchers have hypothesized what caused the series of blasts to occur. Volcanologist Melissa Scruggs and her colleagues propose a "magma hammer," or magma rushing up and hitting the volcano, then dropping back down into the magma chamber several times. Scruggs said the sudden rise of magma was caused by a sudden drop of pressure, which was observed in the seismic record at the time of the largest eruption.
"This seismic signature isn't common for volcanic eruptions, and has never been described before," Scruggs, a researcher at University of California at Santa Barbara, said in an email. "This isn't to say it's never happened, just that our scientific instruments have never recorded such a signature."
Scruggs also points out that the volcano started erupting in December, but scientists don't know what initially triggered the beginning of the eruption. Purkis said, "This is just what volcanoes do." He said they occasionally come to life, and magma starts moving around in the plumbing of the beast.
Co-author Dan Slayback said he was surprised at "simply how an event of such magnitude can appear with little apparent warning." Smaller eruptions had been occurring since December 2021 but had slowed down by mid-January.
Infographic explaining the volcano eruption when it happened in 2022
The big bang
"I don't think anyone particularly foresaw the big bang on the [Jan.] 15," said Slayback, a research scientist at NASA. He called the event a 1-in-500-year eruption for this region.
Given the magnitude and duration of the event, Purkis said the death toll could have been a lot worse, as with Krakatau. Yet he credits a quick response from the community and a low number of tourists in the area because of the coronavirus pandemic. It also happened during the day when people were out and about. The main city in Tonga is also sheltered behind an island in a lagoon, which provided protection from an incoming tsunami.
"This was a very serious event, but won't be remembered in the same way as Krakatau. But that's a good thing," Purkis said.
The study "did exceptional work moving the research on the 2022 local tsunami forward," said coastal engineer Ignacio Sepulveda Oyarzun, who was not involved in the study. He commended the authors' use of information from citizens, such as the report of blasts and plume timing and broken glass, to constrain the model.
"I did not know about the tsunamigenic potential of a possible fifth blast," said Sepulveda Oyarzun, a professor at San Diego State University, who has been studying the mechanism of even farther-reaching tsunami waves from the volcanic eruption, which led to two deaths in Peru and damage as far away as California and New Zealand.
"The effort for understanding volcanic-induced tsunamis extend beyond Tonga, since 1883 for the Krakatau tsunami," he said. "Ultimately, our goal is to clearly understand these tsunamis and to develop predicting tools which will be of great utility for early warning systems and community preparedness."
The tsunami that killed hundreds of people on Sumatra and Java in Indonesia in December 2018 also appeared to have been caused by volcanic activity.
There had been an eruption on the volcanic island of Anak Krakatoa about half an hour before.
In an earthquake, displacement of water can occur when the ground moves as a fault breaks. This was the mechanism by which a 9.1 magnitude earthquake on Dec. 26, 2004, off Aceh Province in northern Indonesia, spawned large waves that traveled across the Indian Ocean and killed 225,000 people.
Tsunamis typically follow earthquakes. An example is the September 2018 tsunami that devastated the city of Palu on the Indonesian island of Sulawesi after a 7.5 magnitude quake.
Volcanic activity creates a tsunami differently. One possibility is an explosive eruption, or general weakening of the flanks of a volcano by hot magma passing through. Either way, part of the volcano can collapse, creating a landslide that displaces water.
Another possible mechanism is the collapse of a magma chamber below the volcano as it empties during an eruption.
Volcano-related tsunamis are common. An eruption in 1792 in Japan created waves that were several hundred feet high. Landslides during the 1980 eruption of Mount St. Helens in Washington spawned large waves in a lake nearby.
The most famous volcanic disaster in history, the 1883 eruption of Krakatoa triggered tsunamis that killed tens of thousands of people. ("Anak Krakatoa" island has built up in place of Krakatoa, which was obliterated in the 1883 event.)