Massive Tongan eruption claimed few lives due to “quirk”

The eruption of Hunga Tonga-Hunga Ha’apai was the largest natural explosion in a century, and produced a major tsunami with the aftermath felt for more than an hour after the event, according to new scientific reports.

Research from Miami University, US, found the 15-megatonne eruption – a power greater than that of the United States’ largest nuclear blast – resulted in 45-metre waves near Tofua Island, part of the Tonga archipelago, and major swells witnessed as far as the South American coastline.

At the time of the January 2022 event, it was considered the most powerful since the eruption of Mount Pinatubo some 30 years earlier and estimated at around 10 megatonnes.

Fortunately, the eruption and tsunami which lashed nations around the Pacific resulted in few casualties, a likely combination of routine preparation by Tongan authorities, such as nationwide evacuation drills first held in 2017, and the location of the underwater volcano relative to populated areas.

The blast occurred amid COVID-19 restrictions, which meant fewer people on some of the resort islands near the eruption point could be impacted. Overall, six lives were lost to the event, including two women caught in swell off the Peruvian coast resulting from the volcanic tsunami.

And yet similar historic events – like Krakatoa’s 1883 eruption that claimed more than 36,000 lives – have been catastrophic.

The Tongan eruption’s less severe impact comes down to what lead researcher Professor Sam Purkis from Miami’s Rosenstiel School calls a “quirk of location”, where shallow seafloors in the vicinity acted as tsunami wave traps for an hour after the eruption. This may have reduced the threat of the largest waves, but may also have led to smaller waves increasing in size for an hour after the explosion.

How the Tonga eruption spread across the world.

Purkis’s team used a mix of satellite imagery, drone mapping and field observations taken by colleagues at the University of Auckland to simulate the eruption. While fatality numbers were low, they warn other underwater volcanoes pose a major risk to Pacific Island communities.

“Other submarine volcanoes possess the capacity to spawn a future tsunami at the same scale,” Purkis says. “This eruption holds important lessons for both past and future tsunami in Tonga and beyond.”

Purkis is hopeful the investigation into the tsunami will enhance the modelling of similar volcanic events around the world, including historic events.  Hunga Tonga-Hunga Ha’apai’s 2022 blast consisted of two minor explosions, followed by three major eruptive events, similar to the four during Krakatoa, and described in accounts of the Theran eruption near Santorini in 1600 BCE.