When Hunga Tonga-Hunga Ha‘apai, a mostly submerged volcanic cauldron in the South Pacific Ocean, exploded on 15 January, it unleashed a blast perhaps as powerful as the world’s biggest nuclear bomb, and drove tsunami waves that crashed into Pacific shorelines. But 3 hours or so before their arrival in Japan, researchers detected the waves of another small tsunami. Even stranger, tiny tsunami waves just 10 centimeters high were detected around the same time in the Caribbean Sea, which is in an entirely different ocean basin. What was going on?
Researchers say there is only one reasonable explanation: The explosion’s staggeringly powerful shock wave, screaming around the world close to the speed of sound, drove tsunamis of its own in both the Pacific and Atlantic oceans. It’s the first time a volcanic shock wave has been seen creating its own tsunamis, says Greg Dusek, a physical oceanographer at the National Oceanic and Atmospheric Administration who documented the phenomenon using a combination of tide and pressure gauges around the world.
But, “It’s almost certainly happened in the past,” says Mark Boslough, a physicist at the University of New Mexico, Albuquerque. The discovery suggests the shock waves generated by explosive eruptions in Earth’s history, and by other violent cataclysms, like the airbursts of comets or asteroids colliding with the planet’s atmosphere, may have also created transoceanic tsunamis, perhaps with considerably bigger waves.
To make a classic tsunami, a sizable amount of water needs to be shoved aside. The sudden displacement of the sea floor in an earthquake, for example, drives most tsunamis. Volcanoes can also generate them, and the Tonga volcano’s eruption clearly accomplished this in some form or the other—either through the underwater part of its explosion, the partial collapse of the volcano, or the aggressive deposition of freshly erupted debris into the sea.
But strong weather events can also create shoreline wave surges, called meteotsunamis. Creating one requires a sustained atmospheric disturbance with a substantial pressure drop or jump. That air pressure wave also needs to move at roughly the same speed as the sea’s waves. As the waves travel together, Dusek says, “You just keep feeding energy into that wave, and it builds up and up and up.”
Dusek says meteotsunamis occur about 25 times a year on the U.S. east coast. Most generate wave heights of only a few centimeters—barely noticeable and decidedly nonthreatening. But occasionally, they can cause chaos. In 2013, for example, a meteotsunami’s 2-meter wave injured three people in New Jersey. And a 3-meter one at Florida’s Daytona Beach injured 75 people in 1992.
Gerard Fryer, an emeritus tsunami researcher at the University of Hawaii, Manoa, says Tonga’s shock wave tsunamis are not true meteotsunamis. “It doesn’t involve weather,” he says. But it did create a pressure wave that tracked with the shoreward motion of sea waves, he says, so it’s broadly in the same physics family.
The speed of the Tongan shock wave also sets it apart from traditional meteotsunamis. At more than 300 meters per second, it was at least an order of magnitude faster than the pressure waves typically associated with weather disturbances, Dusek says. That speed also explains why the meteotsunami appeared in Japan several hours ahead of the volcano’s classical tsunami.
The speed of sea waves is constrained by the water’s depth, with faster waves requiring greater depths. So if those sea waves are to keep up with the air pressure wave and become amplified, they require deep waters. Dusek says that explains why the volcano’s meteotsunami waves were most clearly seen off Japan and in the Caribbean: They have deep ocean trenches.
Boslough believes shock wave tsunamis may have been triggered by even stronger eruptive explosions, such as the 1883 outburst at Indonesia’s Krakatau volcano, or the most explosive phases of Yellowstone’s megaeruption 2.1 million years ago. And although considerably rare on human time scales, he says, a volcanic explosion mighty enough could potentially create “a big tsunami in all ocean basins.”
Powerful, speedy shock waves are also generated by the midair self-destruction of meteors. Boslough has long suspected that these can make their own potentially devastating tsunamis, but the shock wave from Tonga’s eruption has broadened his perspective. “What didn’t occur to me,” he says, is that these shock waves can make tsunamis “on the opposite end of the planet.”
“I was thinking out of the box, but not enough outside of the box, I think.”