The massive eruption of Tonga generated a series of tsunamis that circled the planet and may have started as a single mound of water about the height of the Statue of Liberty.
In addition, the explosive eruption triggered an immense atmospheric shock wave that generated a second set of especially fast tsunamisa rare phenomenon that can complicate early warnings of these often destructive waves, researchers report in October. Ocean Engineering.
As the Hunga Tonga-Hunga Ha’apai submarine volcano erupted in the South Pacific in January, it displaced a large volume of water upwards, says Mohammad Heidarzadeh, a civil engineer at the University of Bath in England (Serial number: 01/21/22). The water in that colossal mound then “ran downhill,” as fluids often do, to generate the initial set of tsunamis.
To estimate the mound’s original size, Heidarzadeh and his team used computer simulations as well as data from deep-sea instruments and coastal tide gauges some 1,500 kilometers from the eruption, much of it in or near New Zealand. The arrival times of the tsunami waves, as well as their sizes, at those locations were key data, Heidarzadeh says.
The team looked at nine possibilities for the initial wave, each of which was shaped like a baseball pitcher’s mound and had a different height and diameter. The best fit to real-world data came from a 300-foot-tall, 7.5-mile-diameter mound of water, the researchers report.
That initial wave would have contained an estimated 6.6 cubic kilometers of water. “This was a really big tsunami,” says Heidarzadeh.
Despite starting out some nine times higher than the tsunami that devastated the Tohoku region of Japan In 2011, Tonga tsunamis killed just five people and caused an estimated $90 million in damage, largely due to their remote source (Serial number: 02/10/12).
Another unusual aspect of the Tonga eruption is the second set of tsunamis generated by a strong atmospheric pressure wave.
That pressure pulse resulted from a steam explosion that occurred when a large volume of seawater seeped into the hot magma chamber below the erupting volcano. As the pressure wave raced across the ocean surface at speeds greater than 300 meters per second, it pushed the water forward and created tsunamis, Heidarzadeh explains.
Along many coastlines, including some in the Indian Ocean and Mediterranean Sea, these pressure-wave-generated tsunamis arrived hours before the gravity-driven waves that rolled out from the 300-foot-high mound of water. Gravity-driven tsunami waves typically travel through the deepest parts of the ocean, away from the continents, at speeds between 100 and 220 meters per second. When waves reach shallow water near the shore, they slow down, the water pools, and then washes ashore, where destruction occurs.
Tsunamis generated by pressure waves have been reported for only one other volcanic eruption: the 1883 eruption of Krakatau in Indonesia (Serial number: 8/27/83).
Those faster-than-expected arrival times, plus the fact that pressure-wave tsunamis from the Tonga eruption were comparable in size to gravity-driven ones, could complicate early warnings of these tsunamis. That’s worrying, says Heiderzadeh.
One way to tackle the problem would be to install instruments that measure atmospheric pressure with the deep-sea equipment already in place to detect tsunamis, says Hermann Fritz, a tsunami scientist at Georgia Tech in Atlanta.
With that setup, scientists could discern whether a passing tsunami is associated with a pressure pulse, thus providing a real-time clue as to how fast the tsunami wave might travel.