The eruption of the Tonga volcano caused a megatsunami

Mathilde Fontez, editor-in-chief at the magazine Epsiloon, today tells us about the terrible eruption of the Tonga volcano, which shook the Tonga Islands on January 15, 2022, and which caused a megatsunami.

franceinfo: Have scientists analyzed the tsunami that this gigantic eruption caused more than a year ago?

Mathilde Fontez: Yes, remember, a little over a year ago: an underwater volcano suddenly erupted in the Tonga archipelago in the Pacific. A record eruption – the most powerful since that of Krakatoa in 1883. It was heard as far away as Alaska, more than 8000 km from the volcano. It caused an earthquake measuring 5.8 on the Richter scale; its plume rose 58 kilometers above sea level – beyond the stratosphere; the shock wave circled the Earth several times.

In short, geophysical specialists immediately began to study this extraordinary phenomenon. And there, a team from the University of Miami has just produced the analysis of the tsunami associated with the eruption.

Was there a global tsunami?

Yes, and that the researchers had measured it immediately: There was first a first wave of 1m20 high, which hit the capital of Tonga 20 minutes after the explosion. Then a wave of a few tens of centimeters crossed the Pacific at the speed of sound – it was detected as far as the Mediterranean.

But the analysis of the local tsunami, in the lagoon of the Tonga archipelago, was missing. This is what these researchers have just done: they have recovered all the satellite data taken before and after the eruption, drone images, photos, field observations. They studied 118 sites on 10 islands in Tonga. And they were able to reconstruct the tsunami in a simulation.

What they see is that on the north side of Hunga Tonga, the waves reached 85 meters high, a minute after the explosion. 65 meters on the south side. 20 minutes after the explosion, they recorded waves 45 meters high, all along the coast of Tofua Island, 90 kilometers to the north. In short, data that places the phenomenon in the category of megatsunamis. Again, the Tonga eruption is totally out of the ordinary.

This megatsunami is linked to the record power of the explosion?

Yes, but not only. The researchers also point to the geography of the places. This tsunami grew in power because the shock waves were trapped in the lagoon: a shallow area. What the researchers are discovering is that this type of area is a double-edged sword: this shallow depth initially dampened the waves, but also trapped them for more than an hour, and ultimately amplified them – the waves are accumulated.

This is the first time we have seen this phenomenon. And this should lead to a reassessment of the risk of tsunamis in certain areas that were thought to be protected.