One year on: reflections on the Hunga Tonga-Hunga Ha’apai volcanic eruption

The tsunami from Hunga Tonga-Hunga Ha’apaiVolcano (HTHHV) in Tonga on the 15 January 2022 was the first from a violent eruption for over 130 years, the last being the 1883 Krakatau volcano eruption in Indonesia in 1883. HTHHV was also the first dual eruption tsunami since the Krakatau event, and the first recorded by modern technology. The resulting shockwave was the most significant ever recorded and the volcano’s plume was the highest on record.

The HTHH event was comparable to the Papua New Guinea (PNG) submarine landslide tsunami of 1998, which resulted in 2,200 fatalities, and the Indian Ocean (IOT) earthquake tsunami of 2004 when over 250,000 people died. Although the mechanisms are different, all identify a previously unrecognised tsunami hazard. The Papua New Guinea submarine landslides generated massively destructive tsunamis, while the Indian Ocean event saw great magnitude earthquake tsunamis striking along convergent margins outside of the Pacific Ocean.

Volcanic activity along the Tonga convergent margin is not unexpected. HTHH last erupted in 2015, but the magnitude and violence of the 2022 eruption was a complete surprise. The unexpected nature of the eruption reveals that global hazard from large volume volcanic eruptions is underestimated and also identifies a global unpreparedness for the impacts from these events. Globally, there are 42 volcanoes with the potential to erupt on a similar scale to Krakatau and HTHH, so the events of 15January 2022 should serve as a ‘wake-up’ call to the potential hazard from other violent eruptions. Many of these volcanoes, unlike HTHH, are close to high density coastal populations. So far there has been little published on the eruption or on local tsunami mechanisms.

So far over one hundred papers on the event have been published. Most focus on the satellite data recordings of the atmospheric disturbances generated from the shock wave and ‘far field’ tsunamis., both due to changes in atmospheric pressure on the ocean surface that initially generate small waves that increase through a process termed ‘resonance’ which is determined by the relationship between the speed of the shockwave and the speed of the tsunami.

There are still fewer papers on the local, or near, tsunamis and the mechanism of these is still uncertain. There are several possibilities as to what generated these tsunamis including collapsing pyroclastic density currents (PDCs), caldera collapse and phreatomagmatic explosions.

One fear from the eruption was an impact on climate, possibly from sulphur dioxide (S0₂), but measurements showed that the presence of S0₂ was quite low volume. More surprisingly was the volume of water pumped into the atmosphere, and recent research shows this to be a high volume with possible impact on climate.

Regarding the ‘wake up’ call from the impact of the event, so far there has been little reaction, which could be attributed to the volume of unpublished papers (such as seabed mapping of the area). The as-yet unpublished papers could well impact the uncertainty over the local tsunami mechanism. The source of the cataclysmic culminating explosion is also to be determined, whether it was mixing of magmas beneath the caldera or the entry of cold water into the edifice, during the final phase. There is still much to do, and many lessons to take from the Hunga Tonga-Hunga Ha’apai eruption and following events but once the results are in, hopefully the real mitigation will begin.

Further reading:

About the author