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New Zealand; Pacific
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Near-field simulation of the 2022 Tonga megatsunami highlights unexpected hazards across the Tonga Archipelago
A near-field simulation of the megatsunami generated by the January 15, 2022 Hunga Tonga-Hunga Ha’apai (HTHH) submarine volcanic eruption describes the aftermath of a series of powerful blasts that caused wave heights of up to 45 meters across Tonga Archipelago’s coastlines, despite claiming few lives. Validated against real-world observations, these findings highlight how complex underwater terrain and wave dynamics can influence tsunami hazards, Sam Purkis and colleagues say, with the potential to inform future hazard assessments for Tonga and other high-risk tsunami zones. The HTHH was one of the most powerful volcanic eruptions of the last century. Researchers have since used far-field observations to investigate HTHH’s eruptive sequence and the resulting seismic waves that reverberated through the Earth, atmosphere, and oceans. But inadequate near-field data has made it challenging to describe the local megatsunami generated by the eruption’s massive displacement of seawater. Now, Purkis et al. have simulated the HTHH megatsunami in the Tonga Archipelago using high-resolution bathymetry (water depth) maps and a diverse range of real-world observations. With the help of barometer readings, ear/eyewitness accounts, and records of windows breaking, the researchers estimated the energy and timing of three key blasts that could justify the tsunami wave runups observed by remote sensing and tide gauge data at 118 sites across the archipelago. They observed diverse and often unexpected wave heights produced by complex wave dynamics and bathymetry in the Tonga Archipelago, illustrating the variability of tsunami risks across short distances. “These results highlight how a single tsunami can remain ‘captured’ by an archipelago, how waves from multiple blasts interact even when separated by hours, and how wrap-around behavior can result in sizable tsunami beaching in areas where they would not necessarily be expected,” the authors write. For reporters interested in trends, a 2023 study published in Science Advances used far-field observations to describe seismic waves that propagated globally in the Earth and atmosphere following the 2022 Tonga eruption: https://www.science.org/doi/10.1126/sciadv.add4931
Expert Reaction
These comments have been collated by the Science Media Centre to provide a variety of expert perspectives on this issue. Feel free to use these quotes in your stories. Views expressed are the personal opinions of the experts named. They do not represent the views of the SMC or any other organisation unless specifically stated.
Cyprien Bosserelle, Hydrodynamics Modeller, NIWA
This analysis is good because it takes a look at the Tonga scale rather than the Pacific basin scale or global scale as most papers published so far do. I like that they are able to link the 4-5 blasts to the sequence of inundation – all this using a rather dispersive tsunami model. These results are consistent with our own unpublished analysis and other work we have presented in conferences in the past year. The paper reflects well where all the research teams are at.
The biggest remaining enigma is how we can explain the later part of the tsunami. Especially the timing of that 5th blast (a bit before UTC 5:00) is hard to understand and future work will probably focus on alternative source mechanism (perhaps a column collapse).
In the paper they call it a "fifth blast", and I’m not sure I fully agree that it is a blast. However, the point is that the largest tsunami wave to reach the west coast of Tongatapu arrived just after UTC 5:00. We know that from witness accounts, but mostly the Tonga Met weather station in Kanokupolu that reported data at 5:00-5:10 UTC but was subsequently destroyed by the tsunami (all we found from it were pieces of the batteries 50m away at the bottom of a mango tree).
However there is no clear trace of an explosion at the caldera just before 5:00 UTC (takes 15-18min for the wave to reach). The paper does acknowledge the lack of seismic evidence for a blast but has some evidence in the atmospheric remote sensing. The paper shows that a 5th blast can “explain” the tsunami signal. But the simulation they show only explains the general trend of the water level record not the high frequency oscillations. In other words, the evidence that this largest tsunami wave is indeed from an eruption blast is still pretty thin, but by default it is the best explanation. Other tsunami generation mechanism needs to be explored (an possibly eliminated if they can’t produce the right waves). We are working on that at the moment.
I believe the analysis is a nice demonstration of the progress of our understanding of the tsunami to date but it also highlights that there is more work to do both on understanding the mechanism of the tsunami in Tonga and the timing which ultimately will highlight the tsunami hazard associated with these type of volcano in Tonga and elsewhere. However future work will need to better use some of the data available in the region (the presented paper could have extended their model to include the DART buoys and the Vavau tide gauge to validate their model further).
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