The 2022 Hunga Tonga-Hunga Ha’apai Volcano Air-Wave Generated Tsunami

The tsunami that occurred after the Hunga Tonga-Hunga Ha’apai volcano eruption on 15 January 2022 was unusually fast and large, particularly at large distances from the source. Here we use an observation-calibrated air-wave model to generate ocean waves using a tsunami model. We used pressure data observed at 94 stations in Niue, the Cook Islands, and New Zealand’s main and outer islands to obtain a simple air-wave model. The modelled air-wave travels at an approximated constant speed of 317 m/s with an amplitude that decays with distance from the volcano. We then simulated the generation and propagation of tsunami due to the propagating air-wave in the atmosphere above the ocean. The leading sea surface displacement excited by the pressure disturbances travels at the same speed as the air-wave. This leading wave is then followed by subsequent water waves that travel in the same direction as the leading wave but at the conventional tsunami propagation speed. In the model, the air-wave was more effective at generating a tsunami when it travelled over a deep bathymetric feature like the Kermadec-Tonga Trench. The tsunami amplitudes observed at gauges do not decay as rapidly with distance from the volcano as would be expected for a localized tsunami source. This is due to the continuous excitation of the tsunami as the air-wave propagates across the ocean. In shallow water, the leading water surface displacement can often be much smaller than the later waves that were most likely to have been generated in the deep ocean. A better understanding of the complexity of tsunami generation and propagation of this kind is important to help improve future tsunami disaster mitigation from such sources.