Volcanic eruptions can trigger tsunamis that pose significant threats to nearby coastal communities. The mechanisms responsible for the formation of tsunamis after volcanic eruptions are still poorly understood. Some possibilities include submarine landslides, pyroclastic flows (fast-moving current of hot gas and volcanic matter that flows in the ocean), caldera column collapses (massive blocks of rock near the top of the volcano sliding down into the volcano), deep-ocean explosions, volcano-tectonic earthquakes, or atmospheric air-pressure waves. More research is needed to better understand these mechanisms so that better early warning systems can be developed. The Hunga-Tonga Hunga-Ha′apai volcano is situated in the South Pacific Ocean about 1000 km south of Fiji and Samoa islands (Figure b). It erupted on January 15, 2022 creating the biggest atmospheric explosion ever recorded². The eruption triggered a tsunami³ affecting Tonga, Fiji, American Samoa, Vanuatu, New Zealand, Japan, the United States, the Russian Far East, Chile and Peru, resulting in at least four casualties.

A recently published study investigated the mechanisms causing the tsunami after the 2022 Tonga volcanic eruption using simulations from a dedicated ocean-atmosphere model and satellite altimetry that can capture sea surface height variations associated with a tsunami. Figure b shows, as black solid lines, the different altimetry satellite tracks that captured the tsunami on top of a simulation of the volcanic tsunami. Sentinel-6 Michael Freilich (S6MF) flies over the same area every 10 days to  monitor sea level changes. Figure c shows different time series of sea surface height from S6MF before (green and dark blue solid lines), during (red solid line), and after (cyan and magenta solid lines) the passage of the tsunami. The red line shows that during the eruption, the sea level captured by S6MF was higher than before the eruption and consistent with the ocean-atmosphere model prediction (blue dotted line).

Using this model, the study finds that the volcanic eruption caused a lateral movement of air forming air-pressure waves (in red in Figure a) followed by a proportional displacement of water in the ocean to fill in the crater now formed (in blue in Figure a) that then triggered the tsunami. Figure d shows the amount of time that passed between the air-pressure waves formation and the arrival of the tsunami in surrounding islands (10 minutes for the Tonga island and 50 min for the Fiji island). This research significantly improves our understanding of the physics of tsunami generation and can help improve early warning systems. For example, this study suggests that air-pressure sensors could be used in early warning systems which could give people warning 10-50 minutes before a tsunami.