Ocean Surface Gravity Waves Excited by the 2022 Eruption of Hunga Tonga‐Hunga Ha'apai Volcano

On 15 January 2022, a massive underwater eruption occurred at the Hunga Tonga‐Hunga Ha'apai Volcano. The plume reached the mesosphere, and the eruption excited a significant atmospheric Lamb wave, which forced the tsunami. The complicated tsunami waveforms due to ocean‐atmosphere coupling prevented inferring the force history of the excitation. To address this, we analyze ocean surface gravity waves (OSWs) from 15 to 40 mHz, which are decoupled from the Lamb wave due to their slower phase velocities. Modeling these OSWs, we infer that the excitation started at 4:00 UTC with an amplitude of 1010 $1{0}^{10}$ N and lasted for 5 hr, followed by a sub‐event at 8:40 UTC. The observations suggest an initial blowout of seawater above the summit and a subsequent outflow that excited a tsunami below 5 mHz. The 2 hr delayed OSW excitation from 6 to 15 mHz may indicate seawater inflow into the crater. Plain Language Summary On 15 January 2022, the Hunga Tonga‐Hunga Ha'apai volcano experienced a massive underwater eruption. This eruption sent a plume into the atmosphere and produced a powerful atmospheric wave that caused an early arrival of the tsunami. The complex tsunami propagation coupled with the atmospheric wave prevents us from understanding the excitation processes. To address this, we focussed on ocean surface gravity waves (OSWs) in an oscillatory period from 25 to 67 s, also interpreted as short‐period tsunamis. These OSWs are less affected by these atmospheric waves because of the slow propagation speed. Because the OSWs are less influenced by ocean depth, they are easier to model. The observed OSWs show that the eruption started at 4:00 UTC with a vertical force of approximately 1010 $1{0}^{10}$ N and continued for about 5 hr. Another significant sub‐event occurred around 8:40 UTC. Furthermore, the OSWs in an oscillatory period from 67 to 167 s arrived about 2 hr after the short‐period OSWs. These observations suggest that the eruption blew seawater above the volcano's summit and a subsequent outflow that excited a tsunami. The 2 hr delay in low‐frequency OSW could indicate that seawater flowed into the crater. Key Points Ocean surface gravity waves at 15–40 mHz were excited at 4:00 UTC when an underwater eruption occurred at Hunga Tonga‐Hunga Ha'apai Volcano A significant sub‐event at about 8:40 UTC followed the main event, and the ocean surface gravity waves lasted for half a day The observations of ocean surface gravity waves suggest a blowout of seawa