Examination of the International Monitoring System (IMS) hydroacoustic data recorded during the 15 January 2022 eruption of the Hunga Tonga–Hunga Ha’apai (HTHH) volcano shows that the in-water hydroacoustic sensors recorded a long-duration (40 min) activity starting before and ending after the paroxysmal HTHH eruption at about 4:15 UTC. The main eruption at 4:15 UTC simultaneously generated low-frequency [0.001–0.05] Hz seismic body waves and large infrasound waves in the atmosphere, as well as a tsunami. The peak-to-peak pressure observed at the closest IMS station, IS22, 1850 km away from the source, was 340 Pa, dwarfing the largest previously observed record for the signal from the Chelyabinsk meteor. The infrasound waves included a Lamb wave containing periods at least up to 2000 s. The Lamb wave and tsunami are well recorded on infrasound and near-shore seismometers. The air wave generated a precursor tsunami ahead of the main tsunami, but we do not observe it on the hydroacoustic IMS network. The dispersive tail of the main tsunami is observed on four in-water hydroacoustic hydrophone stations, one of them as far as 16,120 km away in the Atlantic Ocean.

This paper focuses on events linked to controlled underwater explosions of World War 2 (WW2) ordnances which were included in the Reviewed Event Bulletin (REB). Data used for the study were provided by seismic stations of the International Monitoring System (IMS) in 2020. Examined events were triggered by devices of different charge size and took place in several locations in Europe. There were also other, previously detected WW2 ordnance underwater explosions which could be compared to events in 2020. It is shown that these relatively small underwater explosions listed in the REB, with good coupling to the ground, are located by the IMS network within 20 km from the ground truth. Charge size of explosive material was related to event magnitude. Results were compared to magnitudes published for underwater explosions of larger sizes. The conclusion is that an in-water explosion will result in seismic waves with amplitudes equivalent to the amplitudes of seismic waves from an in-ground explosion with 17.2 times the yield in kT.