Urgency Response to the 15 January 2022 Hunga Tonga - Hunga Ha'apai Volcanic Eruption -understanding the Volcanic Mechanism and Impact of the tsunami

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On January 15th, 2022, a month into the eruption of Hunga Tonga - Hunga Ha'apai volcano, located in the Tonga Island arc in the southwest Pacific, there was a short lived (two hour), extremely high intensity explosive eruption that destroyed most of the subaerial parts of the volcanic edifice. The resulting pressure wave and tsunami impacts were local and global, and the most far-reaching since the eruption of Krakatau volcano in 1883. Tsunami waves striking the nearby (65 km) low-lying coasts of Tongatapu Island, were up to 15 m high. Because of well organised early warning and evacuation, there were very few (three) fatalities, although there was significant destruction along coastal areas. The low-lying islands of the more distant Nomuka Group were completely overwhelmed, and villages destroyed. Farther afield from the Tongan islands, the tsunamis were caused by the massive atmospheric pressure wave, that is the first instrumentally recorded eruption-generated event of this scale, which affected the entire global atmosphere and ionosphere, causing the observed infrasound waves and unusual long-period seismic resonances. The Hunga Tonga - Hunga Ha'apai eruption was a major surprise and there remains a major uncertainty over the mechanism(s) and the generation of the local tsunamis striking the nearby Tongan islands coasts, with three possibilities considered 1) pyroclastic density currents resulting from the collapse of the 50 km high ash column resulting from the explosion, 2) submarine mass sediment movements associated with the destruction of the volcanic edifice, and/or, 3) the massive shock wave resulting from a phreatomagmatic explosion as sea water entered the fractured volcanic caldera. The objectives of the proposed research, therefore, are to identify from very high resolution (one metre) satellite images the volcanic mechanism (s) of the eruption and the impact (inundation, height, and destruction) of the tsunami on the nearby Tonga islands. To achieve these aims, we will use the satellite imagery, 1) to map the morphological changes of Hunga Tonga - Hunga Ha'apai volcano in the build-up to and during the eruption; 2) as basis for observations on the eruption mechanism and its impacts, particularly information on the timing of the events, and 3) identify the height and inundation of the tsunami on the Tongan islands together the resulting destruction. After previous recent tsunamis, such as those of 2018 in Indonesia, the impacts on coastal areas were mapped soon after the event from field work by multinational teams that recorded the destruction and inundation. A major challenge with the Hunga Tonga - Hunga Ha'apai event, however, is the ongoing lack of information on the tsunami impact from Tonga because the eruption fractured the internet communications cable connecting the country to the outside world - so since the eruption the Island Kingdom has been largely isolated. It is also because Tonga is in Covid lockdown, with no access for foreign (non-Tongan) visitors. Our use of satellite imagery to map the tsunami impact, therefore, is a novel approach, not used previously in mapping tsunami inundation immediately after an event. The eruption has resulted in a major programme of scientific research carried out by many scientific organisations with who we are co-ordinating. We are also co-ordinating as far as possible, with local scientists, who will provide observational information on the tsunami impact to validate the interpretations of satellite data. These interpretations, in addition to validating the tsunami modelling, will also be used to underpin mitigation of the tsunami impact.