Deformation and morphological changes before the 2021–2022 explosive eruption at Hunga Tonga-Hunga Ha’apai submarine caldera revealed by satellite remote sensing

On December 19, 2021, the Hunga Tonga-Hunga Ha'apai (HTHH) submarine volcano in the Tonga-Kermadec island arc initiated a new eruptive phase that lasted for approximately 1 month after 7 years of dormancy. This eruptive phase culminated in the January 15, 2022, VEI ≈ 6, Plinian eruption and the associated destructive tsunami. To analyze the complete eruption sequence of the HTHH volcano from 2021 to 2022 and to relate it to the previous eruptions, satellite optical images and synthetic aperture radar images are analyzed revealing the morphological changes of the volcanic island(s) during the time. The preparatory phase preceding the first eruption on December 19, 2021, is studied by considering descending Sentinel-1A images from 2020 to 2021. The obtained surface deformation of the HTHH volcano before the December 2021 eruption shows an up to nearly 6.4 cm on the line of sight. Results reveal that a possible intrusion of magma started in May 2020 and gradually increased until December 2021, leading to a 19-month long deformation phase before the eruption. The observed deformation is possibly explained by (a) a NNW-SSE striking, magma-filled dike upraising since the beginning of 2020 from a 5-km-deep spheroid-like source located at 5000 m depth, i.e., the magma chamber, and (b) gravity instability phenomena along the flanks of the tuff cones. Our study highlights the relevance of satellite monitoring of small volcanic islands resenting the top or caldera rims of larger submarine volcanoes. The capability to analyze the deformation of such small islands may provide valuable information for the detection of unrest episodes.