GNGTS 2024 - Atti del 42° Convegno Nazionale

Session 1.3 GNGTS 2024 Monitoring the Tonga volcano startng from the unrest of 2014/2015 to the 2021/2022 explosion with Sentnels C. Braitenberg Department of Mathematics, Informatics and Geosciences (MIGe), University of Trieste, Trieste, Italy, This study explores the dynamic evoluton of the Tonga volcano, Tonga, throughout its volcanic cycle, with a partcular focus on the formaton and subsequent disappearance of a new island between Hunga Tonga (HT) and Hunga Ha’Apai (HH) between 2014 and 2023. The island frst increased its surface extension in 2015 and vanished in January 2022, by a signifcant erupton, characterized by a 50 km high ash plume and Tsunami. Leveraging remote sensing techniques, specifcally multspectral imaging from the satellite missions Sentnel 2 and SAR imaging from Sentnel 1, the research employs a supervised Random Forest classifcaton algorithm to track the changing subaerial surface area of the volcano. This approach enables the documentaton of size variatons in the islands, especially during weeks surrounding volcanic unrests. The classifer, trained on nearly cloud-free multspectral images, automatcally defnes surface area changes over the years. The temporal resoluton of area change is constrained by images with less than 5% cloudiness, resultng in 113 Sentnel 2 images between 2016 and 2023, selected from 506 available images. The Sentnel 1 Synthetc Aperture Radar (SAR) images penetrate clouds, and are therefore complementary to the multspectral observatons, with the drawback of observing the refectvity only on up to two bands. Nonetheless the classifer is successful in distnguishing land from ocean, albeit with greater noise. The Sentnel 1 observatons start in 2014, extending the tme series to cover the volcano unrest of 2014/2015, totalling 213 images. The processing is fulflled in the Earth Engine cloud computng data facility. Analysis reveals a slight decrease in area change afer emplacement of a new island in 2015 and identfes the disappearance of the island bridge connectng HT and HH, along with two smaller islands south of HTHH in 2022. The 2022 unrest is preceded by an increase in island