Nyiragongo Crater Collapses Measured by Multi-Sensor SAR Amplitude Time Series

Crater morphology undergoes rapid changes at active volcanoes, and quantifying these changes during volcanic unrest episodes is crucial for assessing volcanic activity levels. However, various limitations, including restricted crater access, cloud cover, haze, and intra-crater eruptive activity, often impede regular optical or on-site crater monitoring. To overcome these challenges, we utilize multi-sensor satellite Synthetic Aperture Radar (SAR) imagery to generate dense time series of quantitative indicators for monitoring crater morphological changes. By combining images from diverse satellites and acquisition modes, we achieve high temporal resolution. Nevertheless, due to variations in acquisition geometries, direct image comparisons become impractical. To address this, we develop PickCraterSAR, an open-access Python tool that employs basic trigonometry assumptions to measure crater radius and depth from SAR amplitude images in radar geometry. We apply our methodology to study the crater collapse associated with the May 2021 and January 2002 eruptions of Nyiragongo volcano. Following the 2021 collapse, we estimate the maximum depth of the crater to be 850 m below the rim, with a total volume of 84 +/- 10 Mm(3). Notably, the post-2021 eruption crater was 270 m deeper but only 15%-20% more voluminous compared to the post-2002 eruption crater. Additionally, we demonstrate that the 2021 crater collapse occurred progressively while a dike intrusion migrated southward as a consequence of the drainage of the lava lake system. Overall, our study showcases the utility of multi-sensor SAR imagery and introduces PickCraterSAR as a valuable tool for monitoring and analyzing crater morphological changes, providing insights into the dynamics of volcanic activity.