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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 Mm3. 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. Plain Language Summary: Changes in crater morphology provide important hints to assess the activity of a volcano. In addition to optical and thermal imagery, radar images are useful to monitor the crater as they are not limited by daylight and cloud cover conditions. Unlike in optics, where pixels are located in the picture depending on their viewing angle, pixels are located on radar images depending on their distance to the satellite. In the presence of topography, this causes geometric distortions (shortening and layover), which complicates the interpretation of the images. PickCraterSAR, an interactive Python tool, was designed to assist the interpretation of those images and, based on simple trigonometry assumptions, to extract indicators of the crater radius and depth. Dense time series are obtained by mixing images acquired by various sensors in different viewing geometries. We apply this methodology to measure changes in Nyiragongo crater associated to the 2021 eruption. We show that the crater collapse is significantly deeper but only 15%–20% more voluminous than the 2002 collapse. Moreover, we can quantify the progression of the crater collapse that occurred after the 2021 eruption, while a magma intrusion was migrating underground southward for about a week. Key Points: Multi‐sensor Synthetic Aperture Radar (SAR) amplitude images provide dense time series of Nyiragongo crater during the 2021 progressive collapseNyiragongo crater was 270 m deeper after the 2021 eruption than after the 2002 eruptionPickCraterSAR is a simple open‐access interactive tool in Python to analyze images in multiples SAR geometries [ABSTRACT FROM AUTHOR]

Surface deformation accompanying dike intrusions is dominated by uplift and horizontal motion directly related to the intrusions. In some cases, it includes subsidence due to associated magma reservoir deflation. When reservoir deflation is large enough, it can form, or reactivate preexisting, caldera ring‐faults. Ring‐fault reactivation, however, is rarely observed during moderate‐sized eruptions. On February 21, 2015 at Ambrym volcano in Vanuatu, a basaltic dike intrusion produced more than 1 m of coeruptive uplift, as measured by InSAR, synthetic aperture radar correlation, and Multiple Aperture Interferometry. Here, we show that an average of ∼40 cm of slip occurred on a normal caldera ring‐fault during this moderate‐sized (VEI < 3) event, which intruded a volume of ∼24 × 106 m3 and erupted ∼9.3 × 106 m3 of lava (DRE). Using the 3D Mixed Boundary Element Method, we explore the stress change imposed by the opening dike and the depressurizing reservoir on a passive, frictionless fault. Normal fault slip is promoted when stress is transferred from a depressurizing reservoir beneath one of Ambrym's main craters. After estimating magma compressibility, we provide an upper bound on the critical fraction (f = 7%) of magma extracted from the reservoir to trigger fault slip. We infer that broad basaltic calderas may form in part by hundreds of subsidence episodes no greater than a few meters, as a result of magma extraction from the reservoir during moderate‐sized dike intrusions. Plain Language Summary: Many volcanoes feature large depressions, called calderas. Calderas form when enough magma leaves a deep reservoir, and the solid rock lid above this reservoir can no longer support its own weight. Caldera faults, or cracks surrounding the reservoir which extend from the reservoir to Earth's surface, form as the lid collapses. Ambrym volcano (Vanuatu) has a 12‐km wide caldera, and researchers propose it formed during an explosive eruption 2,000 years ago. However, in 2018, Ambrym's caldera sunk along caldera faults during a nonexplosive eruption. This observation questions whether an explosive eruption was necessary to form Ambrym's caldera in the first place. Furthermore, in February 2015, an eruption 10 times smaller than in 2018 also caused the ground to sink along caldera faults. Utilizing ground motion data obtained from satellite radar systems to model magma reservoir outflow and fault displacement, we conclude that, in 2015, the ground sank along caldera faults. This sinking is explained by the removal of as little as <7% of the stored magma from the reservoir. We therefore propose that Ambrym's wide caldera may have formed as a result of many frequently occurring medium‐sized eruptions. This challenges the thought that wide calderas mainly form as a result of large eruptions. Key Points: Ground displacement at Ambrym in February 2015 was caused by a dike intrusion, deflating reservoir, and normal slip on a caldera ring‐faultExtracting at most 7% of the magma from Ambrym's reservoir suffices to reactivate the caldera ring‐faultsNormal slip along Ambrym's ring‐fault can occur during moderate‐sized eruptions, resulting in subsidence and further caldera development [ABSTRACT FROM AUTHOR]

Magma propagation is an unsteady process controlled by magma‐crust interaction. To provide information on its dynamics, we invert complementary ground deformation data spanning the 8 hr preceding the 26 May 2016 eruption at Piton de la Fournaise (PdF) volcano (La Réunion, France). Data are inverted using 3‐D boundary element models combined with a Monte Carlo inversion method. The final geometry of the displacement source is determined based on four interferograms spanning the whole propagation phase while the dynamics of the propagation is inferred from temporal inversion of continuous Global Navigation Satellite System (GNSS) data, using the final geometry as an a priori to constrain the source. The best modeled magma path consists in a 2,700‐m‐long sill located 800 m above sea level and connected to the eruptive fissure by a subvertical dike. The quick opening of the horizontal part of the intrusion could have been favored by limited flank sliding during the early stage of propagation. The intrusion then stalled for ∼5 hr, while pressure increased slightly, until final upward propagation and eruption. Volume budget suggests that the eruption was fed by a single batch of magma quickly disconnected from its source. The delay prior to the eruption may reflect a limited magma supply. Finally, two mechanisms, potentially acting together, might have favored the eruption: a driving role of magmatic gas and/or, as often observed at Piton de la Fournaise, an eastward flank slip. Plain Language Summary: Basaltic magma stored beneath volcanoes reaches the surface by fracturing the Earth's crust. As experienced in May 2018 at Kilauea volcano (Hawaii, USA), magma can travel kilometers from the reservoir and fissure opening may threaten man‐made structures. Anticipating where and when eruptive fissures open requires better understanding of the factors controlling magma propagation. During the 26 May 2016 eruption of Piton de la Fournaise, Réunion Island,the preeruptive crisis spanned 8hr25min from the first signal recorded by the observatory to the eruption onset.We determine the magma paths and propagation timing, which led to this eruption using complementary satellite data of ground surface displacement, combining radar interferometry, which provides high spatial resolution, with GPS, which provides high temporal resolution. We highlight complex magma propagation within the subareal volcano, showing two direction changes, an arrest and an acceleration. Flank slip and magma degassing seem to play a key role in controlling both the geometry and the timing. Based on this scenario, this event was close to turn into a failed eruption as there was a 5‐hr pause in propagation before magma finally reached the surface. Understanding such unusual eruptions is a challenge for observatories as it may lead to repeated "false" alerts. Key Points: Magma feeding the May 2016 eruption propagated laterally as a sill before turning into a dikeThe sill propagation is stepwise with an initial acceleration followed by a 5‐hr pauseThe eruption was fed by a single batch of magma quickly disconnected from its source [ABSTRACT FROM AUTHOR]