A document by Raphael Grandin. Click on the document to view its contents.

Crater morphology at active volcanoes can change rapidly. Quantifying changes during the course of a volcanic unrest episode may help assess the level of volcanic activity. However, limitations such as crater accessibility, cloud cover or intra-crater eruptive activity may hamper regular optical or on-site crater monitoring. Here we use multi-sensor satellite Synthetic Aperture Radar (SAR) imagery to produce dense time series of quantitative indicators of crater morphological changes. High temporal resolution is achieved by combining images from a variety of sensors and acquisition modes, though the diversity of acquisition geometries (incidence angle, viewing direction, resolution…) prevents direct comparison between the different images. Using basic trigonometry assumptions, we develop PickCraterSAR, an open-access tool written in Python 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. After the 2021 collapse, we estimate the maximum depth of the crater to be 850 m below the rim and the total volume to be 84$\pm$10 Mm$^3$ (270 m deeper but only 15-20 \% more voluminous than the post-2002 eruption crater). We also show that the 2021 crater collapse occurred progressively while a dike intrusion was migrating toward the south.

Stratospheric sulfate aerosols play a key role on atmospheric chemistry and Earth's radiation budget, but their size distribution, a critical parameter in climate models, is generally poorly-known. We address such gap for the 2022 Hunga Tonga-Hunga Ha'apai (HT-HH) volcanic eruption by exhaustively analyzing a set of satellite observations (TROPOMI, IASI, AHI, CALIOP) together with photometric ground observations from the worldwide open-access AERONET network. We document a rapid growth of HT-HH sulfate aerosols in the days following eruption, faster than observed for 1991 Pinatubo eruption, likely due to the exceptional hydration of the stratosphere by this phreatomagmatic eruption. An unusual aerosol fine mode (peak radius in 0.3-0.5 µm) is identified at 20 stations of the southern hemisphere until May 2023 (time of writing). Nevertheless, 1.4 years after eruption, HT-HH sulfate aerosols remain smaller than Pinatubo particles. Smaller aerosols backscatter more efficiently visible light and sediment more slowly than larger particles, implying stronger and longer-lasting negative radiative forcing.

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 pre-existing, caldera ring-faults. Ring-fault reactivation, however, is rarely observed during moderate-sized eruptions. On February 21st, 2015 at Ambrym volcano in Vanuatu, a basaltic dike intrusion produced more than 1 meter of co-eruptive uplift, as measured by InSAR, SAR correlation, and Multiple Aperture Interferometry (MAI). 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 million cubic meters and erupted ∼9.3 million cubic meters 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.

The 2004-2009 caldera uplift is the largest instrumentally recorded episode of unrestat Yellowstone caldera. We use GPS and InSAR time series spanning 2004-2015, witha focus in the aforementioned event to understand the mechanisms of unrest. InSARdata recorded∼25 and∼20 cm of uplift at the Sour Creek (SCD) and Mallard Lake(MLD) resurgent domes during 2004-2009, and∼8 cm of subsidence at the NorrisGeyser Basin (NGB) during 2004-2008. The SCD/MLD uplift was followed by sub-sidence across the caldera floor with a maximum at MLD of∼1.5-2.5 cm/yr and nodeformation at NGB. The best-fit source models for the 2004-2009 period are two hori-zontal sills at depths of∼8.7 and 10.6 km for the caldera source and NGB, respectively,with volume changes of 0.354 and -0.121 km3, and an overpressure of∼0.1 MPa. TheInSAR and GPS time series record exponentially increasing followed by exponentiallydecreasing uplift between 2004 and 2009, which is indicative of magma injection intothe caldera reservoir. However, magma extractionfrom NGB to the caldera is unable to explain the subsidence coeval with the calderauplift. Models of magma injection can also explain other episodes of caldera uplift likethat in 2014-2015. Distributed sill opening models show that magma is stored acrossthe caldera source with no clear boundary between MLD and SCD. Since the magmaoverpressure is orders of magnitude below the tensile strength of the encasing rock,historical episodes of unrest like these are very unlikely to trigger an eruption.

The 2004-2009 uplift episode is the largest recorded episode of unrest at Yellowstone caldera. We use GPS and InSAR time series spanning 2004-2015, with a focus in the aforementioned event to understand the mechanisms of unrest. InSAR data recorded ~25 and ~20 cm of uplift at the Sour Creek (SCD) and Mallard Lake (MLD) resurgent domes during 2004-2009, and ~8 cm of subsidence at the Norris Geyser Basin (NGB). The SCD/MLD uplift was followed by subsidence across the caldera floor with a maximum at MLD of ~1.5-2.5 cm/yr and no deformation at NGB. The best-fit source models are two horizontal sills at depths of ~8.7 and 10.7 km for the caldera source and NGB respectively, with volume changes of 0.354 and -0.121 km3, and an overpressure of ~0.1 MPa. The InSAR and GPS time series record an exponential increase followed by exponential decrease in the uplift, which is indicative of magma injection into the caldera reservoir, with no need for other mechanisms. However, magma extraction from NGB to the caldera is unable to explain the subsidence coeval with the caldera uplift. The GPS time series of the 2014-2015 episode of caldera uplift can also be explained by a magma injection model. Distributed sill opening models show that magma is stored across the caldera source with no clear boundary between MLD and SCD. Since the magma overpressure is orders below the tensile strength of the encasing rock, historical episodes of unrest like these are very unlikely to trigger an eruption.

On the 22nd of May 2021, although no alarming precursory unrest had been reported, Nyiragongo volcano erupted and lava flows threatened about 1 million of inhabitants living in the cities of Goma (Democratic Republic of Congo) and Giseny (Rwanda). After January 1977 and January 2002, it was the beginning of the third historically known flank eruption of Nyiragongo volcano and the first ever to be recorded by dense measurements both on the ground and from space. In the following days, seismic and geodetic data as well as fracture mapping revealed the gradual southward propagation of a shallow dike from the Nyiragongo edifice underlying below Goma airport on May 23-24, then Goma and Gisenyi city centers on May 25-26 and finally below the northern part of Lake Kivu on May 27. Southward migration of the associated seismic swarm slowed down between May 27 and June 02. Micro seismicity became more diffuse, progressively activating transverse tectonic structures previously identified in the whole Lake Kivu basin. Here we exploit ground based and remote sensing data as well as inversion and physics-based models to fully characterize the dike sized, the dynamics of dike propagation and its arrest against a structural lineament known as the Nyabihu Fault. This work highlights the shallow origin of the dike, the segmented dike propagation controlled by the interaction with pre-existing fracture networks and the incremental crater collapse associated with drainage which led to the disappearance of the world’s largest long-living lava lake on top of Nyiragongo.