*4.4. Detection of SO2 Gas Flux (UV)*

On 18 February 2019 a new eruption of the Piton de la Fournaise began. According to OVPF Reports, 14 <sup>±</sup> 5 Mm3 of lava were erupted during the 18 days of activity, fed by several eruptive fissures located on the upper east flank of the volcanic cone. Following a phase of gradual increase in volcanic tremor and the intensification of surface activity, the eruption ended abruptly on March 10 (OVPF Reports).

Figure 9 shows the time series of SO2 mass burden recovered from S5P (Figure 9b), complemented by thermal anomalies recorded from S2 SWIR data (Figure 9c). The SO2 mass obtained from MOUNTS is compared with the SO2 mass computed by NASA as a measure of correlation between the two datasets. In addition, thermal anomalies detected by S2 are overlaid with the VRP data provided by MIROVA (Figure 9c) to show correspondence between the active flow area and the heat radiated by the flow surface. Once suitably calibrated, the combination of SO2 and thermal data provides a synoptic view of the gas and magma fluxes during the course of the February–March 2019 eruption which can be used to track eruptive trends and patterns in real time [82]. Notably for this specific case, the two datasets show consistent trends, indicating a gradual intensification of the effusive and degassing activity during the final phases of the eruption. Thermal anomalies recorded after 10 March and in the absence of gas emission are attributed to the cooling of the lava field. The last SO2 detection (10 March 2019 09:38 UTC) is attributed to the gas plume, no longer fed from the eruptive vent but still inside the 500 × 500 km AOI as it slowly drifts away from the island.

**Figure 9.** Example of SO2 emission detection during the February 2019 eruptive crisis at Piton de la Fournaise (Reunion Island). (**a**) SO2 images from Sentinel-5P at selected dates (planet boundary layer PBL, 500 × 500 km mask). The detected pixels contaminated with volcanic SO2 are overlaid with a semi-transparent gray mask. (**b**) SO2 mass recovered by MOUNTS (purple markers) and by NASA (black markers, data available at https://so2.gsfc.nasa.gov/pix/daily/0319/reunion\_0319tr.html, computed on 1000 <sup>×</sup> 1000 km mask). (**c**) Number of hot pixels (×106) detected in the S2 SWIR image (orange curve, computed by MOUNTS), and Volcanic Radiative Power (VRP) recovered from MODIS data (black markers, computed by MIROVA).

#### *4.5. Combining Ground-Based and Space-Based Sensors*

Magma migration within the crust generates stresses, which can result in earthquakes as the surrounding rocks are displaced or fractured. This seismicity, commonly known as volcano-tectonic (VT) seismicity, is often recorded both prior and during volcanic eruptions, within and around the volcanic edifice [83]. High magnitude VT earthquakes can be recorded and located by global seismological networks, even when the nearest seismic stations are installed several hundreds of kilometers away. In turn, their timing, location, magnitude, and sometimes focal mechanism, are stored in open access global earthquake databases, particularly GEOFON and USGS catalogues. MOUNTS facilitates the interrogation of such catalogs, recovering potential earthquakes recorded in a region centered around the monitored volcano. This data can support the analysis of the volcanic phenomena, especially when the volcano is not equipped with ground-based monitoring instrumentation.

Figure 10 shows the recent eruption of Ambrym (Vanuatu), and illustrates how combining ground-based and space-based sensors helps understand the eruptive dynamics of this volcano located in a very remote and cloud-prone region. On 15 December 2018 and in the days that followed, a swarm of volcano-tectonic earthquakes were recorded in the vicinity of the volcano, with magnitudes ranging between ~4.5 and 5.5 (Figure 10d). The volcano was known until then for its persistent activity characterized by two active volcanic lakes (Figure 10(b.1)), responsible for high heat and gas fluxes (Figure 10b,c respectively), [84,85]. Analysis of the SAR intensity images immediately before and after this swarm reveal profound morphological changes (Figure 10(d.1,d.2)), in particular the collapse and enlargement of the summit crater. DInSAR analyses indicate very strong ground deformation during this period (Figure 10a red curve, Figure 10(a.1)), related to dyke intrusion and caldera subsidence [86]. Simultaneously, the decorrelation in the coherence map increases (Figure 10a blue curve), due to both the ground deformation and perhaps also pyroclastic deposits. Once stabilized, the coherence map reveals the presence of a new eruptive vent (Figure 10(a.2)), from which lava was most likely emitted, as suggested by the SWIR image acquired on 15 December 2018 (Figure 10(b.2)). Interestingly, following this event the volcano completely changed dynamics: the summit lava lakes were most likely drained, as suggested by the absence of thermal anomalies and the cessation of SO2 gas emissions.

#### **5. Discussion**

The key to detecting volcano unrest and understanding the underlying mechanisms is to be able to recognize when a volcano is deviating from its background level of activity. Once the eruption starts on the other hand, the key to decipher the eruptive dynamics and to mitigate the related hazards, is to integrate multiparametric dataset streaming from both space- and ground-based sensors, in order to provide the most comprehensive view of the eruptive phenomena. Both require "monitoring", i.e., observing the volcanic activity over long periods of time, during both quiescent and eruptive phases. As such, the aim of monitoring platforms such as MOUNTS is twofold: (1) a scientific one, aiming at deepening our understanding of volcanic processes and patterns at stake at active volcanoes, by processing in a systematic way large amounts of data in an effort to construct global databases, and (2) a societal one, aiming at producing more successful eruption forecasts, and providing additional information to the operational community (e.g., local volcano observatories and civil protection) in order to mitigate the risks related to volcanic hazards.

The results presented in this paper intend to demonstrate how the monitoring platform MOUNTS can contribute to both scientific and operational aims. We here discuss the benefits, limitations, and future developments of the system.

**Figure 10.** Multiparametric dataset combining space-based and ground-based sensors, which help decipher the recent eruptive dynamics of Ambrym (Vanuatu). The eruption onset on 15 December 2018 is indicated by a red vertical line across the plots. Time series of: (**a**) decorrelation score COH (pixels with coherence <0.5) in blue indicating surface reflectivity change, and deformation score DEF in red; (**b**) thermal anomalies from S2 SWIR analysis in orange (N hot pixels <sup>×</sup>106) and MODIS MIR analysis in black (VRP), processed by MOUNTS and MIROVA, respectively; (**c**) SO2 gas mass in the atmosphere from Sentinel-5P analysis in purple and OMPS analysis in black, processed by MOUNTS and NASA respectively; (**d**) magnitudes of the earthquake recorded in the vicinity of the volcano, recovered from the USGS global earthquake catalog. Images: (a.1) wrapped interferogram computed between SAR images d.1 and d.2, revealing very strong ground deformation (areas with strong phase decorrelation are masked to show only where deformation fringes are visible); (a.2) interferometric coherence, on which a new eruptive vent is identifiable; (b.1) S2 SWIR image prior to eruption onset, showing the persistent lava lakes activity; (b.2) S2 SWIR image after eruption onset, showing the lava flow emplacement from the new eruptive vent; (c.1) S5P image showing SO2 emissions prior to eruption onset; (c.2) S5P image showing strong decrease of SO2 emissions after eruption onset; (d.1,2) S1 intensity images immediately before and after the eruption onset, revealing profound morphological changes. Spatial extent of images a.1,2, b.1,2 and d.1,2 = 10 × 10 km, extent of images c.1,2 = 500 × 500 km.

*5.1. Benefits of MOUNTS*

The benefits of the developed system are the following:


The operational community such as volcano observatories can use MOUNTS and contribute to its development in a number of ways. The IGEPN (Instituto Geofísico de la Escuela Politécnica Nacional) for example, responsible for volcano monitoring in Ecuador, suggested to add Sangay to the list of monitored volcanoes in order to contribute to the surveillance of this remote edifice. The data available on the platform was used freely, and a collaborative exchange was initiated upon request to provide more specific data processing. The resulting material was further analyzed by IGEPN staff according to their needs, and was used in the activity reports describing the ongoing crisis for public information [87]. (Disclaimers on the data usage and appropriate acknowledgements can be found on the website). Scientific collaborations to investigate specific volcanic processes, or to develop specific methods (based on either the dataset available on the website, or on datasets resulting from more complex analysis) are also welcomed.
