## 4 Results
### 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  
Figure 9: Example of SO\({}_{2}\) emission detection during the February 2019 eruptive crisis at Piton de la Fournaise (Reunion Island). (**a**) SO\({}_{2}\) images from Sentinel-5P at selected dates (planet boundary layer PBL, \(500\times 500\) km mask). The detected pixels contaminated with volcanic SO\({}_{2}\) are overlaid with a semi-transparent gray mask. (**b**) SO\({}_{2}\) mass recovered by MOUNTS (purple markers) and by NASA (black markers, data available at [[https://so2.gsfc.nasa.gov/pix/daily/0319/reunion_0319](https://so2.gsfc.nasa.gov/pix/daily/0319/reunion_0319) tr.html]([https://so2.gsfc.nasa.gov/pix/daily/0319/reunion_0319](https://so2.gsfc.nasa.gov/pix/daily/0319/reunion_0319) tr.html), computed on \(1000\times 1000\) km mask). (**c**) Number of hot pixels (\(\times 10^{6}\)) 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).  
(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 \(\sim\)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 SO\({}_{2}\) gas emissions.