<p>During the last two decades, Sangay has been one of the most active Ecuadorian volcanoes. However, because of its remote location and logistically difficult access, monitoring Sangay is a challenging task. The IG-EPN tackled this problem by expanding its terrestrial monitoring network and complementing it with the available satellite data. On 7<sup>th</sup> May 2019, the most recent and ongoing eruptive episode commenced. Compared to the previously monitored and observed eruptive activity at Sangay since the 2000&#8217;s, this episode is by far the most intense and the first to affect populated areas due to ash fallouts and numerous lahars. Surface activity is generally characterized by frequent low-to-moderate magnitude ash emissions and a semi-continuous viscous lava flow extrusion. This activity is punctuated by occasional lava flow collapse events, probably associated with pulses of high lava extrusion and that produced long-runout pyroclastic density currents towards the southeastern flank.</p><p>Here, we present the most complete data set of long-term instrumental observations performed at Sangay. SO<sub>2</sub>&#160;degassing, seismic activity, ground deformation, ash emissions and thermal anomalies are depicted as a multiparametric sequence to better understand the link between these parameters and the dynamism and eruptive style of this isolated volcano. &#160;</p><p>Correlations between the depicted parameters are not straight-forward, making it hard to identify patterns that might lead to enhanced eruptive activity. High values of SO<sub>2</sub>&#160;recorded by the DOAS instruments as well as the TROPOMI satellite sensor seem to coincide with periods of increased eruption rate. Nevertheless, increases in SO<sub>2</sub> flux do not occur systematically before or after these episodes. Seismic activity, characterized by daily counts of individual seismic events, does not demonstrated a clear precursory pattern either. These results indicate that none of the available monitoring parameters currently allow for a timely forecast of the largest and potentially most dangerous eruptions. However, looking at the entire time series we are able to distinguish a slightly but progressive change in the ground deformation displacement associated with a higher number of earthquakes per day prior to the 20 September 2020 paroxysmic event. This eruption produced regional ash fallout which affected significant swaths of farming lands and livestock. Since then, a different ground deformation pattern has taken hold, and coincides with a step decrease in the number of daily earthquakes and a significant increase in the SO<sub>2</sub>&#160;mass measured by TROPOMI.</p><p>This behavior matches an open-vent system, where punctual increases in eruptive activity show few precursory signals. The observed increase in all the parameters compared to previous eruptions before 2019 allows us to propose that this eruptive phase is fed by batches of deep and volatile-rich magma which rise to the surface at high ascent rates. The interpretations presented here are an important step towards a better understanding of the dynamism and eruptive style of this very active and isolated volcano. Moreover, the various monitoring parameters from terrestrial to satellite provide a better picture of the behavior of Sangay that could be applied to other remote and open-system volcanoes.</p>
Migration of seismic activity associated with phreatic eruption at Merapi volcano, Indonesia
