Sangay volcano (Ecuador): multiparametric analysis of the December 2021 eruptive activity including the opening of new vents, a drumbeat seismic sequence and a new lava flow

Sangay is a 5286 m high stratovolcano located in the southern part of the Ecuadorian Andes, about 200 km south of the capital city of Quito. Sangay is the last active volcano to the south of the Northern Andes, and has been characterized by an almost constant and continuous activity with variable periods of quiescence. During historical times, the written reports describe at least 9 major eruptions since 1628. Sangay has been instrumentally monitored by the Instituto Geofísico of the Escuela Politécnica Nacional (IG-EPN) since 2013. In May 2019, Sangay began a new eruptive period, which is still ongoing and has been categorized as the most intense in the last six decades. The main phenomena produced during this period are small explosions, ash and gas emissions, lava fountaining, lava flows and associated pyroclastic currents and secondary lahars.

On 1 December 2021, from around 19:20 UTC, the seismic recordings of SAGA station began to show transient events occurring regularly. These events persisted for the next 13 hours with an irregularly accelerating rate of occurrence and increasing amplitude before merging into tremor at around 08:20 on 2 December. This sequence was rapidly followed by two explosive emissions, which were observed by the GOES-16 satellite, the first one at 09:02 and the second at 09:13. The emissions produced a 14.5 km-high gas-rich, ash-depleted eruptive column without any associated regional fallout reported. This drumbeat sequence was produced after a series of morphological changes observed through satellite images (Planet and Sentinel 2). Specifically, during the short time period considered in this study: 1) two new vents opened; 2) a landslide affected the northern flank of the volcano; 3) the first drumbeat sequence was recorded at Sangay; and 4) a new lava flow was emitted through the new northern vent. The drumbeat sequence is interpreted as being caused by the forced extrusion of this new lava flow through the new opening northern vent. Timely communication of this kind of volcanic events is favored by the creation and strict following of internal protocols within volcano observatories and the appropriate use of social networks allowing thousands of people to be reached in very short time period. The corresponding short report produced by the IG-EPN reached more than 300.000 people.