Developing hazard scenarios from monitoring data, historical chronicles and expert elicitation: case study of Sangay volcano, Ecuador

Abstract

Sangay volcano is widely considered as one of the most active volcanoes worldwide. Nevertheless, due to its remote location and low-impact eruptions, its eruptive history and hazard scenarios are poorly constrained. In this manuscript, we addressed this issue by combining an analysis of monitoring data and historical chronicles with expert elicitation. The first allowed us to reconstruct the eruptive history of the last 400 years, subdividing it into periods of quiescence, weak and enhanced eruptive activity, punctuated by eruptive pulses. Expert elicitation enabled us to constrain the probability of occurrence and eruption source parameters for eruptive pulses, with a quantification of uncertainty. Sangay volcano has been mainly active since the 17th century, with weak eruptive activity the most common regime, although there have also been several periods of quiescence. During this period, eruptive pulses with VEI 1-3 occurred mainly during enhanced eruptive activity and produced far-reaching impacts due to ash fallout to the west and long-runout lahars to the south-east. Four eruptive pulse scenarios were considered in the expert elicitation: strong ash venting (SAV), violent Strombolian (VS), Subplinian and Plinian. The SAV scenario is considered the most likely event and is opposed to the Plinian scenario. The experts expressed great uncertainty about the probability of occurrence of the VS and Subplinian scenarios. Large uncertainties were also observed for eruption duration and bulk fallout volume for all eruptive pulses, while average column height was characterized by a relatively smaller uncertainty, particularly for SAV and VS. We interpreted these results as a consequence of the lack of volcano-physical data, which could be reduced with further field studies. Finally, this study shows how historical reconstruction and expert elicitation can help develop hazard scenarios with uncertainty assessment for poorly known volcanoes, as a first step towards the elaboration of appropriate hazard maps and subsequent planning.