Developing probabilistic tephra fallout hazard maps for Cotopaxi and Guagua Pichincha volcanoes (Ecuador) with uncertainty quantification

Abstract

Tephra fallout is a major hazard for many human activities, spanning from aviation safety (ash concentration in the air) to building stability (tephra load) as well as health and respiratory problems. Long-term planning is therefore an important task for civil authorities, especially in highly populated areas. Probabilistic hazard maps are useful tools to address these complex issues, and the quantification of the uncertainty associated to such maps represents an equally important element of hazard assessment. In this work, we present a tephra fallout hazard assessment study related to two volcanoes (Cotopaxi and Guagua Pichincha) directly threatening the capital city of Ecuador, Quito. This work resulted in the production of hazard maps of different formats, in order to meet the needs of both the scientific community and the local authorities. The quantification of the uncertainty was performed in two steps. First, an elicitation session involving 20 experts from different countries and areas of expertise was performed in order to quantify: a) the probabilities of occurrence of different eruptive styles at the two studied volcanoes; b) the range of uncertainty of some key eruptive input parameters (total fallout mass, eruption duration, plume height, total grain size distribution). These latter estimations were used to produce hazard maps with different eruptive scenarios. Second, it was quantified the uncertainty of the model used for the development of the hazard maps (the plume model PLUME-MoM coupled with the tephra dispersal model HYSPLIT) using recent eruptions in the Andean context as test cases. Using a combination of eruptive parameters from such uncertainty quantification,, a coefficient of uncertainty has been derived and applied to the final maps to actually quantify the uncertainty related to the model.