Controls on flank instability at southern Andes volcanoes constrained by field, laboratory, and numerical evidence: Applications to San José and Chillán volcanoes

Beyond eruptive activity, volcanic hazard over surrounding communities also relates to their edifices' gravitational stability and capacity to generate massive destructive processes with related cascading hazards. Lateral collapses are gigantic (> 1km3) landslides producing volcanic debris avalanches that may instantaneously kill thousands and transform the local landscape in the long term (i.e., thousand years). Understanding the conditions that increase volcanic edifice instabilities is central to determining the potential areas that may fail and, ultimately, the collapse hazard of volcanoes. Past studies have shown that the conjugation of multiple conditions is therefore required to propitiate lateral collapses, including the geometry and dimensions of the volcanic edifice, the weakening of volcanic rocks and edifice substratum, and the own magmatic-volcanic activity. However, the complex and heterogeneous nature of volcanic successions and the broad diversity of textures existing in volcanic rocks are considered to obscure the mechanical behavior and failure modes of volcanoes. Consequently, assessing the volcanic instability and collapse potential of volcanoes requires multiple lines of research.The main goal of this study is to determine the distribution and geometry of slope instability and its controlling factors (i.e., textural and compositional characteristics of rocks, their mass structure and discontinuities, and their mechanical rock properties) at the SJVC and NCVC. Specific objectives involve encompassing the production of thematic maps to represent the factors contributing to instability (e.g., magmatic intrusions, hydrothermal alteration areas, observed landslides, etc.) and defining and characterizing lithotechnical units based on field observations. In addition, it should determine the mechanical rock properties of natural samples based on laboratory tests that represent different scenarios. Lastly, these results should be incorporated into numerical models to represent unstable areas, failure planes, and conditions for failure.

The fast growth of young volcanic edifices, the layering of competent (lava) and incompetent (ash) strata and the corrosive action of volcanic fluids (gasses and hydrothermal) make volcanoes unstable and prone to collapse. The specific conditions that favor the collapse are not fully understood, despite the potential for catastrophic consequences. This proyect aims to further the understanding of the conditions that lead to the collapse of volcanic edifices.