Considering the ductile behavior of altered volcanic rocks in the flank stability of volcanoes and their response to earthquake

Volcanic flank collapses are among the most destructive natural hazards, with the potential to cause widespread devastation and trigger catastrophic events such as tsunamis 1. Understanding the factors contributing to their instability is crucial. These large flank collapses occur cyclically throughout the life of a volcano. One potential cause is hydrothermal alteration, where reactive fluids and heat interact with the host rocks, altering their mechanical properties 2. In most volcanoes, this process negatively impacts the brittle-ductile transition of volcanic rocks, leading to a more ductile failure behavior instead of a brittle one. The mechanisms behind large volcanic flank collapses remain unclear, particularly when hydrothermal alteration is involved 3. The impact of the transition in mechanical behavior is rarely considered in volcanic stability assessments. We performed Finite Element Method simulations under both dry and wet conditions on 2D and 3D models of the Tutupaca volcano as it existed before its collapse in the late 18th century. To assess stability, we applied the strength reduction method to each configuration, allowing us to determine the factor of safety and identify the most critical failure mechanism. The collapse was most accurately reproduced when the volcanic rocks were modeled as a Mohr-Coulomb material with a compressive cap. This compressive cap accounts for the low brittle-ductile transition observed in previous experimental studies of altered volcanic rocks. Importantly, incorporating the compressive cap significantly enhances the model's ability to predict volcanic stability against earthquake-induced ground acceleration. Our results highlight that hydrothermal alteration affects volcanic stability by altering the brittle-ductile transition. This research provides a foundation for understanding volcanic instabilities influenced by hydrothermal alteration and suggests that incorporating variations in the brittle-ductile transition could enhance future volcanic hazard assessments. References 1 L. Siebert, “Large volcanic debris avalanches: Characteristics of source areas, deposits, and associated eruptions,” Journal of Volcanology and Geothermal Research, vol. 22, no. 3–4, pp. 163–197, Oct. 1984, doi: 10.1016/0377-0273(84)90002-7. 2 M. Detienne, “Unravelling the role of hydrothermal alteration in volcanic flank and sector collapses using combined mineralogical, experimental, and numerical modelling studies,” Université catholique de Louvain, Louvain-la-Neuve, 2016. 3 M. J. Heap and M. E. S. Violay, “The mechanical behaviour and failure modes of volcanic rocks: a review,” Bull Volcanol, vol. 83, no. 5, p. 33, May 2021, doi: 10.1007/s00445-021-01447-2.