Hydrothermal alteration shifting brittle-ductile transition promotes volcanic flank collapses

Volcanoes are inherently unstable, causing tremendous catastrophes, such as cities destruction or large tsunamis creation. These large flank collapses are not one-time events, it happens cyclically over the life of a volcano. One of the potential causes of such phenomena is hydrothermal alteration: Reactive fluids coexisting with a heat source interact with the host rocks and modify their mechanical properties. In most volcanoes, these hydrothermal alterations have a negative impact on the brittle-ductile transition of volcanic rocks, promoting a ductile failure behavior instead of a brittle one. However, the mechanisms behind large volcanic flank collapse are still obscure, especially when hydrothermal alteration is involved. The influence of the transition of mechanical behavior is rarely considered when the stability of the volcano is assessed. We performed Finite Element Method simulations, in dry and wet conditions, on 2D and 3D geometries of the Tutupaca volcano before its collapse at the end of the 18th century. To assess the stability, the strength reduction method was applied for each configuration allowing the obtention of the factor of safety and the most critical failure mechanism. The collapse is best reproduced when the volcanic rocks are modeled as a Mohr-Coulomb material with a compressive cap. The cap offers the consideration of the low brittle-ductile transition observed in previous experimental studies of altered volcanic rocks. Our results demonstrate that hydrothermal alteration influences the stability of a volcano through the brittle-ductile transition variation. These results are an entry point into assessing the instabilities of volcanoes because of hydrothermal alteration. They consider silicic and argillic alterations, but many others exist, and they might alter the rocks in a different way than reducing the brittle-ductile transition. These preliminary results are a basis to start adding complexity with, for example, external events such as earthquakes or meteorologic changes.