Brittle-Ductile Transition: Hydrothermal Alteration's Role in Avalanche Calderas

Avalanche calderas, characterized by the horseshoe-shaped crater they leave behind, are common volcanic catastrophes causing tremendous damage and fatalities (e. g. , Unzen in 1792, Bandai in 1888, Bezymianny in 1956, Mount St. Helens in 1980. ). These deep-seated volcanic flank collapses produce high-volume (> km³) landslides that spread over large areas or provoke tsunamis. One contributing factor to such phenomena is hydrothermal alteration. Reactive fluids coexisting with a heat source interact with host rocks, modifying their mechanical properties. However, the mechanical implications of large volcanic flank collapses, particularly involving hydrothermal alteration, remain obscure. We performed Finite Element Method (FEM) simulations under dry and wet conditions on 2D and 3D geometries of the Tutupaca volcano before its avalanche caldera event in the late 18th century. To assess stability, we applied the strength reduction method to each configuration, obtaining the factor of safety and identifying the most critical failure mechanism. Our simulations best reproduced the collapse when the volcanic rocks were modeled as a Mohr-Coulomb material with a compressive cap. This cap considers the low brittle-ductile transition observed in experimental studies of altered volcanic rocks and proved crucial for predicting failure under horizontal ground acceleration caused by earthquakes. Our results demonstrate that hydrothermal alteration influences the stability of a volcanic edifice by varying the brittle-ductile transition. These findings provide an entry point for assessing volcanic instabilities, considering silicic and argillic alterations. Future work will explore additional complexities, such as meteorological changes, dykes, and eruptions, to build on these results.