Strength reduction method applied to Hardening multi-plasticity model to numerically study flank collapses of volcanoes submitted to hydrothermal alteration

Volcanic flank collapses are catastrophic events capable of causing significant destruction and triggering secondary hazards such as tsunamis. Strength reduction is a widespread numerical method assessing the stability of a geological edifice by gradually reducing its strength parameters. The reduction factor provoking the instability is called the safety factor. 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. In most volcanoes, this process negatively impacts the brittle-ductile transition of volcanic rocks, leading to a more ductile failure behavior, mainly driven by compression, instead of a brittle one, led by shear. The mechanisms behind large volcanic flank collapses remain unclear, particularly when hydrothermal alteration is involved. The impact of the ductile-brittle transition in mechanical behavior is rarely considered in flank stability assessments. Moreover, the compressive plastic cap behavior, notably present in altered volcanic rocks, is seldom modeled, even more so as strength reduction method is followed. We conducted Finite Element Method simulations on 2D and 3D models of the Tutupaca volcano, representing its pre-collapse state in the late 18th century, under both dry and wet conditions. Using the strength reduction method, we evaluated the stability of each configuration by calculating the factor of safety and identifying the most critical failure mechanisms. The collapse was best replicated when volcanic rocks were modeled using a Mohr-Coulomb material with an additional compressive cam-clay cap, consistent with the low brittle-ductile transition seen in experimental studies on altered volcanic rocks. This inclusion of the compressive cap improved the model's predictive accuracy for collapse triggered by earthquake-induced ground acceleration. Our findings demonstrate that hydrothermal alteration impacts volcanic stability by modifying the brittle-ductile transition of volcanic rocks. This study offers key insights into the role of hydrothermal processes in volcanic flank instabilities and highlights the importance of accounting for variations in the brittle-ductile transition in stability assessments. Incorporating these variations could significantly improve future models for predicting volcanic hazards.

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UCLouvainSST/IMMC/GCE - Civil and environmental engineering

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Niclaes, J., Delmelle, P., & Rattez, H. (2025). Strength reduction method applied to Hardening multi-plasticity model to numerically study flank collapses of volcanoes submitted to hydrothermal alteration. MOOSE International Workshop 2025, Idaho Falls, Idaho, United States of America. https://hdl.handle.net/2078.5/256260