Thermo-mechanical modelling of a gravitational slope deformation overlying an active hydrothermal system (Ischia, Italy) in a multiple hazard framework

The interaction between geothermal and mechanical processes can be considered relevant for the quantification of scenarios of slope stability in volcanic islands. The present research focuses on Ischia Island, which experienced a history of volcanic flank collapses in the last 10 ka. In the most uplifted sector of the resurgent caldera, a gravity-induced slope deformation took place and evolved in the area of Mt. Nuovo, where ultimate failure conditions have not yet been reached. To better understand the factors controlling the evolution of this slope deformation and to validate the hypothesised future instability scenario, a multiscale and multiphysical modelling approach was developed. This involved fine-tuning a 2D model of the deep hydrothermal system and combining it with a thermomechanical stress–strain model of the deforming slope. The numerical analysis examined the mechanics and rheology of tuffs, already constrained in laboratory, assuming present-day geothermal conditions and verifying the kinematics of the slope deformation process with respect to geomorphological time-volume targets. A strain-softening approach and viscosity thermal weakening were thus incorporated to back-analyse the rock mass creep process, reconstructing the size and mechanism of the slope deformation while aligning with geomorphological and temporal constraints. The study points out the preparatory role of geothermal anomalies in the evolution of large slope scale deformations, suggesting that thermally and strain-controlled viscosity reductions could lead to an acceleration of the process by approximately 10,000 years. Despite the formation of localised shear zones, no global paroxysmal failure was observed. This indicates that external or internal triggers, such as seismic or volcanic-hydrothermal transient events, are required to cause a slope collapse.