GNGTS 2016 - Atti del 35° Convegno Nazionale

270 GNGTS 2016 S essione 1.3 the lateral extension of aquifers and geothermal reservoir, locally representing preferential pathways for upwards migrations of fluids. As documented in the literature (McKenna et al. , 2004; Bachler et al. , 2003), structural alignment may control the geometry of convective cells, fluid velocity, and thermal anomalies distributions in geothermal system. The volcano-tectonic evolution of Ischia Island also affected the gravitational response of the edge of the resurgent block. Slope instability-related deposits including rock falls, slides, toppling as well as lahars and large scale debris flows and debris avalanche at Ischia has been described by many authors inland and above continental shelf all over the island (Chiocci et al. , 2006; De Alteriis et al. , 2009). Some of the volcanoclastic deposits are intercalated to primary volcanic, outlining a close relationship among slope instability and volcano-tectonic activity (de Vita et al. , 2006). The most important events, occurred since 3 ka, are characterized by volumes of hundreds Mm 3 , representing the major catastrophic mass movements documented in the island since the Holocene. Ongoing deformations still involve the edge of the resurgent block in Mt. Nuovo area over a volume of about 160-190 Mm 3 (Della Seta et al. , 2011, 2015). This gravitational slope deformation was reported as triggered by a catastrophic volcano-tectonic event that took place around 460÷470 BC in the external portion of the edge of resurgent block (Della Seta et al. , 2011 and references therein), where historical occurrence of large debris avalanche has been documented. The spatial distribution as well as the morphological and geo-structural similarities between Mt. Nuovo slope deformations and already detached debris avalanche (Della Seta et al. , 2015) strengthen the analogy between the evolution of the occurred landslide and the gravitational ongoing-deformation, leading to assume such landslides as the local evolution to collapse of a wide deforming sector. Based on a high-resolution engineering-geological model a preliminary conceptual model of the slope-scale deformations involving the Mt. Nuovo sector was already proposed (Della Seta et al. , 2015). This conceptual model highlights possible relations between hydrothermal system and ongoing slope deformation. In fact, increased pore pressures can induce transient stress field changes, so increasing the instabilities of the slope instabilities and driving fastly toward a generalized failure. Multiphysics 2D numerical modelling of coupled hydrothermal-slope system. In order to investigate possible thermo-mechanical interactions between the Ischia hydrothermal system and the deep gravity-driven slope deformation a numerical modelling was designed. Starting from the conceptual model of the geothermal reservoir proposed by Carlino et al. (2014), a 2D thermo- fluid dynamic simulation was performed by the use of FEM COMSOL® code. A multilayer aquifer was defined, fixing a reservoir located at depth ranging between -150 and -800 m a.s.l.. A numerical analysis was performed, assuming a stationary, conductive and convective geothermal heat flux coupled with fluid flow in porous medium and fixing thermal boundary conditions. The 2D thermo-fluid model was validated by matching experimental thermal data obtained from deep wells (AGIP, 1987), i.e. verifying the coherence of the numerical thermal output respect with the path of measured temperature. The model of the deep hydrothermal system was combined with the engineering-geological model of the Mt. Nuovo slope, applying the output of the deep model as thermal conditioning of the slope-system thermal model. The free-convection in porous medium was solved by introducing a Bousinnesq buoyancy term to the Brinkman’s momentum equation, linking resulting fluid velocity to the heat transfer equation, thus accounting for the lifting force due to thermal expansion (Hossain and Wilson 2002). In order to assess the role of structural elements in the development and characteristics of thermal convection, fault zones were implemented, by introducing linear permeable elements. The modelling results highlight a well developed steady-state convection in the shallow reservoir of Ischia western sector,