GNGTS 2016 - Atti del 35° Convegno Nazionale

GNGTS 2016 S essione 1.3 269 Hydrothermal system influencing slope-scale deformations at Mt. Nuovo (Ischia, Southern Italy): preliminary results from 2D-multiphysics numerical modelling G.M. Marmoni 1 , A. Calabriso 2 , S. Martino 1 , D. Borello 2 , M. Della Seta 1 , C. Esposito 1 , M. Fiorucci 1 , P. Venturini 2 1 Dip. Scienze della Terra, “Sapienza” Università, Roma, Italy 2 Dip. Ingegneria Meccanica ed Aerospaziale, “Sapienza” Università, Roma, Italy Introduction. Gravitational slope instabilities can be strictly related to volcanic and hydrothermal systems as widely documented in several case studies (Reid et al. , 2001, 2004; Lopez et al. , 1993). Such relations are due to interactions between slope system and inner forces (Bozzano et al. , 2013), produced by a renewal of the volcanic activity, magma or dikes emplacement, volcanic-related seismicity, hydrothermal pressurization (Reid et al. , 2004). This close relation is strengthened by alterations effect produced by aggressive hydrothermal fluids which can compromise the mechanical properties of rocks and soils (John et al. , 2008; Frolova et al. , 2014; del Potro et al. , 2009) often jeopardizing mechanical properties (Heap et al. , 2012) and hence flank stability. Inorder to evaluate the role of hydrothermal systems in conditioning slope-scaledeformations, a 2D multiphysics numerical model was implemented for the Mt. Nuovo case study (Ischia Island, Southern Italy), by coupling hydrothermal and mechanical solutions. Geological framework of Ischia Island. Ischia Island is one of the most adapt volcanic systems for studying relations between geothermal systems and development of slope-scale instabilities, because of the presence of a well developed and stable geothermal system and the occurrence during the Holocene of massive rock slope failures (Della Seta et al. , 2011). These occurrences is closely related to volcano-tectonic dynamics, mainly driven by magma emplacement and volcanic activity renewal. In a cyclical recurrence pattern (de Vita et al. , 2013), slope instabilities would represent the surficial response to the general gravitational disequilibrium induced by deep deformation and caused by magmatic intrusion or pressurization. The main volcano-tectonic event recorded in the geological succession of Ischia Island is represented by an asymmetric resurgence able to produce an uplift up to 900 m over a period of 30 ka, as testified by the present elevation above sea level of marine sediments (Tibaldi and Vezzoli, 1998). According to Rittmann (1930) such a resurgence could have been generated by the emplacement of a shallow laccolith, or by an increase of volume and pressure in the magma chamber (Tibaldi and Vezzoli, 1998), which furthermore produced a vapor dominated hydrothermal system. The Ischia hydrothermal system is characterized by high heat flow (200- 400 mW/m 2 ) and geothermal gradients ranging between 180 and 220 °C/km (Cataldi et al. , 1991; AGIP, 1987). The principal surface evidences of the presence of a vigorous hydrothermal circulation are several thermal springs and fumaroles with temperature up to 100°C, among which most vigorous localized in the western sector of the island, in the Donna Rachele fumarolic field. The geothermal system of Ischia is also characterized by important pressure fluctuations, as testified by several wells eruptions, occurred after the wells closure. The underground fluid circulation mainly occurs within tuffs and lavas through a dense cracks network in a multilayered aquifer, controlled by the presence of fractured porous media separated by low-permeability layers and impermeable horizons (Carlino et al. , 2014). According to Di Napoli et al. (2011), the hydrothermal system in the Mt. Nuovo sector is fed by rainwater with significant seawater inputs, as clearly shown by chemical and isotopic markers. The tectonic discontinuities represented by the network of sub-vertical NW-SE oriented faults which drove the resurgence, strongly controlled the hydrothermal circulation, conditioning