GNGTS 2017 - 36° Convegno Nazionale

210 GNGTS 2017 S essione 1.3 The thermo-rheological model of Ischia Island and its implications for distribution of the natural seismicity and ground deformation pattern P. Tizzani 1 , R. Castaldo 1 , G. Gola 2 , A. Santilano 2 , V. De Novellis 1 , S. Pepe 1 , M. Manzo 1 , A. Manzella 2 1 Istituto per il Rilevamento Elettromagnetico dell’Ambiente, Consiglio Nazionale delle Ricerche (IREA-CNR), Napoli, Italy 2 Istituto di Geoscienze e Georisorse, Consiglio Nazionale delle Ricerche (IGG CNR), Pisa, Italy We developed a model that joint explain the ground deformation behaviour and distribution of natural seismicity at Ischia Island. The model is employed to investigate the driving forces and physical parameters of the crust that controls the subsidence of the Island. To this purpose, we integrate and homogenize in a Finite Element (FE) environment a large amount of data derived from several and different observation techniques. In detail, the main steps of the multiphysics model are: (i) the generation of a 3D geological model of the crust beneath the Island by merging the available geological and geophysical information; (ii) the optimization of a 3D thermal model by exploiting the available thermal measurements; (iii) the computation of the 3D Brittle/Ductile transition by using the temperature distribution of the crust and the physical information of the rocks; (iv) the pictures of seismicity cutoff; (v) the numerical optimization modelling of the ground deformation by considering the spatial and temporal information detected via satellite multi-orbit C-Band SAR (Synthetic Aperture Radar) measurements acquired during the 1992–2010 time period. The achieved results allow investigating the physical process responsible for the observed ground deformation pattern and the distribution of natural seismicity. Specifically, they reveal how the rheology modulates the spatial and temporal evolution of the long-term subsidence phenomenon, highlighting a coupling effect of the viscosities of the rocks and the gravitational loading of the volcano edifice. The achieved results provide a very detailed and realistic velocity field image of the subsurface crust of the Ischia Island. The model results support the open hypothesis that the observed ground deformations are controlled by the coupling effects of crust rheology and the gravitational loading of the volcano edifice: the subsidence phenomenon is modulated by the viscosity contrast between the rocks of the ductile and brittle region. Several aspects relevant to the volcano behaviour emerge from the performed multiphysical FE optimized modelling (Castaldo et al ., 2017). We analyzed the thermic observations in the deep wells and found that the convective heat transfer processes control the temperature distribution within the deep- seated geothermal reservoir of Ischia. We explored the crustal temperature distribution resulting from conductive and convective processes in a limited but realistic range of permeability and temperature of the magmatic intrusion in the FE environment. The magnitudes of the heat source and the reservoir permeability largely influenced the simulated thermal field, also at a level deeper than the hydrothermal system. We evaluated different scenarios and the retrieved FE solutions pointed out that, in order to recognize the thermal effects due to the free convection, reservoir permeability larger than 10−16 m 2 is needed. The optimized permeability value of 5.0 · 10−15 m 2 is in agreement with other estimates found in literature (Carlino et al ., 2014), and the optimized temperature of the heat source of 600 °C is lower than the melting temperature of the intrusion of about 1000–1200 °C (Carlino et al ., 2014). Our analysis confirms that the temperature fields as well as the rheology properties of the crust have a strong influence on the seismogenic processes occurring beneath the Ischia Island. More specifically, all the earthquake hypocenters are located above the B/D transition that is about 2.5 km of depth beneath the Mt. Epomeo resurgent block; below this transition the seismicity is absent. However, below the shallower B/D transition it is possible to observe renewed brittle conditions characterized by deeper seismicity (Ranalli, 2001). Generally, this sandwich-like rheological shape affects the continental crust where tensional delamination processes are not well developed. Instead, in the case of the thinning continental crust, characterized by a hot anomalous thermal state, it is possible to observe a lack of seismicity below the B/D transition.