Temperature-dependent viscoelastic modeling of ground deformation: Application to Etna volcano during the 1993–1997 inflation period

Abstract

We used the Finite Element Method (FEM) for modeling time-dependent ground deformation due to volcanic pressure sources embedded in a viscoelastic medium. Especially in volcanic areas, the presence of heterogeneous materials and high temperatures produce a lower effective viscosity of the Earth's crust that calls for considering the thermal regime of crustal volume surrounding the magmatic sources. We propose a thermo-mechanical numerical model for evaluating the temperature dependency of the viscoelastic solution. Both temperature distributions and ground deformation are evaluated by solving an axi-symmetric problem to estimate the effects of thermo-viscoelastic response of the medium. The thermo-mechanical model permits to evidence that viscoelastic relaxation is responsible for significant time-dependent variations in long-term deformation. These effects may be relevant for the interpretation and quantitative assessments of the pressure changes within magmatic sources. With this in mind, we reviewed the ground deformation observed on Etna volcano during the 1993–1997 inflation period by setting up a fully 3D temperature-dependent viscoelastic model. Since 1993 different geodetic measurements (EDM, GPS, SAR and leveling data) identified an inflationary phase characterized by a uniform and continuous expansion of the overall volcano edifice that was not perturbed by eruptive activity. The numerical model, including significant viscoelastic material and reduced crustal rigidity around the magmatic source, enables to produce deformation comparable with those obtained from elastic model, requiring a significantly lower pressure. For a purely elastic model with the same geometry and rigidity the pressure change necessary to describe the 1993 through 1997 inflation is around 320 MPa, whereas for the viscoelastic model a pressure increase of about 200 MPa is required.