Abstract
Employing a 3D finite element method, we develop an algorithm to calculate gravity changes due to pressurized sources of any shape in elastic and inelastic heterogeneous media. We consider different source models, such as sphere, spheroid and sill, dilating in elastic media (homogeneous and heterogeneous) and in elasto-plastic media. The models are oriented to reproduce the gravity changes and the surface deformation observed at Campi Flegrei caldera (Italy), during the 1982-1984 unrest episode. The source shape and the characteristics of the medium have great influence on the calculated gravity changes, leading to very different values for the source densities. Indeed, the gravity residual strongly depends upon the shape of the source. Non negligible contributions also come from density and rigidity heterogeneities within the medium. Furthermore, if the caldera is elasto- plastic, the resulting gravity changes exhibit a pattern similar to that provided by a low effective rigidity. Even if the variation of the source volumes is quite similar for most of the models considered, the density inferred for the source ranges from ?400 kg/m3 (super critical water) to ?3300 kg/m3 (higher than trachytic basalts), with drastically different implications for risk assessment.
Highlights
Volcanic activity produces deformation and gravity changes that may be used as precursors of future eruptions
We develop a numerical technique to calculate gravity changes due to deep inflating sources using FE modelling
Very few analytical solutions are available in literature, while the FE models may be characterized arbitrarily
Summary
Volcanic activity produces deformation and gravity changes that may be used as precursors of future eruptions. The detection of ground deformation allows to constrain location and type of deep reservoirs, while gravity monitoring is recognized as a valuable tool for mapping subsurface density distributions. Both of them contribute to quantify the change in subsurface mass. Fernandez and Rundle (1994) developed a method to compute gravity variations due to point magma intrusion in a horizontally layered elastic-gravitational media. The introduction of rigidity layering and plastic rheology in the local crust causes a shrinking of the deformation shape and enhances the maximum values within the caldera This behavior allows to estimate a source depth of 5 km (plastic medium) instead of 3 km (elastic medium), adopting the same overpressure and in accordance with informations from groundwater studies and petrological data. The models are discussed as a preliminary approach to reproduce the gravity changes observed at Campi Flegrei during the 198284 unrest episode
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