On the relative importance of self‐gravitation and elasticity in modeling volcanic ground deformation and gravity changes

Abstract

[1] Elastic half-space models, widely used to interpret displacements and gravity data in active volcanic areas, usually compute the displacement response to dilatational sources that simulate a change in pressure of the magma chamber. Elastic-gravitational models allow the computation of gravity, deformation, and gravitational potential changes due to pressurized magma cavities and intruding masses together. This type of model takes into account the mass interaction with the self-gravitation of the Earth through coupling between model equations. We perform a dimensional analysis of the elastic-gravitational model estimating the magnitude of intrusion mass and coupling effects at the space scale associated with volcano monitoring. We show that the intrusion mass cannot be neglected in the interpretation of gravity changes while displacements are primarily caused by pressurization. Therefore the intrusion of mass, together with the associated pressurization of the magma chamber, produces distinctive changes in gravity that could be used to interpret gravity changes without ground deformation and vice versa, depending on what type of source plays the main role in the intrusion process. Theoretical experiments indicate that mass and self-gravitation could produce changes in the magnitude and pattern of predicted gravity that may be above microgravity accuracy. Application of the elastic-gravitational model to interpret geodetic precursors observed at Mayon volcano (Philippines) prior to the eruption of 2001 shows that inversions increase in precision by using this model. Therefore our elastic-gravitational model is a refinement of purely elastic models and can better interpret gravity and deformation changes in active volcanic zones.Citation: Charco, M., J. Fernández, F. Luzón, and J. B. Rundle (2006), On the relative importance of self-gravitation and elasticity in modeling volcanic ground deformation and gravity changes,

Some Insights into Topographic, Elastic and Self-gravitation Interaction in Modelling Ground Deformation and Gravity Changes in Active Volcanic Areas

“…BATTAGLIA and SEGALL (2004) and CHARCO et al (2006) performed a dimensional analysis and scaling of the elastic-gravitational problem in order to gain insight into the approximations and solutions without actually attempting a formal solution. From dimensionalized parameters and using typical parameter values for volcanic areas, they found that terms depending on G and g in the equations (1) and (2) are negligible in the computation of the displacement field.…”

4D volcano gravimetry

“…When computing volcanic loading effects, coupling between gravity and elasticity is negligible for displacements in the spatial scale associated with volcano monitoring, but the absolute effects of the existing gravity field become important when rigidity decreases ͑i.e., viscoelastic media͒ ͑Fernández et al, 1997; Battaglia and Segall, 2004;Charco et al, 2006͒. Coupling is a second-order effect that cannot be ignored in flat half-space models when a mass source term…”

FEM and ANN combined approach for predicting pressure source parameters at Etna volcano

“…Moreover, for spherical sources the topography effect is due primarily to the distance of the free surface from the magma source rather than the local shape of the free surface (Williams and Wadge, 1998). As a result, no differences are found between numerical and analytical solutions, if the topography effect is approximated by replacing in the analytical expressions the flat reference surface with the real elevation of the observation point Charco et al, 2009). At the observation point (x, y, z) the analytical solution of the thermomagnetic change T due to a source centered at (x 0 , y 0 , z 0 ) is expressed as follows (Blakely, 1995):…”

“…The last two contributions have a significant effect on the magnitude of the predicted gravity changes when the source resides at shallow depths (Bonafede and Mazzanti, 1998;Charco et al, 2006;Currenti et al, 2007). In case of deep pressure sources these contributions are below microgravity accuracy (Battaglia and Segall, 2004;Currenti et al, 2007) and are negligible compared to those produced by the input of new mass.…”

“…Neglecting these features that strongly affect the solutions, an inaccurate estimate of source parameters is obtained (Currenti et al, 2009;Wadge, 1998, 2000;Trasatti et al, 2008). To overcome this intrinsic limitation and provide more realistic models, numerical solutions based on Finite Element Method (FEM) have been investigated (Cayol and Cornet, 1998;Williams and Wadge, 2000;Currenti et al, , 2009Trasatti et al, 2003;Lungarini et al, 2005). However, the use of numerical forward models in iterative methods is computationally expensive since the estimate of the objective function requires to perform a full FEM analysis at every iteration step.…”