Abstract. Volcanic activity produces deformation and gravity changes that many times can be used as precursors of future eruptions. Applying geodetic techniques to monitoring activity involves interpretation using deformation models. Usually, the observed changes of the deformation and gravity fields are interpreted seperately, not in a joint inversion. It can be difficult, if not impossible, to interpret the data coherently or correctly in terms of the characteristics of the intrusion or the deflation derived from the gravity changes with purely elastic models, as in the case of Mayon Volcano, Phillipines. We show that elastic-gravitational models can be used to interpret these cases simultaneously leading to a result that is more plausible on the basis of the available information. Thus, we may need to change the philosophy normally used to interpret geodetic observations. Interpretation as proposed in this work can significantly improve the possibility of predicting future eruptions.

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[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,

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This paper describes design, observation methodology, results and interpretation of the GPS surveys conducted in the areas of the N-W of Tenerife where deformation was detected using InSAR. To avoid undesirable antenna positioning errors in the stations built using nails, we designed and used calibrated, fixed-length metal poles, allowing us to guarantee that the GPS antenna was stationed with a height repeatability of the order of 1 mm and of less than 3 millimeters on the horizontal plane. The results demonstrate that this system is ideal for field observation, especially to detect small displacements that might be masked by accidental errors in height measurements or centering when observed with a tripod. When observations were processed, we found that using different antenna models in the same session sometimes causes errors that can lead to rather inaccurate results. We also found that it is advisable to observe one or two stations in all the sessions. The results have reconfirmed the displacement in the Chı´o deformation zone for the period 1995-2000 and indicate a vertical rebound from 2000 to 2002. They also confirm that the subsidence detected by InSAR to the south of the Garachico village has continued since 2000, although the magnitude of the vertical deformation has increased from around 1 cm to more than 3 cm a year. Detected displacements could be due to groundwater level variation throughout the island. A first attempt of modelling has been made using a simple model. The results indicate that the observed deformation and the groundwater level variation are related in some way. The obtained results are very important because they might affect the design of the geodetic monitoring of volcanic reactivation on the island, which will only be actually useful if it is capable of distinguishing between displacements that might be linked to volcanic activity and those produced by other causes. Even though the study was limited to a given area of Tenerife, in the Canary Islands, some conclusions apply to, and are of general interest in similar geodynamic studies.