The potential of discrete and continuous microgravity observations as a tool to monitor and study active volcanoes. The case of Mt. Etna

Résumé: Discrete gravity measurements have been carried out at Mt. Etna since 1987, along a network that nowadays is composed of 70 benchmarks. Many gravity variations, often not accompanied by measurable ground deformation, were detected and interpreted as the effect of underground mass redistributions linked to the ensuing volcanic activity. Some case studies will be illustrated to show how, under a complex context such as Etna, where a close interplay exist between volcanic and tectonic systems, microgravity studies, in conjunction with other geophysical and geochemical observables, allow to assess the complete picture of the deep processes which can trigger eruptions and other paroxysms. Continuous gravity studies at active volcanoes have been infrequently carried out in the past because of the logistic difficulties of running the delicate spring gravimeters against the harsh environments. In 1997 experiments of continuous gravity measurements through spring gravimeters were started at Mt. Etna and, since 1998, three continuous gravity stations have been installed on the volcano at distances from the summit zone of the volcano between 1 and 10 km. Atmospheric perturbations have an important effect on continuously recording gravimeters which is frequency- and instrument-dependent and thus it is difficult to remove from gravity sequences longer than about 1 month. A way to check whether the signal was suitably reduced for these effects is to compare it with discrete gravity data from an array with one or more points close to the continuous station. Shorter gravity sequences (within a few days) are not significantly affected by atmospheric perturbations and thus do not need to be processed through complex algorithms. Still, they have considerable potential to both investigate the internal dynamic of a volcano and improve the confidence of volcanic hazard assessment, especially when cross-analyzed with other geophysical and volcanological data. In particular, explosive activity is preceded by the occurrence of phenomena that can trigger important changes in the density profile within the shallower levels of the volcanos plumbing system. These density changes were proved to take place over short periods (minutes to days) and, even though they surely induce variations in the gravity field measurable at the surface, they could not provoke recognizable seismic signals and/or important deformation of the ground surface. ------------------------------------------------------------------------------------------ Atelier de Dynamique des Systèmes Géologiques ------------------------------------------------------------------------------------------