The Deformation Style of Somma-Vesuvius

The Somma-Vesuvius volcano is one of the most dangerous on the Earth due to its proximity to the city of Napoli (Southern Italy). The volcanic edifice has a typical asymmetric shape: the truncated cone of Mt.  Somma topped by the Vesuvius “Gran Cono”. Somma-Vesuvius last erupted in 1944 and is currently quiescent, experiencing fumarolic activity, low-energy seismicity and slow ground deformation (subsidence of the edifice itself and uplift in the surrounding area). Understanding the deformation style of Somma-Vesuvius and the corresponding long-term structural evolution allows inferences about volcanic activity and associated hazards. A large amount of data has already been collected about Somma-Vesuvius. Nevertheless, the deformation style affecting its volcanic edifice is still matter of debate. We present results of an integrated numerical-analogue modeling approach aimed at refining the current state of deformation of this volcano. Numerical models were built using a Finite Element (FE) method, implemented with a three-dimensional time-dependent fluid-dynamic approach, representative of both 1:100,000 and 1:1 scales. A wide range of laboratory analog models were built at a scale of 1:100,000, using sand mixtures as brittle medium and polydimethylsiloxane as a ductile one. A comparison with the actual Somma-Vesuvius deformation velocity patterns, obtained by differential interferometric synthetic aperture radar (DInSAR) and GPS measurements, allowed the selection of a pair of analog/numerical models that faithfully reproduced the field and remote sensing observations. The modeling procedure adds new constrains supporting a combined gravitational spreading-sagging process governing the deformation of the Somma-Vesuvius volcano. This conclusion has a critical consequence: the recognized deformation processes support the presence of a tensional regime. This has the potential implication of reducing the loading stress on the magmatic reservoir system and, consequently, of decreasing the Volcanic Explosive Index of eruptive events. The refined knowledge of the actual deformation process affecting Somma-Vesuvius should be a key contribution to a reliable volcanic surveillance system.