Internal stress field at Mount Vesuvius: A model for background seismicityat a central volcano

We propose a model to explain the background seismicity occurring at Somma- Vesuvius in its present, mostly quiescent period. A finite element procedur e has been used to simulate the stress field due to gravitational body forc es in an axisymmetric volcano characterized by a central high-rigidity anom aly. Results emphasize the important effect of axial high-rigidity, which c oncentrates at its borders stresses resulting; from the gravitational load of the volcanic edifice, as well as external (regional) stresses. The joint effect of the gravitational loading: and of the presence of the anomaly pr oduces stresses very close to or above the critical rupture threshold. The observed spatial concentrations of seismicity and moment release correlate well with peaks of computed maximum shear stress. Seismicity is then interp reted as due to small stress perturbations concentrated around the high-rig idity core and added to a sa;stem already close to the failure threshold. T his model call explain the widely observed occurrence of background seismic ity at. central volcanoes worldwide.