Medium and long period ground oscillatory pattern inferred by borehole tiltmetric data: New perspectives for the Campi Flegrei caldera crustal dynamics

We analyse tiltmetric time series recorded at borehole instruments recently installed at Campi Flegrei caldera (Italy), a volcanic area subjected to the phenomenon of bradyseism, which consists in fast ground uplift phases alternated to slow subsidence. For the first time, we evaluate the crustal response in terms of ground tilting of the entire caldera to external excitations such as long/medium-period tidal constituents, by adopting Independent Component Analysis. Indeed, we recognize diurnal (solar) and long-period (fortnightly and monthly) components, superimposed to the normal deformation trend of the area. They show well defined polarization directions and are associated with an oscillatory deformation pattern with the same periodicity of the corresponding tidal constituents. The comparison with the local geology evidences that the tidal tilting is controlled by the local stress field distribution and the rheology, thus inducing structural and thermoelastic site effects