Subsurface deformations induced by rainfall and atmospheric pressure: tilt/strain measurements in the NE-Italy seismic area

Tilt and strain measurements are collected in a cave at a depth of 60 m in the Friuli seismic area (NE-Italy) where the topography suddenly reaches a height of 600 m a.s.l. The data are analysed to find a possible correlation between crustal deformations on one side and precipitation and barometric pressure variations on the other. Once the significance of the correlation is established, an attempt is made to give a physical explanation of the detected correlation in the frame of the elasticity theory. A significant areal reduction in strain observations occurs together with a marked rotation of the principal axes in the horizontal plane at the time of rainfall events while no evident signals are revealed in the tilt components. The atmospheric pressure has small influence on the tilt–strain observations, when compared with the effects of rainfall and consequent underground waterflow: referring to large perturbations, atmospheric pressure appears to influence the areal deformations in terms of about 1 nstrain per mbar, an order of magnitude less then that of the hydrologically induced deformation rate, which reaches up to 600 nstrain (in about 1 day) per 120 mm/day rainfall.