Monitoring Crustal Deformation by Continuous Gravity and GPS Observations

At Medicina, near Bologna, Italy, a superconducting gravimeter, periodically controlled by absolute gravity observations, is acquiring continuous data series since the beginning of measurements in October 1996. A permanent GPS receiver was also installed at the station and is providing continuous observations since mid 1996. The time series are about 8 years long and they constitute, together with a remarkable ensemble of environmental parameters, a unique data set. The results of the gravity data analysis are interpreted to separate seasonal oscillations from a long-term trend and are compared with the height variations estimated by means of GPS. Noticeable seasonal oscillations are present both in the gravity and GPS time series and they were modeled by accounting for atmospheric, hydrological and non-tidal oceanic effects. Interannual variability is observed. A mean seasonal model derived by stacking of the observations is computed both for the gravity and GPS data series and it is compared with that obtained from the models. A moderate long-term trend has been identified in gravity, which is confirmed by the GPS height variations.