Height and gravity variations by continuous GPS, gravity and environmental parameter observations in the southern Po Plain, near Bologna, Italy

• Published in: Earth and planetary science letters 2001-10, Vol.192 (3), p.267-279
• Main Authors: Zerbini, Susanna, Richter, Bernd, Negusini, Monia, Romagnoli, Claudia, Simon, Dietrich, Domenichini, Francesco, Schwahn, Wolfgang
• Format: Article
• Language: English
• Subjects: environmental effects; Global Positioning System; gravity methods; oscillations
• ISSN: 0012-821X; 1385-013X
• DOI: 10.1016/S0012-821X(01)00445-9

During 1996, a Global Positioning System (GPS) receiver and a superconducting gravimeter (SG) were installed at Medicina, Italy, in order to monitor ground deformation and signals related to global/regional processes and local environmental effects. First results provided by this combined observing strategy are presented, by focusing on common patterns as well as on the differences observed in the two time series. A marked annual signal of increasing amplitude, present in both series, is becoming quite recognizable towards the end of 1997. Seasonal loading effects induced by air pressure, the ocean and surficial water table were estimated and modeled for both data sets. For the gravity series, 12-h data of balloon radio sounding launches have been used to estimate the attraction effect of the seasonal vertical density distribution of the air pressure column above the station. Mass effects due to the surficial water table and the ocean were also estimated and accounted for. The comparison between the observed and modeled series, both in the case of GPS heights and gravity, shows a remarkable agreement and provides a convincing explanation for the observed seasonal fluctuations. In the July–September 1997 time frame, a sudden gravity increase occurred. This anomaly is in the order of 3 μGal and is present in the data series after accounting for the seasonal fluctuation. This step-like feature is responsible for most of the estimated positive linear trend present in the data series. The anomalous increase in gravity has been interpreted as due to mass/density variations, likely associated with the uprising of deep-seated waters in relation with local stress field changes. During the second half of 1997, concurrent seismic activity occurred both in the southern Po Valley and in central Italy (Umbria seismic crisis).