S. Zerbini et al., Height and gravity variations by continuous GPS, gravity and environmentalparameter observations in the southern Po Plain, near Bologna, Italy, EARTH PLAN, 192(3), 2001, pp. 267-279
During 1996, a Global Positioning System (GPS) receiver and a superconducti
ng gravimeter (SG) were installed at Medicina, Italy, in order to monitor g
round deformation and signals related to global/regional processes and loca
l environmental effects. First results provided by this combined observing
strategy are presented, by foe-using on common patterns as well as on the d
ifferences observed in the two time series. A marked annual signal of incre
asing amplitude, present in both series, is becoming quite recognizable tow
ards the end of 1997. Seasonal loading effects induced by air pressure, the
ocean and surficial water table were estimated and modeled for both data s
ets. For the gravity series, 12-h data of balloon radio sounding launches h
ave been used to estimate the attraction effect of the seasonal vertical de
nsity distribution of the air pressure column above the station. Mass effec
ts due to the surficial water table and the ocean were also estimated and a
ccounted for. The comparison between the observed and modeled series, both
in the case of GPS heights and gravity, shows a remarkable agreement and pr
ovides a convincing explanation for the observed seasonal fluctuations, In
the July. September 1997 time frame, a sudden gravity increase occurred. Th
is anomaly is in the order of 3 mu Gal and is present in the data series af
ter accounting for the seasonal fluctuation. This step-like feature is resp
onsible for most of the estimated positive linear trend present in the data
series. The anomalous increase in gravity has been interpreted as due to m
ass/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 Valle
y and in central Italy (Umbria seismic crisis). (C) 2001 Elsevier Science B
.V. All rights reserved.