GNGTS 2013 - Atti del 32° Convegno Nazionale

are stable or in some cases are decreasing, as a consequence of the drastic reduction of water withdrawal. In particular the data of two regions (the industrial area north of Bologna town and Po Delta, east of Rovigo), which were involved in a very strong subsidence in the past, with maximum rate of 60-70 mm/y, confirm the decrease of the subsidence velocity. European Vertical Reference frame. In order to investigate if the absolute values of the computed vertical velocities of the CGPS sites located in the Italian region have a real geophysical significance, some careful considerations have to be made: the vertical velocities estimated are obtained in the International Terrestrial Reference Frame (ITRF2005) realized by the combination of different space techniques (Altamini et al. , 2007). Considering that the central part of Europe lying north of the Alps is generally seen as a relatively tectonically stable area of Eurasia (Campbell and Nothnagel, 2010), we analyze the vertical velocity field of this area obtained by aligning our data to the EUREF Permanent Network solution at 1600 (2010/09/06) GPS week (Bruyninx and Kenyeres, 2008). Additional information about the present vertical velocity field in the Alps and the European area can also be inferred from the Regal permanent GPS array (Calais et al. , 2000, 2002), and from the AMON network (Austrian Monitoring Network; Höggerl et al. , 2011), whose solutions are given in the International Terrestrial Reference Frame (ITRF2005) realized by the combination of different space techniques (Altamini et al. , 2007). In order to evaluate eventual bias due to different procedures for aligning the four networks in the ITRF frame, we performed a seven Helmert transformation assuming as reference the EUREF solution at 1600 GPS week (2010/09/06). The Helmert transformation parameters are listed in Tab. 1. These parameter have been estimated using the velocity of the common sites included in the EUREF and in the other solution (AMON, Regal, our) included in the analysis. So the AMON, Regal and our solution Have been aligned to the EUREF - ITRF2005 ones using the values listed in Tab. 1. Moreover, the number of the common stations included in EUREF and of the other networks, along with their characteristics (mean and root mean square RMS of the velocity differences), are listed in Tab. 2. The four networks considered are all aligned in the ITRF2005 reference frame; the mean of the difference are lower than 1 mm/yr with a distribution (RMS) of about the same value (Tab.1). The vertical velocities obtained after the 7 parameter Helmert transformations are shown in Fig. 3. It can be noted that the global vertical velocities field obtained in this Reference Frame is in good agreement with the vertical displacements measured by the tide gauges network (e.g. Bouin and Wopplelmann, 2010), and shows also an overall agreement with the Glacial Isostatic Adjustment (GIA); this process could affect the Europe south of Fennoscandia, inducing a prevalent uplift north of the 48th parallel. The small subsiding areas shown in figure and located almost above the 48th parallel of latitude, are generally inferred by one or few velocity data that do not seem to be consistent with nearby stations, and may be connected with regional and local tectonic structures (LODZ and CATO stations, Mid Polish Trough and its sedimentary infill) and oil/gas production (GWWL station, western Poland). Tab. 1 – Helmert transformation parameters in Cartesian coordinate system estimated assuming as reference the EUREF solution at 1600 GPS week (2010/09/06). T and R are respectively the translation and rotation velocity vector, S is the velocity scale factor. Source Tx (m/yr) Ty(m/yr) Tz(m/yr) Rx(mas/yr) Ry(mas/yr) Rz(mas/yr) S (x10-9) Amon (-0.010 ± 0.017) (-0.004 ± 0.004) (0.018 ± 0.004) (0.3 ± 0.1) (-0.594 ± 0.001) (0.055 ± 0.007) (-0.84 ± 0.03) Regal (-0.004 ± 0.001) (-0.002 ± 0.001) (0.002 ± 0.001) (0.06 ± 0.02) (-0.127 ± 0.003) (-0.007 ± 0.003) (0.22 ± 0.01) This paper (-0.004 ± 0.04) (-0.001 ± 0.03) (-0.020 ± 0.020) (-0.01 ± 0.08) (-0.035 ± 0.003) (-0.035 ± 0.003) (0.19 ± 0.02) 166 GNGTS 2013 S essione 1.2