GNGTS 2013 - Atti del 32° Convegno Nazionale

We considered the GPS data of the longest time series from both networks, starting from 2002 till present. The resulting data set is composed by the data of: ZOUF, AFAL, ACOM, MPRA, UDI1, MDEA, TRIE from FReDNet, and AMP0, MOG0, POR0, PAL0 and TRI0 from the Marussi network. We processed the data using GAMIT/GLOBK, eliminated the outliers, and filled the eventual short gaps in the data through linear interpolation. A strong annual component is present, as expected, due to seasonal variations of hydro-meteorological parameters (e.g., Blewitt and Lavallee, 2002). We tested, however, whether this annual term is affected by the so-called draconitic term, i.e. the time between two passages of the object through its ascending node, or the point of its orbit where it crosses the ecliptic from the southern to the northern hemisphere. From an accurate spectral analysis of the time series, it resulted that only a couple of station show, on one of the components, relevant amplitude associated to the draconitic term. A low-band pass filter allowed obtaining the time-series cleaned from the components with frequencies higher than 1.5 year, so to eliminate the annual and quasi-annual terms, and the highest frequencies. The so-obtained time-series for the two horizontal components are dominated by a linear trend, as expected, to which clear oscillations of apparent period of a few years (2-4) are superimposed. Oscillations are present also in the vertical component. The data are shown in European plate reference frame, obtained from ITRF08 after rotation using the Euler vectors of Altamimi et al. (2012). The resulting velocity field from the analysis of the linear trend suggest crustal shortening, with values ranging between 0.6 and 2.8 mm/ year, decreasing from South to North and, less pronounced, from East to West. This is in agreement with preceding observations and with the geodynamic character of the region, located in the area of convergence between Adria microplate and Eurasia (Fig. 1). Deviations from the linear trend: characteristics and analysis. A said above, in the various sites, superimposed on the linear trend, 2-4 year period oscillations are present, and show higher amplitude with respect to the annual terms (Fig. 2). To better analyse the oscillations, we first applied a band-pass filter (1.5–3 years) to the data, and then calculated the signal component along directions, spaced 15°, from N to N165E. This procedure allowed to evidence a sort of transient, of “period” of roughly 2.0 years, causing a bending in all the stations considered, distributed over the whole region, mainly along a direction about coincident with the Dinaric trend, N120E. Only in a few cases, the transient causes a bending toward an anti-dinaric direction which is also a dominant tectonic direction in the region (e.g. Bressan et al. , 2003, 2007). Fig. 3 shows the curves relative to the signal along the direction in which the maximum is recorded at each station, as well as the correspondent vertical GPS signal. Fig. 2 – Three of the amplitude spectra of the longest time series from the FReDNet and Marussi networks: the numbers on the peaks are indicative of the periods in years and fractions of year. 199 GNGTS 2013 S essione 1.2