GNGTS 2013 - Atti del 32° Convegno Nazionale

bounded by west-dipping buried high-angle normal faults (Boncio and Lavecchia, 2000b; DISS working group, 2010; Rovida et al. , 2011). Nevertheless which of the known fault systems play a major role in accommodating the extension, and which are the modes (seismic VS aseismic deformation) this extension is taken up, is still a debated topic. In particular recent studies about the northernmost part of Umbria-Marche region show seismic and tectonic activity (Chiaraluce et al. , 2007, Hreinsdóttir and Bennet, 2012, Mirabella et al. , 2011) on correspondence of Alto Tiberina (AT) low-angle normal fault (LANF), which is widely documented by geological data (Brozzetti, 1995; Boncio et al. , 2000; Collettini, 2000) and deep seismic reflection profiles (CROP03, Barchi et al. , 1998). The supposed detachment of AT fault is an interesting case in which crustal extension could be driven by a LANF, considered by “Andersonian” theory as averse to faulting. During last years on Umbria-Marche Apennines close to Gubbio fault (GuF) a dense network of continuous GPS stations, belonging to the RING-INGV network, has been installed, improving significantly the spatial resolution of the detectable geodetic gradients. Using kinematic block models to reproduce GPS velocity field, we define the optimal fault boundaries accommodating the tectonic extension. GPS data processing. To estimate the present-day deformation on Northern Apennines, we analyzed data from a dense network of continuous and survey-mode GPS stations. Survey- mode stations are those installed in the framework of the RETREAT project (Bennett et al. , 2012). The processing follows a three-step approach, as described on Serpelloni et al. (2006), which includes: 1) raw phase data reduction, 2) combination of loosely-constrained solutions and reference frame definition, and 3) time series analysis. In the first step, we use the GAMIT (V10.4) software (Herring et al. , 2010) on daily GPS phase observations to estimate geodetic parameters applying loose constraints. We apply the ocean-loading and pole-tide correction model FES2004, and use the parameterized version of the VMF1 mapping function, the GMF for both hydrostatic and non-hydrostatic components of the tropospheric delay model. We use the IGS absolute antenna phase center model for both satellite and ground-based antennas. Continuous GPS data are divided into several subnets and processed independently; each subnet share a set of high quality IGS stations, which are subsequently used as tie-stations in step 2. Survey-mode GPS networks are processed separately, adding a larger number of high quality cGPS stations, in order to reduce the average baseline lengths. In the second step we use the ST_FILTER program of the QOCA software (Dong et al. , 2002) to combine all the daily loosely constrained solutions, for both cGPS and sGPS subnets, with the global and regional solutions made available by SOPAC (http://sopac.ucsd.edu ), and simultaneously realize a global reference frame by applying generalized constraints (Dong et al. , 1998). Specifically, we define the reference frame by minimizing the horizontal velocities of the IGS core stations (http://igscb.jpl.nasa.gov ), while estimating a seven-parameter transformation with respect to the IGS08 realization of the ITRF2008 frame (Altamimi et al. , 2011). In the third step we analyze the position time series in order to estimate velocities and uncertainties. For the cGPS and sGPS stations we estimate a constant velocity term together with annual and semi-annual seasonal components and, if present, offsets at specific epochs, and adopt a white + flicker noise model, following Williams et al. (2004). We incorporate data from cGPS and sGPS stations with an observation period longer than 2.5 years, as shorter intervals may result in biased estimates of linear velocities (Blewitt and Lavallée, 2002). We use velocities and uncertainties of cGPS stations located on tectonically stable domains of the Eurasian and Nubian plates in order to estimate their Euler rotation poles. The final GPS velocity field is calculated w.r.t. Eurasia fixed frame and in this study we use overall 594 velocities (from continuous and survey-mode stations), located on Italian peninsula and European region. 148 GNGTS 2013 S essione 1.2