GNGTS 2014 - Atti del 33° Convegno Nazionale

northward displacement of the Gran Sasso Arc and the Laga units which might have influenced the shocks that successively occurred at the NCGGA �� fault system (Norcia-Valnerina 1979, M=5.9; Gubbio 1984, M = 5.7; Colfiorito 1997, M=5.7, 6.0, 5.5, 5.7) and also the 2009 L’Aquila earthquake (M=6.3) at the inner extensional border of the Gran Sasso Arc. GPS velocity field and seismotectonic implications. The post early Pleistocene kinematic pattern of the Apennine belt, described in the previous section, has been compared with the velocity field obtained by the analysis of space geodetic (GPS) measurement achieved by a fairly dense geodetic network in the Italian peninsula, comprising �� 262 scientific and commercial permanent stations having an observation time span longer than 2.5 year� �� . The characteristics of the geodetic network and the details of data processing are described by Cenni et al. (2012, 2013). The resulting velocity field (Fig. 2a) �� is fairly consistent with the kinematic pattern shown in Fig. 1. The relatively high density of GPS sites provides a good spatial definition of the structural domains characterized by different velocities, indicating that the outermost sector of the RMU wedge moves significantly faster, �� and with a greater eastward component �� (4-6 mm/y, roughly NE ward)� �� , with respect to the inner part of the belt �� (1-2 mm/y, roughly N/NW ward). Insights into the possible connection between the present kinematic field and the distribution of seismic activity in the study area are provided by Fig. �� 2b, which points out that the narrow zone lying between the mobile RMU sector and the almost fixed inner belt (corresponding to the NCGGA fault system) includes the epicenters of the strongest historical earthquakes. Implications for the present seismic hazard. The above considerations suggest that during the ongoing seismic sequence (since 1930) the migration of the outer sector of the Apennine belt has mainly involved the MS wedge, the ELA wedge (Gran Sasso Arc), the Laga units and the southern sector of the outermost RMU wedge (Tuscany-Umbria Apennines). The effects Fig. 2 – �� A) Geodetic velocity field (with respect to Eurasia) reported on a digital elevation model of the study area. The green marks the zone where GPS velocities are higher than 3.5 mm/y. Red circles indicate the epicenters of major earthquakes occurred since 1000AD. B) �� Velocities of the GPS stations (red dots) lying within the box shown inA. The lower diagram reports the pattern of total seismic energy (x10 19 erg) released in the NCGGA fault system, computed with the standard relationionship E = 10 (11.8+1.5M) , where M is the magnitude of earthquakes occurred since 1000 AD (Rovida et al. , 2011). 184 GNGTS 2014 S essione 1.2