GNGTS 2016 - Atti del 35° Convegno Nazionale

34 GNGTS 2016 S essione A matrice active extension across the Apennines (Anzidei et al., 2005; Serpelloni et al., 2006; D’Agostino et al., 2008; Galvani et al., 2012). Here, we use continuous and episodic GPS (Global Positioning System) measurements to infer the fault geometry and the amount of coseismic slip. In particular, around the epicentral area, several continuous GPS stations, managed by different private and public institutions, were operating before and after the seismic sequence. In addition, in the following hours and days after the August 24 mainshock, several GPS instruments have been installed at geodetic markers belonging to the CaGeoNet discrete network (Anzidei et al., 2008a, 2008b; Galvani et al., 2012) and at sites belonging to other episodic networks managed by the Italian Civil Protection Department (DPC) and by the Italian National Institute for Environmental Protection and Research (ISPRA). Finally, the first rapid solution has been integrated with data from continuous GPS network managed by DPC and ISPRA. In particular, the CaGeoNet GPS sites are crucial to properly resolve the near-field coseismic deformation patter, allowing direct observations of the details of coseismic displacement, as it was in the case of the 2009 Mw 6.3 L’Aquila earthquake (Anzidei et al., 2009; Cheloni et al., 2010). GPS data were analysed using the BERNESE, GAMIT and GIPSY-OASIS software (e.g., Avallone et al., 2010), following the procedures described in Devoti (2012), and adopted for the 2012 Emilia (northern Italy) seismic sequence (Serpelloni et al., 2012). In particular, the final coseismic displacement field was obtained from the combination of the three individual solutions by solving for the combined offsets in a least-squares sense (Devoti , 2012). The combined horizontal and vertical coseismic displacements for the August 24 main shock show that the calculated offsets are qualitatively consistent with a normal fault, striking NW-SE and dipping to the SW. The largest displacements were measured at Accumuli (ACCU) which moved to the SE by about 6 cm and subsided by about 17 cm. We model the observed horizontal and vertical near- and far-field coseismic displacements, using a rectangular dislocation in an elastic, homogeneous and isotropic half-space (Okada, 1985). The best-fitting fault parameters and their relative confidence intervals were found using a global optimization algorithm and a Monte Carlo simulation technique as described in Cheloni et al. (2010). In the uniform slip solution, the causative fault strikes ~NW-SE and dips ~50° to the SW, with an average slip of ~0.6 m (rake -75°), which (using a value of 30 GPa for rigidity) gives an estimated seismic moment of 2.16 10 18 Nm (equivalent to a M W 6.2). The derived source model is in agreement with the mainshock focal mechanisms and the surface ruptures observed in the field (EMERGEO Working Group, 2016). To test variable slip on the fault plane, we subdivided the uniform slip fault in patches of increasing size with depth. The variable model shows two main asperities with ~1.5 m of maximum slip, mostly contained within the uniform slip fault. The largest asperity lies at a downdip depth of 5-10 km, roughly below the observed surface ruptures seen at the base of the uppermost branch of the Mt. Vettore fault. The second asperity, located SE-wards of the hypocentre, show a more compact slip distribution confined between 3-7 km depth. References Anzidei, M., P. Baldi, A. Pesci, A. Esposito, A. Galvani, F. Loddo, P. Cristofoletti, A. Massucci, S. Del Mese (2005), Geodetic deformation across the Central Apennines from GPS data in the time span 1999-2003, Annals of Geophysics, 48 (2), 259-271. Anzidei, M., P. Baldi, S. Del Mese, A. Esposito, P. Cristofoletti, A. Galvani, F. Loddo, A. Massucci (2008a), La rete geodetica GPS dell’Appennino centrale CA-GeoNet, Quaderni di Geofisica, 54 . Anzidei, M., P. Baldi, E. Serpelloni (2008b), The coseismic ground deformations of the 1997 Umbria-Marche earthquakes: A lesson for the development of new GPS networks, Annals of Geophysics, 51 (2-3), 27-43. Anzidei, M., E. Boschi, V. Cannelli, R. Devoti, A. Esposito, A. Galvani, D. Melini, G. Pietrantonio, F. Riguzzi, V. Sepe, E. Serpelloni (2009), Coseismic deformation of the destructive April 6, 2009 L’Aquila earthquake (central Italy) from GPS data, Geophys. Res. Lett., 36 , L17307, doi:10.1029/2009GL039145. Avallone, A. et al. (2010), The RING network: improvements to a GPS velocity field in the central Mediterranean, Annals of Geophysics, 53 (2), doi:10.4401/ag-4549.