GNGTS 2018 - 37° Convegno Nazionale

GNGTS 2018 S essione 1.1 151 the highest emergence of this extensional structure. Field data (Villani et al. , 2018b), indicate that these splays exhibit a high dip angle at the surface (>75° on average), so we speculate they become less steep at depth, and eventually merge to the seismogenic fault (dipping nearly 45°, see Chiaraluce et al. , 2017; Scognamiglio et al. , 2018). The basin-bounding fault splay F1 is located at the base of the long-term cumulative fault scarp of Mt. Vettore-Mt. Redentore. The ERT T1 indicates that the fault zone is ~40-50 m wide. These results in addition with the absence of coseismic surface breakages following the 2016 Norcia earthquake along fault F1 (Civico et al., 2018) suggest that this structure is experiencing a very low degree of activity. The geological throw of fault F1 is nearly 1400 m, in accordance with some previous estimates (Calamita et al. , 1992; Pizzi et al. , 2002). In the hangingwall of fault F1, the VF is a high-angle active splay (Galadini and Galli, 2003; Villani and Sapia, 2017) with a total throw of ~100 m. The surface fault scarp is ~2.3-2.8 m high, likely related to the last ~12 kyr of activity. As suggested by ERT models, the VF fault zone displays relatively low resistivity values (ρ < 100 Ωm), due to the presence of fluids within the sub-vertical and 10-30 m-wide granular damage zone. The fault F3 does not show evidence of recent faulting, however it played an important role during the Quaternary in shaping the PGC basin. In order to accommodate the sudden deepening of the top-bedrock surface between TDEM soundings td9 and td8, we suppose an additional E-dipping fault F6, in the hangingwall of fault F3. In the PGC basin the dominant direction of fast S-wave polarization is consistent and changes accordingly with the strike of faults. Moreover, we also observe a secondary direction of the fast S-wave parallel to the maximum horizontal stress SH max of the regional extensional stress field suggesting a combined effect of stress-induced and structure-induced anisotropy in the upper crust beneath the PGC basin. In the western side of the basin, NNE-SSW fast directions well match the local strike of main faults F2 and F3. We suppose that the anisotropy could be primarily controlled by the geometry of fracture systems and by the small-scale structures developing into the fault damage zones (Liu et al. , 2015). Moving towards the eastern side of the PGC basin the fast polarization directions trend NNW-SSE and are clearly in accordance with both the maximum horizontal stress and the strike of the VBFS. This is often observed in extensional regimes as in the central Apennines, where SH max direction coincides with the strike of the main active normal faults (Sibson et al. , 2011) and, thus, the fast direction could be oriented parallel to both the maximum horizontal stress and the fault strike (Hurd and Bohnhoff, 2012; Baccheschi et al. , 2016). It is possible, therefore, that here both the alignment of local fluid-filled micro-cracks and the faults can control the pattern of fast axes. The Quaternary extensional activity in this sector of the Sibillini Mts. involved two differently oriented fault-systems, trending N130°-150° and N30°, respectively. Probably, in their early development, those faults acted together. In the first stage of the PGC basin formation, the N30°-trending faults may have played an important role, testified by the fact that the long axis of the basin is parallel to faults F2 and F3 (which are >5 km and >3.5 km-long, respectively). Subsequently, the N130°-150°-trending faults become dominant, as the result of the progressive growth and linkage of several aligned segments that now compose the ~25 km-long VBFS, which is a currently active and seismogenetic crustal-scale normal fault-system (Chiaraluce et al. , 2017; Pizzi et al. , 2017; Scognamiglio et al. , 2018). The interplay of the two fault-systems was responsible for the complex shape of the PGC basin, and it can be inferred from the seismic anisotropy pattern and the bumpy morphology of the basin bottom. The deepest part of the investigated portion of the PGC basin is ~300 m b.g.l. Such depth is the result of the combined action and interference of two different fault-systems, and unfortunately the available data do not allow deriving any long-term fault throw rates. We cannot reconstruct with our data the throw rates of the individual fault splays within the PGC. With regard to the VF splay, Villani and Sapia (2017) infer a post-23 ka of 0.22 ± 0.07 mm/yr. With the results reported in this work, we estimate ~100 m total throw accrued by