GNGTS 2019 - Atti del 38° Convegno Nazionale

GNGTS 2019 S essione 1.1 131 (as suggested by borehole data and electrical resistivity profiles by Villani et al. , 2019). Steeply dipping reflections coupled with relatively high Vp (> 3000 m/s) indicate alluvial fan bodies from the basin borders. These data put additional constraints on the location and geometry of several subsurface faults, which are detected by reflection truncations or disrupted zones. Fig. 3 - Seismic profile Vettore (looking to the north). Top panel: high-resolution Vp tomographic model (the red arrows points to the Valle delle Fonti – Prate Pala fault, that ruptured the surface during the Norcia earthquake). Bottom panel: migrated depth-converted reflection section. As an example, we show preliminary results from seismic profile Vettore (Fig. 3). The high- resolution tomographic model (Fig. 3 upper panel) indicates a complex velocity field with several lateral variations that we relate to the occurrence of subsurface faults. The limestone basement corresponds to the deep high-Vp region (Vp > 4000-4500 m/s). The reflection section (Fig. 3 lower panel) shows a complex continental infill with variable dip and the occurrence of different seismic facies related to interfingering coarse and fine-grained deposits, likely representing different generations of alluvial fans. The red arrow points to the surface rupture trace of the Norcia earthquake, in correspondence of the Valle delle Fonti - Prate Pala active splay (VF), characterized by a surface fault scarp ~2.3-2.8 m high, likely related to the last ~12 kyr of activity (Galadini and Galli, 2003). Previous investigations (Villani and Sapia, 2017; Villani et al. , 2019) suggest that this fault has total throw of ~100 m. Our new tomographic image shows that the VF fault zone is characterized by a wide low velocity zone (Vp < 2500 m/s) deepening down to ~300 m below the surface. Moreover, the reflection image highlights the internal complexity of the VF fault zone, characterized by at least three shallow and closely spaced sub-vertical splays. Therefore, the 2016 ruptures are the surface expression of a normal fault with complex architecture and being responsible for ~200 m throw affecting the limestone basement. We hypothesize that the PGC basin structure is characterized by several fault splays distributed in a >3 km-wide deformation zone. In addition, our tomographic and reflection images point out the occurrence of other splays with different orientations with respect to the N150° general trend of the VBFS. In most cases, a 50 m to 100-m thick cover of deposits seals