## 3 Results
### The Val d'Agri Case Study  
Conversely to the effect of trapped waves, which are spatially limited to small fault sections (e.g., [9; 85]), ground motion directional amplification and polarization due to fractures can involve larger extensions. As an example, in the Val d'Agri sector [34] found that such effects are consistently observed in a several kilometre-wide areas. The Val d'Agri area hosts the largest oil fields of the southern Apennines, whose high productivity was  
Figure 6: The Campo Imperatore fault, Abruzzi (central Italy). (**A**) Detailed structural geological survey redrawn by [76], showing the fault core (FC, yellow), the low strain damage zone (LS-DS, in blue) and the high strain damage zone (HS-D, red). Grey rose diagrams represent the distribution of fracture strikes, while lower hemisphere polar Schmidt plots indicate poles in each zone. The right-lower side depicts one temporary ambient noise measurement station that is close to the fault. On the upper right side, the dip of the associated parallel fractures is shown. (**B**) Rose diagrams from the covariance matrix analysis, their dimension being scaled according to the HVSR amplitude peak. They provide the mean horizontal polarization at each station and are colored depending on the dominant horizontal polarization: red rose diagrams are related to a transverse NE-SW fault; yellow rose diagrams to NNW-SSE; and green rose diagrams E-W. (**C**) A sketch representing the expected cleavages in the section view. (**D**) Transversal return between ground motion polarization (sky rose diagrams), fracture strike, and tectonic lineaments (grey rose diagrams). related to the presence of open and permeable fracture networks held open by the tectonic stress. Here, a 30-km wide Quaternary graben basin was filled by up to 500 m of continental deposits, and is bordered by two systems of NW-SE trending, high- angle normal faults: (i) the Eastern Agri fault system (EAFS), with subparallel SW dipping strands and a vertical displacement of \(\sim\)500 m; and (ii) the NE dipping Monti della Maddalena fault system (MMFS in [86]), which is the main structure accommodating the active extension in the area. In Figure 7A, they are represented with black and blue lines, respectively. They are both consistent with the local active stress field inferred borehole breakout, seismological, and hydrocarbon production data. In this area, wavefield polarization was investigated using a network of 20 stations installed on a rock outcrop (yellow dots in Figure 7A) using signals by almost 200 seismograms (the database was created by [87; 88]). We redrew [33] the findings in terms of rose diagrams representing ground motion polarization (Figure 7A). They are obtained by merging all of the analyzed seismic events at each station. At most stations a persistent horizontal polarization is observed along the NE-SW direction, transverse to the general trend of Quaternary normal faults and to the maximum horizontal stress related to the present extensional regime. This amplification effect is confirmed by HVSR calculated at stations AG08 and AG13 by using confirmed seismic events and ambient noise (Figure 7B). In fact, as evidenced by the high number of data from hydrocarbon exploration and production, in this sector the acting local stress regime has a primary role in opening cracks and fractures which strike NW-SE, parallel to the SH\({}_{\text{max}}\) direction, which are open and saturated by fluids (water and oil). Conversely, cracks and fractures with an orthogonal strike tend to be closed ([PERSON], 1999). Figure 7: The Val d’Agri extensional basin (southern Italy). (**A**) Redrawn by [33]. Location of seismic stations and systems of NW-SE trending, high-angle normal faults: the Eastern Agri fault system (EAFS), represented by black lines, and the NE dipping Monti della Maddalena fault system (MMFS), represented by blue lines [86]. The red rose diagrams obtained by merging all of the analyzed seismic events at each station represent ground motion polarization.