## 5 Final Remarks  
Much effort has been devoted in recent decades to the study of peculiar directional amplification effects across fault zones. In this paper, we discussed our main findings at several fault zones, characterized by different geological settings and kinematics. The common feature of all case studies is the occurrence of DA and ground motion polarization perpendicular to the predominant fracture field. These latter findings were defined through both the modelling of the fracture pattern in the fault damage zone (package FRAP [59]), and the detailed structural surveys. Such an orthogonal relation between ground motion polarization and fractures was also confirmed by a comparison with the results of an S-wave splitting analysis, which was used as a proxy to define the orientation of open fractures at a larger depth [23; 33; 34]. DA and polarization effects orthogonal to fractures completely differ from fault-zone trapped waves, with the latter showing polarization oriented parallel to the fault strike. Moreover, while the effect of trapped waves is spatially limited to small fault sections (e.g., [9; 85], ground motion DA and polarization due to fractures can involve relevant areal extents up to several kilometers wide, as suggested by the Val d'Agri case study (see also [33]). The site-effects produced by the presence of fractures at rock sites can have significant implications. First, the existence of amplification effects at rock sites can raise relevant implications both in terms of seismic hazard and in engineering applications, as in GMPEs (ground motion prediction equations) exploitation, where they are considered to be free from resonance and amplification effects (e.g., [93; 94]), and in ground motion simulation techniques made for ground motion prediction purposes. The current state of knowledge is limited to the relative amplification between horizontal and vertical components, and further estimates are needed to evaluate absolute amplitude and to understand to what extent this effect could be important for seismic hazard and engineering applications. Another implication involves the choice of a reference site for site-response estimates, as, for example, in the standard spectral ratio method [36]. Several authors have already stressed how rock sites can have their own site response, an arbitrary choice of a reference rock site leading to a bias on the estimates of the site effect (e.g., [95; 96; 97; 98]). [PERSON] in [94] highlighted that the amplification effects at rock sites have an influence on the prediction of the expected motion among the recording stations of the Italian Accelerometric Archive (ITACA, [[http://itaca.mi.ingv.it](http://itaca.mi.ingv.it)]([http://itaca.mi.ingv.it](http://itaca.mi.ingv.it)), last accessed on 16 April 2023, [99]). Finally, rock sites are also considered to be preferred locations for permanent seismic stations, where the occurrence of DA may lead to higher amplitude levels and to incorrect earthquake magnitude estimates. Recent papers involving stations of permanent networks have highlighted that the DA effects can involve a high number of rock sites among stations of permanent seismic networks [83; 100; 101]. **Supplementary Materials:** The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app13106060/s1](https://www.mdpi.com/article/10.3390/app13106060/s1)]([https://www.mdpi.com/article/10.3390/app13106060/s1](https://www.mdpi.com/article/10.3390/app13106060/s1)). Table S1--Data selection used in the Hayward fault, downloaded through the IRIS Wilber3 tool ([[http://ds.iris.edu](http://ds.iris.edu)]([http://ds.iris.edu](http://ds.iris.edu)), last accessed on 19 April 2021): hypocentral parameters and station record. Figure S1--The Mattinata fault, Gargano Promontory (Puglia, southern Italy). The geological map of MF given in [PERSON] et al. ii (1999), marking the pull apart basin and transpresional sector in yellow and blue, respectively. The top picture shows some sectors of the fault, with pervasive fracturing.