## 3 Results
### The Pernicana Fault Case Study  
The Pernicana Fault (PF) is the most relevant tectonic lineament of the Mt Etna volcano, located on the Ionian coast of Sicily. The fault extends for a total length of 18 km, from the NE Rift to the coastline [71]. At elevations between 950 and 1530 m, the fault morphology is represented by a scarp (up to 80 m high); the lack of a clear morphological signature in the other zones has been related to the most recent lava cover [72]. Fault activity (slip rates at a centennial scale ranging from 1 to 2.7 cm/yr) results in continuous damage to man-made structures (see Figure 4A) [73; 74]. The fault displacement varies from pure left-lateral (to the eastern side near the coastline) to trans tensional (near the intersection with the NE rift, on the Mt Etna summit area). The fault movement is mainly accommodated through creeping [72], but large slip episodes also occurred during shallow (<3 km) earthquakes [74]. The top picture in panel A (taken in 2013) shows visible damage produced by the continuous fault movement to man-made structures.  
The PF also represents the northern boundary of a large sliding movement of the E and SE sectors of the volcano [75; 76]. The sliding movement occurs towards E to ESE, as indicated by the white arrows in Figure 4A [77; 78]. The relation between the flank sliding and the PF is highlighted by the relative downthrow of the S sector prevailing over the movement of the fault.  
The directional amplification at Piano Pernicana was investigated by [5] using ambient noise measurements and local earthquakes. The main results of their paper are redrawn in Figure 4B. They found variations across the PF damage zone, with an abrupt rotation of the azimuth by about 30\({}^{\circ}\) across the fault, varying from N166\({}^{\circ}\) to N139\({}^{\circ}\) from the northern to the southern fault side on the south (Figure 4B). Such variations in ground motion polarization between the two sides of the fault were interpreted in terms of different deformation and kinematic conditions, resulting in different fracture patterns. A combined numeric and analytic approach was applied using the FRAP Package [59] to determine the most likely expected cleavage on the two sides of the PF (synthetic cleavages, antithetic cleavages, or extensional fractures), as well as their orientations. The two sides of the fault at Piano Pernicana were separately modeled: in the northern side of the PF the left-lateral strike-slip movement prevails, whereas the southern side is also subjected to sliding, and there is a dominant extensional stress regime. Therefore, while in the former the synthetic cleavagespredominate, with an overall N74\({}^{\circ}\) trend, in the latter the extensional fractures are more diffuse, with an overall N42\({}^{\circ}\) trend. They both show a near-orthogonal relation (\(\sim\)88\({}^{\circ}\) in the northern sector and \(\sim\)83\({}^{\circ}\) to the south) with the azimuth of the observed directional resonance (Figure 4C).  
Therefore, as in previous sections, even [79] concluded that the direction of the largest resonance motions is sensitive to and has a transversal relationship with the dominant fracture orientation (Figure 4D).  
We finally stress that such findings were recently confirmed by a controlled-source seismic experiment undertaken in the same area to investigate the subsoil capability in generating polarized motion [35]. This study suggested that the propagation of surface waves is more efficient along the observed main polarization direction. In fact, when the shear excitation is orthogonal to the pre- dominant site polarization, ground excitation lost its initial polarization less than 50 m away from the source position.