## 3 Results
### The Campo Imperatore Fault Case Study  
A similar study was led out by [26], who assessed the ambient noise amplification across the Vado di Como Fault (VCF, Campo Imperatore, central Italy, where a very detailed structural geological survey was recently led out [84], revealing the high anisotropy of the fault that is affected by a complex network of faults and fractures with a dominant WNW-ESE strike. A summary of their results is redrawn in Figure 6A. In the same area, Ref. [26] measured seismic noise along a \(\sim\)500 m long transect perpendicular to the average fault strike. Ambient noise signals were processed using the covariance matrix analysis (see Section 1). Rose diagrams from the covariance matrix analysis are given in Figure 6B, their dimension being scaled according to the HVSR amplitude peak. They provide the mean direction of motion horizontal polarization at each station. The results were grouped in three classes on the basis of the dominant horizontal polarization: NNW-SSE (yellow rose diagrams); NE-SW fault-transverse (red rose diagrams); E-W (green rose diagrams). The majority of the recordings showed a predominant NNE-SSW to NE-SW amplification of the horizontal component of the seismic waves, transverse to the average strike of the fault-fracture network.  
Figure 5: The Mattinata fault on the Gargano Promontory (Puglia, southern Italy). (**A**) In subpanel (**a1**), we show the variation of polarization across the fault plotted along the transect AA\({}^{\prime}\), obtained by using the transect diagrams computed with the package Daisy [59]. The color scale indicates the frequency of occurrence (red is related to the most frequent values). On the ordinates we report polarization azimuth, ranging from N-90\({}^{\circ}\) to N90\({}^{\circ}\) (both corresponding to EW strike), where N 0\({}^{\circ}\) corresponds to N-S direction. In subpanel (**a2**) we show the fault trace and the main cities. In subpanel (**a3**) we give in a representation similar to subpanel a1, the measured pressure solution cleavage in [59], represented through a bw scale (black is related to the most frequent values) (**B**) Pictures of recording stations and rose diagrams representing (from the top to the bottom): ground motion polarization in the strike-slip sector (mean value N175\({}^{\circ}\)) and in the pull apart basin (mean value N220\({}^{\circ}\)), and measured fracture cleavages (mean value N131\({}^{\circ}\)) in [59].  
Seismic signals were also processed calculating the HVSRs and using the time frequency polarization analyses. The results at four representative stations are given in Figure S3 (CAM6, CAM11, CAM15, CAM24). The HVSR calculated using the Geopsy software show a resonance peak at about 1 Hz, with amplitudes over a factor of 5, and ground motion amplification occurring along the azimuth N20\({}^{\circ}\), according to ground motion polarization assessed through a covariance matrix analysis. The direction of maximum amplification varies to N160\({}^{\circ}\) at station CAM24. The results by time- frequency (TF) polarization analysis (see Section 2) show consistent results, with the ellipticity plot vs frequency showing minimum values at frequency ranges corresponding to the amplification at HVSR, giving an indication of the linear polarization. Moreover, the strike polar plot indicates polarization at the azimuth compatible with the HVSR and covariance matrix analyses.  
Finally, numerical simulations of earthquake ground motions in [26] ruled out the possibility that the observed effect is produced by the topographic irregularity, confirming that the higher amplitudes of ambient noise observed in the fault-perpendicular direction were due to a site effect (i.e., the fracture network and the resulting stiffness anisotropy of the rock mass).