## 2 Methods of Analysis  
The methods adopted to assess ground motion polarization and directional amplification to this point have involved the use of the ambient noise wavefield and earthquake data. The oldest technique to get information on amplification-prone sites is based on the spectral ratios SSRs (standard spectral ratios) ([36; 37], and later papers) of horizontal ground motions recorded during an earthquake at close sites with different near-surface geological structures. SSRs require the presence of a reference site not affected by any site amplification effects. Therefore, its use is somewhat suspect when studying tectonically active areas where it is difficult to find a reference site in the proximity of the studied fault zone sector. Therefore, it is not applied in the present study. In a similar approach, another technique involves the calculation of spectral ratios between horizontal and vertical components (HVSR), generated by earthquakes or ambient seismic noise at individual stations [38; 39]. The theoretical basis of this method is still debated [40]; however, it is a very efficient tool for mapping the different seismic motions of nearby sites. When dealing with the ambient noise, DA might be caused by many factors, such as the local geological setting or the sources of the ambient noise. For example, [41] observed the DA in the frequency band 0.15-0.6 Hz, finding that the location of the sources affects the polarization of the ambient noise [42]. In order to identify directional amplification, both SSR and HVSR are calculated by rotating the two horizontal components. The use of rotated spectral ratios was first introduced by [43] and was subsequently exploited by several authors to detect the horizontal polarization of ground motion in fault zones (e.g., [4; 5; 16]). When using seismic events, both SSR and HVSR values are calculated at each station separately for each event, considering a portion of the signals with a varying length (depending on the event magnitude), but always including the S and early coda waves. The spectra of horizontal motions are computed after rotating the NS and EW components by steps of 10\({}^{\circ}\), from 0\({}^{\circ}\) to 180\({}^{\circ}\). A taper (i.e., Hannning) and a smoothing filter (such as a running mean box or triangular filter; or a Konno-Ohmachi filter) [44], are generally applied. The spectral ratios are then graphed, calculating the mean over the considered seismic events. An example of an HVSR calculation is shown in Figure 1B, clearly showing the amplification in a broad frequency band (from roughly 3 to 7 Hz), with amplitudes over a factor of 4; amplification is the maximum for the N40\({}^{\circ}\) rotation angle (NE-SW). Because the spectral ratios (SSR and HVSR) may be biased by anomalies in the spectrum of the vertical component (at the denominator), in the time domain the covariance matrix method [45; 46] has been widely exploited to estimate the ground motion polariza  
Figure 1: Examples of the analysis methods generally employed to assess ground motion polarization. (**A**) results obtained by the covariance matrix analysis. They are modified after [34]. They include, from the top to the bottom: filtered signals of the three components of ground motion; values of polarization azimuth vs time; eight examples of polarization ellipsoids with different shape (elongation, horizontality, and flatness), and are related to different seismic phases. The employed color scale related to the reliability scale is based on the hierarchical criterion introduced in [17] to give a higher weight to time windows associated with more horizontal and elongated polarization ellipsoids. To show the influence of the use of this hierarchical criterion, two rose diagrams are given: the “HC” one is made by applying the hierarchical criterion, while the “NO-HC” rose diagram is produced by weighting the polarization azimuth values (1).