## 3 Data and Methodology  
### Horizontal-to-Vertical Spectral Ratio Technique (HVSR)  
The HVSR (or H/V) technique [25] allows retrieval of the resonance frequency of the soil at a given site, using single station ambient vibrations measurements. The theoretical aspect of this technique consists of calculating the ratio between the amplitude spectra of the vertical and horizontal components of ambient vibrations. As a result, an HVSR curve is obtained. The HVSR frequency peak is well correlated with the soil resonance frequency.  
Although this technique is very effective in estimating the soil resonance frequency [26], the scientific community is reluctant about its ability to estimate the amplification factors [27; 28; 29]. This issue is due to the contribution of different seismic waves to the wavefield. As for now, it is very difficult to quantify the ratio between body and surface waves. [PERSON] et al. [30] showed that in the case of high impedance contrasts between the bedrock and the sediments, the HVSR curve is mainly controlled by surface waves. The relatively low amplitude of the body waves is not sufficient to correctly estimate the amplification factor. Moreover, [PERSON] et al. [31] and [PERSON] et al. [32] have proven that the HVSR peak amplitude varies considerably with time. The amplitude of the frequency peak obtained from the HVSR technique in this study is interpreted as a relative indicator and not as a true amplification factor value.  
Ambient vibration single-station measurements were carried out in October 2021 at 71 sites in the cities of El-Attaf (24 sites), Ain-Defla (33 sites), and El-Abadia (14 sites) (Figure 3). Some measurement points [15] were taken from a previous study [21]. Recordings were performed at night and in calm weather, as recommended by the SESAME project [33]. The acquisition time was 20 min. At some sites, the recording time was extended to 30 min due to anthropogenic noise from human activity. The equipment used for the recording was a pair of Tromino seismographs, with a sampling rate of 512 samples per second.  
The HVSR technique was processed following the recommendations of the SESAME project [33]. The whole data were processed in the same way using the Geopsy software [34]. First, the signals were divided into several windows of 30 s each, tapered by a 5% cosine function. The window selection was made automatically using an anti-triggering algorithm, which allows selecting windows where the ambient noise is stationary. After that, the Fast Fourier Transform (FFT) is computed for each component (vertical and both horizontals) and the amplitude spectrum of both horizontal components is combined by an RMS (root mean square) average computation. After that, the ratio between the amplitude spectrum of both vertical and horizontal components is calculated. The HVSR curve is calculated for each window. The resulting curve is smoothed using the Konno-Ohmachi algorithm [35],with a smoothing coefficient of 40. Finally, the curves are averaged and the final HVSR curve is retrieved in the frequency range between 0.2 and 20 Hz. The resulting HVSR curve may contain one or several frequency peaks, which are directly linked to impedance contrasts at the soil column.