## 5 Results and Discussions  
### Soil Frequency Peaks and the Corresponding Amplitudes  
The fundamental frequency variation map obtained from the H/V analysis is shown in Figure 6. The frequency values vary from 1.8 to 15 Hz. This wide frequency range is due to the different outcropping formations within the study area (Figure 2). The results show different types of curves: curves with one clear peak, curves with two clear peaks, and curves with no clear peaks (flat curves) (Figure 4). All around the city center, the predominant frequency varies between 1.8 and 3 Hz, with the existence of a secondary frequency peak between 6.5 and 15 Hz (Figure 7). This frequency peak should correspond to shallow impedance contrasts in the soil column. Frequencies between 5 and 7 Hz are identified in a small area in the eastern and northwestern parts of the city, like a transition zone between the first range and the higher frequencies (>10 Hz). The frequency peak values increase considerably towards the central part of the city, to reach a maximum value of 15 Hz on the most elevated point of the city, where the Ante-Neogene bedrock is at a very shallow depth [7].  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Zone**} & \multirow{2}{*}{**Epoch**} & \multirow{2}{*}{**Thickness (m)**} & \multirow{2}{*}{**Vp (m/s)**} & \multirow{2}{*}{**Vs (m/s)**} & \multirow{2}{*}{**Density** (Kg/m3)**} & \multirow{2}{*}{**References**} \\  & & & & & & \\ \hline Z1 & Holocene & 0–40 & 600–1000 & 315 & 1700–1900 & [7] \\  & Pleistocene & 10–70 & 1800–2000 & 500–750 & 1950–2200 & [38] \\  & Miocene & 40–170 & 2500–3500 & 900–1400 & 2100–2400 & [39] \\ \hline Z2 & Pleistocene & 0–50 & 1800–2200 & 500–800 & 1950–2250 & [7] \\  & Miocene & 30–130 & 2500–3500 & 900–1400 & 2100–2400 & [38] \\ \hline Z3 & Pleistocene & 0–40 & 1800–2000 & 500–750 & 1950–2200 & [7] \\  & Miocene & 20–60 & 2500–3500 & 900–1400 & 2100–2400 & [38] \\ \hline Z4 & Pleistocene & 0–15 & 1500–2000 & 450–750 & 1900–2200 & [7] \\  & Miocene & 10–30 & 2500–3500 & 900–1400 & 2100–2400 & [38] \\ \hline \hline \end{tabular}
\end{table}
Table 1: Geological and geotechnical information for each zone of the study area.  
Flat H/V curves (with amplitudes between around 0.5 and 2.0, without any clear peak) have been obtained from some points recorded in the center of the city and the Sidi-Amar cemetery in the southeastern side of Oued-Fodda (Figures 4 and 6), which is assumed to be built on sandstones from Eocene age [7]. Some flat curves have also been obtained in the Sara-El-Maarouf anticline N-W of Oued-Fodda (Figures 4 and 6). Flat HVSR curves reflect, among others, the absence of an impedance contrast in the soil column, which may mean that the signal had been recorded on a rock formation [36]. The amplitude of the fundamental frequency peaks range from 2.5 to 6 (Figure 6), and from 2 to 4 for the second peaks. The largest amplification factor is mainly observed west and south of the city center. Fortunately, in the city center where the constructions are essentially of a single floor (building frequency close to the soil fundamental frequency), the amplification factor is very low (\(<\)2).  
The spatial distribution of the experimental HVSR curves shows that curves with only one clear peak are concentrated only in the central part of the Oued-Fodda city (mainly Zones 3 and 4), while HVSR curves with two frequency peaks are located all around the city center (Figure 4).  
By comparing the results (Figure 4) of the HVSR technique with the geological map of the study area (Figure 2), we notice that the experimental H/V curves with one clear frequency peak are located on Pleistocene formation, while curves with two frequency peaks are on Holocene alluviums. The secondary peak at higher frequencies appears to be related to shallow impedance contrasts between the Holocene and the Pleistocene deposits.