## 6 Results and Discussions  
### Fundamental Frequencies and the Corresponding Amplitudes  
The resonance frequencies and the corresponding amplitudes obtained from the HVSR analysis are mapped in Figure 9. The frequencies range between 0.75 and 12 Hz, while the amplitudes vary from 2 to 6.2. The obtained HVSR curves contain one frequency peak, two peaks, and multiple peaks. Curves with two frequency peaks are predominant, especially in the Quaternary plain.  
In the Oued-Fodda Plain (Zone1), the predominant resonance frequencies of the soil vary between 1.5 and 3 Hz, and the corresponding amplitudes vary between 2 and 6. However, particularly in the center of Oued-Fodda city, higher frequencies, between 5 and 12 Hz, are observed. This high-frequency range is related to the outcropping bedrock in the area [22]. In the village of Zbabdja, about 3 km west of Oued-Fodda city, the frequencies are around 1.5 Hz. In terms of shape, two types of curves are observed in this zone: curves with two frequency peaks are predominant in the Oued-Fodda Plain, while curves with one peak were obtained only from recordings in the central parts of Oued-Fodda city and Zbabdja village. Second peaks at higher frequencies are related to impedance contrasts in the soil column at shallow depths [22]. These peaks are only observed at sites located on Holocene alluviums. This allowed us to deduce that this shallow impedance contrast is responsible for the second frequency peaks that are between the Holocene silt layer and the Pleistocene gravel and conglomerate layers.  
In the Sara-El-Maarouf anticline, the predominant resonance frequency is around 2 Hz, while the amplitudes of the peaks range between 2.5 and 4.5. The second peaks at higher frequencies appear to be related to the impedance contrast between the Upper Pliocene sands and the Lower Pliocene sandstones.  
In the Carnot Plain (Zone 2), the predominant resonance frequencies vary between 0.75 and 5 Hz. The corresponding amplitudes vary from 2.2 to 5.8. HVSR curves with one frequency peak, two peaks, and multiple peaks were observed in this area.  
The variation in shape for these curves is mainly related to the variation in the lithology, the apparition of new sedimentary layers, and the disappearance of others at some places, caused by different factors that affect the sedimentation process as erosions and faults. Concerning the shapes of the peaks, they are mainly controlled by the dipping angles of the different layers [54; 57].  
Peaks at higher frequencies (between 8 and 12 Hz) are observed on the hills overlooking the El-Attaf city from the southwest. These peaks appear to be caused by the outcropping Cretaceous marks. In the city of El-Abadia, the resonance frequency is around 2 Hz, while in El-Attaf city, it is around 3 Hz.  
In the third zone, the resonance frequencies range between 1.3 and 3 Hz. The corresponding amplitudes vary between 2.2 and 3.5. Two types of HVSR curves were obtained: curves with one frequency peak in the southern part, and curves with two frequency peaks in the northern part. The frequency of the second peak varies from 5 to 12 Hz and appears to be related to an impedance contrast between the Holocene and Pleistocene alluviums at shallow depths. In the city of Ain-Defla, the resonance frequency of the soil increases from 1.8 to 3 Hz, from north to south, respectively. The corresponding peak amplitudes are around 5.