## 7 Conclusions  
In the present work, ambient vibration data were used to investigate the soil characteristics and the site effects in the Middle-Chelif Basin, with both single-station and array measurements. The region is home to over 400,000 inhabitants, distributed over its six main cities: Ain-Defla, El-Amra, Rouina, El-Abadia, El-Attaf, and Oued-Fodda. Being located on a zone with moderate to high seismic activity, these cities have suffered important building damages during past earthquakes. The fact that most of them are built on soft sediment layers has played a role in amplifying the ground shaking and the lengthening of its duration. The first tens of meters of the topsoil column play an important role in ground shaking amplification during an earthquake. In our case study, it corresponds principally to the thickness of the Quaternary layers. In the major part of the basin, these layers lie directly over the Miocene sediments, which creates an important impedance contrast. Thus, the thickness of the Quaternary layers and the corresponding Vs values had to be determined with more precision. For this purpose, seismic noise array measurements and electrical resistivity measurements were carried out. The HVSR technique was applied on ambient vibration recordings at 164 sites, and F-K analysis was applied on array recordings at 7 sites. The obtained HVSR curves allowed us to identify the soil resonance frequencies at each site. The F-K analysis allowed us to retrieve the surface wave dispersion curve at each of the 7 sites. The HVSR and dispersion curves were then jointly inverted to obtain the Vs profiles at each site. Figure 16: Bedrock depths of the Middle-Chelif Basin. Red circles represent the cities. OF: Oued-Fodda. AB: El-Abadia. AT: El-Attaf. RO: Rouina. AM: El-Amra. AD: Ain-Defla. The analysis of the HVSR curves showed the existence of different types of curves: curves with one, two, and multiple frequency peaks. The HVSR curves with two peaks are predominant in the Middle-Chelif Plain. The predominant soil resonance frequencies vary between 0.75 and 12 Hz. This wide range of frequencies is explained by the synclinal aspect of the basin, as well as the thinning of the sedimentary layers on its edges. The corresponding amplitudes vary between 2 and 6.2. However, the amplitudes obtained from the HVSR analysis may not be a reliable estimation of true amplification. The high-frequency peaks (between 5.9 and 12 Hz) appear to be related to an impedance contrast between the Holocene and Pleistocene alluviums at shallow depths. In the Middle-Chelif Plain, the topmost layer is composed of thin Holocene alluvial deposits, for which the shear-velocity (Vs) ranges between 210 and 370 m/s. The largest thickness observed for this layer is 21 m. For the Pleistocene deposits, the Vs value varies between 330 and 680 m/s, with a maximum observed thickness of 86 m. The Pliocene deposits are present only in the northern part of the plain and on the Sara-El-Maarouf anticline in the west. The shear-wave velocity varies between 300 and 750 m/s for the Upper Pliocene sands and conglomerates, and between 510 and 900 m/s for the Lower Pliocene sandstones. The largest observed thickness for the Pliocene layers is 145 m, while the Miocene deposits occupy a major part of the sedimentary column, often composed of four distinct layers. Their thickness reaches 550 m around the city of El-Abadia. From the Messinian stiff marly formations to the Serravallian poudingues, the Vs values vary from 640 to 1450 m/s. In the southern part of the Middle-Chelif Plain, Jurassic formations outcrop in succession on the massits of Doui, Rouina, and Temoulga. The Jurassic limestones are dipping, generally north, and constitute the bedrock for the Neogene deposits in this part of the basin. The shear-wave velocity for these formations varies between 2300 and 2670 m/s. However, in a major part of the basin, the sedimentary deposits lie over a Cretaceous bedrock, for which the Vs values range between 1620 and 2300 m/s.