## 1 Introduction  
Northern Algeria is characterized by a series of Neogene basins (e.g., Constantine Basin, Hodna Basin, Soummam Basin, Tizi-ouzou Basin, Mitidja Basin, Medea Basin, and Chelif Basin), elongated in an E-W direction, and surrounded by the Tellian Atlas mountain belts, which act as a substratum for their sedimentary covers [1, 2, 3]. These basins host important seismic activity, mainly in their marginal zones [4]. Active tectonics in the northern part of the country and the related seismicity are due to the fact that this zone is located at the boundary between the African and Eurasian convergent plates [4, 5]. In the northwestern part of Algeria, between the septentronal and meridional Tellian Atlas mountain belts, lies the Chelif Basin, a wide depression of over 450 km in length, which is the largest and the most subsident of the sublittoral basins [2, 3]. The dimensions of the basin and the complexity of its hydrographic network has led several authors to divide it into three sub-basins: The Lower-, the Middle-, and the Upper-Chelif. In this study, we focus on the plain extending from Oued-Fodda to Ain-Defla (Figure 1). However, one must note that this subdivision is not unanimous within the scientific community. While some authors assign our study area to the Middle-Chelif Basin [6, 7, 8], others have assigned it to the oriental part of the Lower-Chelif Basin [2; 3; 9]. In this study, we consider the study area as a part of the Middle-Chelif Basin. The Middle-Chelif plain is home to over 400,000 inhabitants, who are mainly concentrated on its edges, and distributed over its principal cities, which are: Ain-Defla, Rouina, El-Amra, El-Attaf, El-Abadia, and Oued-Fodda. The region has witnessed several destructive earthquakes, such as the 1858 El-Amra earthquake (I\({}_{0}\) = IX, [10]); the 1934 El-Abadia earthquake (I\({}_{0}\) = IX, [10]); the 1954 Orleansville earthquake (Ms 6.7, [10]); and the 1980 El-Asnam earthquake (Ms 7.3, [11]). In the last few decades, techniques based on ambient vibrations have been widely used to study sedimentary basins because of the quick implementation and the lower cost of the process, compared to boreholes or other geophysical prospecting methods (e.g., [12; 13; 14; 15; 16]). It is proven that soft sedimentary layers can amplify the ground shaking and prolong its duration during an earthquake, which can be harmful for buildings. Therefore, the determination of the geotechnical characteristics of the soils is primary for seismic risk and site effects assessments. One of the most important parameters for determining the geotechnical characteristics is the shear-wave velocity structure. The high-velocity contrast between soft sediments and the bedrock, along with the local geological and topographical aspects, are important factors for ground motion amplification. The Chelif Basin has been the subject of several geological and geophysical studies. The majority of these studies have been done on the Lower-Chelif Basin, and only a few of them have included the western part of the Middle-Chelif. It was only at the end of the 1960's that the CGG (Compagnie Generale de Geophysique) carried out a geophysical prospecting campaign by electric methods in the Middle-Chelif Plain in order to study the structure of the sedimentary deposits [7]. Since the 1980 El-Asnam earthquake, and the damage it caused, the Middle-Chelif region has been the subject of a multitude of geological and geophysical studies. The first study was carried out by the Institute of Earthquake Engineering of Skopje University  
Figure 1: Situation of the Middle-Chelif Basin. LCB: Lower-Chelif Basin. MCB: Middle-Chelif Basin. UCB: Upper-Chelif Basin. OF: Oued-Fodda. AB: El-Abadia. AT: El-Attaf. AM: El-Amra. RO: Rouina. AD: Ain-Defla. (Macedonia), which resulted in the elaboration of the code for the repairing and strengthening of damaged buildings in the Chelif region. The code was based on studies of seismic hazards and the geotechnical conditions of the soil.