## 1 Introduction  
Northern Algeria is located in the collision zone between the African and Eurasian plates. It is characterized by moderate to high seismic activity (e.g., [1, 2, 3]), mainly concentrated in the marginal areas of the Neogene basins [4]. The Chelif Basin is located in the northwestern part of Algeria (Figure 1). It is the largest of the northern Neogene sedimentary basins and hosts an important seismic activity. The basin is mainly affected by NE-SW oriented reverse faults [4, 5]. The most important is the El-Asnam fault, a reverse fault about 40 km long [6], that generated several destructive earthquakes during the last century, such as the 1934 Carnot earthquake, now El-Abadia, (\(M_{S}\) = 5.1, [7]); the 1954 Orleansville earthquake, now Chlef (\(M_{S}\) = 6.7, [7]); and the 1980 El-Asnam earthquake, now Chlef (\(M_{S}\) = 7.3, [6]). The latter is the largest and most destructive earthquake recorded in Algeria in the instrumental era. The Chelif Basin is divided into three parts: the Lower-Chelif Basin, the Middle-Chelif Basin, and the Upper-Chelif Basin. The Middle-Chelif extends from Oued-Fodda in the west to Ain-Defla in the east (Figure 1). The cities of the Middle-Chelif suffered important damage during the 1980 El-Asnam earthquake. The cities of Oued-Fodda, El-Abadia, and El-Attaf were almost totally destroyed. In addition, several secondary effects of the earthquake were observed along the rupture zone [8], such as cracks, settlement, and soil liquefaction. Moreover, the coseismic uplift of the western part of the fault has obstructed the flow of the Chelif River, causing a flood which formed a natural dam, where the phenomenon of liquefaction occurred over a wide area west of El-Abadia and El-Attaf [8, 9]. Prior to the 1980 El-Asnam earthquake, several geological and geophysical studies were conducted on the Chelif Basin (e.g., [10,11]). However, these studies were mainly concentrated in the Lower-Chelf Basin due to the presence of oil indices. Only a few studies have been carried out on the Middle-Chelif Basin [12,13]. Right after the 1980 earthquake and given the extent of its damage, the region has finally been the subject of several geological, seismological, and geophysical studies (e.g., [6,8,14,15]). The firm Woodward and Clyde Consultants [15] conducted an important seismic microzonation study in eight cities of the Lower and Middle-Chelif Basin, including El-Abadia and El-Attaf. During the investigations, several holes were drilled using Standard Penetration Tests (SPT). The study provided geotechnical, hydrogeological, landslide, and liquefaction potential maps for each of the eight cities. The Neogene formations of the Middle-Chelif were described in detail in [12], and later in [16]. Furthermore, the structural aspect of the shallow and deep lithological units was recently imaged using land gravity data [17]. In the present study, we used ambient vibrations data to characterize some geotechnical features and to estimate some dynamic properties of the soil column in the cities of El-Attaf, El-Abadia, and Ain-Defla. Ambient vibration-based techniques have been used previously in the Chelif Basin [18-21]. The aim was to determine the resonance frequencies of the ground, the shear-wave velocity structure of the sedimentary layers, and the bedrock depth, where the impedance contrasts with the sedimentary cover and may be the cause for ground shaking amplification during strong earthquakes. This work is a continuation of the ones carried out in the Middle-Chelif basin [20,21]. In the first part of this study, we used ambient vibration data recorded from single stations to estimate the ground resonance frequencies and assess the liquefaction potential in the three cities under study. In the second part, we used ambient vibration data recorded from single station and array techniques to estimate the average shear-wave velocity in the upper 30 m of the soil column (V\({}_{\text{s30}}\)). Finally, a V\({}_{\text{s30}}\) predictive equation for the Middle-Chelif Basin was proposed.