## 1 Introduction  
The north of Algeria is characterized by moderate to high seismicity, as a result of the convergence between the African and Eurasian plates. This seismicity is mainly concentrated on the margins of the Neogene basins [1]. In the Cheliff Basin, located in the northwestern part of Algeria, between the northern and southern Tellian Atlas mountain belts and lying parallel to the Africa-Eurasia plate boundary, in the approximately E-W direction, the active deformation is mainly expressed by reverse faults oriented in a NE-SW direction [1,2]. The Cheliff Basin is a synclinal depression filled with Mio-Plio-Quaternary deposits lying over a Mesozoic basement [2,3]. Due to its width (about 350 km from east to west), [PERSON] [3], divided the basin into three sub-basins: the Low, the Middle and the High Cheliff Basin. The city of Oued-Fodda is situated in the western margins of the Middle Cheliff Basin (Figure 1). During the past century, the Chlef region has experienced several destructive earthquakes, likethe 1934 Carnot (I0 = IX) earthquake [4], the 1954 Orleansville (Ms 6.7) earthquake [5] and the 1980 El-Asnam (Ms 7.3) earthquake [6], with an intensity of IX (MSK), all closeto the city of Oued-Fodda [6]. The epicenters of these three earthquakes are marked in Figure 1. The 1980 El-Asnam earthquake is the largest recorded event in Algeria, and the most destructive, with over 2600 victims and massive building damage. The causative fault is located about 1 km north of Oued-Fodda city. Right after the El-Asnam earthquake, a geotechnical and geological study of the Chlef region was carried out by Woodward Clyde Consultants (WCC) [7] in order to assess the seismic risk in the region. More concretely, the study was conducted in the city of Chlef and eight surrounding cities, including Oued-Fodda city. As results, the WCC report provided different geotechnical and hydro-geological maps, and identified the liquefaction and landslide zones. The report also mentioned that site effects might be the cause of ground motion amplification. Alternatively to geotechnical and geological techniques, seismic methods based on ambient vibrations are being widely used in recent decades (e.g., [8; 9; 10; 11]). These methods are especially suitable for urban areas (non-destructive, low cost, quick implementation, etc.). There are two main categories of experimental techniques that use this kind of data: the first one is based on array measurements, and the second one is based on three-component single-station measurements (horizontal-to-vertical spectral ratio method, HVSR). In both approaches, an inversion of the estimated curve (dispersion curve in array measurements and HVSR curve in single-station technique) can be performed subsequently for estimating the associated S-wave velocity (Vs) profiles. In this sense, stochastic search methods such as genetic algorithms [12; 13] and the neighborhood algorithm [14; 15] are becoming increasingly popular in this kind of geophysical inverse problems. In this regard, a complete site effect investigation has been recently conducted in the city of Chlef [16; 17] in order to improve the study presented in the WCC report [7]. Standard spectral ratio, receiver functions and HVSR methods have been applied for both earthquakes and ambient vibrations. The soil fundamental frequencies (f0) have been identified, along with some subsurface characteristics (e.g., Vs profiles and depth to bedrock). In addition, one of the main results of Layadiet al. [16], is the  
Figure 1: Situation of the Middle Cheliff Basin. Yellow circles represent the major earthquakes in the area: (1) The 1934 Carnot (I0 = IX) earthquake, (2) The 1954 Orleansville (Ms 6.7) earthquake, (3) The 1980 El-Asnam earthquake (Ms 7.3). EAF: El-Asnam fault (black line). OF: Oued-Fodda city. TM: Temoulga Massif. RM: Rouina Massif. DM: Doui Massif. interpretation of the bedrock as Mesozoic and not the outcropping Miocene rock.