## 1 Introduction  
Northern Algeria is characterized by moderate seismic activity, with the occurrence of few strong earthquakes, resulting from the collision between the African and Eurasian tectonic plates. Most of the seismic activity is located at the margins of the northern Neogene Basin [1, 2]. Understanding local site effects is crucial in seismic hazard assessment, especially in sedimentary basins where ground motion amplification can significantly impact infrastructure. Various methods, including the HVSR technique [3] and array-based seismic surveys, have been used globally to characterize subsurface structures and assess seismic vulnerability (e.g., [4]). While several studies have focused on urban areas in high-risk seismic zones (e.g., [5, 6]), northwest Algeria remains very little explored in this regard. To address this gap, this study investigates the sedimentary cover of the city of Ain Temouchent using ambient vibration single station and array methods, providing new insights into the soil characteristics of the Lower Cheliff Basin. Ain Temouchent is a city located at the westernmost part of the Lower Cheliff Basin, the largest of the Neogene basins of northern Algeria [7]. It stretches for 450 km in an EW direction and it is divided into 3 sub-basins: Upper, Middle, and Lower Chelif Basin (e.g., [8]). The city of Ain Temouchent is built on a small plain (the Ain Temouchent Plain), located between the Mleta Plain, the Tessala Mounts, and the Tlemcen Mounts (Figure 1). In the Lower Chelif Basin, northwest Algeria, the seismicity is also moderate. The most important earthquakes were the 1790 Oran earthquake, with an intensity \(I_{0}=\) VIII (European Macroseismic Scale, EMS-98) [9]; the 1999 Ain Temouchent earthquake, with a moment magnitude of Mw = 5.7 ([10,11]); and the 2008 Oran earthquake, with a moment magnitude of Mw = 5.4 [12]. The 1999 Ain Temouchent earthquake (Figure 1) was caused by a reverse fault oriented NE-SW, located around 20 km west of the city center [10]. The final toll from this earthquake was 25 dead and 174 injured. The total number of people affected was around 25,000. More than 600 houses were destroyed and more than 1200 others were severely damaged. The Ain Temouchent region has undergone several geological (e.g., [13,14]) and geotechnical studies (e.g., prospections carried by the LTPO (Laboratoire des Travaux Publics de l'Ouest) and the LNHC (Laboratoire National de l'Habitat et de la Construction)). After the 1999 earthquake, more studies were conducted in the region, including the city of Ain Temouchent. In the first place, several studies have used seismological [10] and InSAR [11] data to characterize the surface rupture and propose an underground fault model. After that, several studies focused on the geotechnical characteristics of the soils in the city of Ain Temouchent. For example, the study carried out in [15] led to the elaboration of a geotechnical map of the city using the geographic information system (GIS). The most important was the seismic hazard assessment study carried out mainly by Geomatrix ([16], unpublished report). It is well known that each region and structure react differently during an earthquake. The fundamental frequencies of the soils and buildings are the main elements  
Figure 1: Location of the study area. The seismicity was obtained from the ISC (www.isc.com) for the period between 1 January 1980 and 31 December 2024.  
involved in this case, so it is crucial to understand their behaviors in the case of a strong earthquake [17; 18; 19]. Several methods based on the analysis of ambient noise data have been developed to study the shear wave velocity structure in shallow sedimentary layers, which is likely to be a factor of amplification during earthquakes. Among these methods, the horizontal-to-vertical spectral ratio (HVSR) technique [3] is one of the most widely used.