# Characterization of Shallow Sedimentary Layers in the Oran Region Using Ambient Vibration Data
## 1 Introduction  
Oran region is located on the coast of the Mediterranean Sea, in the northwest of Algeria. It is part of the Lower-Chelif Basin and is surrounded by the Murdjadjo anticline to the west and by the Arzew massif to the east. In the south, a large Sebkha (saline area) constitutes the southern limit. The Cheflif Basin is a large depression that extends over 450 km in an east-west direction and is located in the midst of the northern and southern Tellian Atlas Mountain Belt (Figure 1). The basin is separated into three sub-basins: the Lower Chelif, the Middle Chelif, and the Upper Chelif basins. The boundary between the convergent African and Eurasian tectonic plates in northern Algeria is well-defined by the seismicity, which is mainly located at the margins of the Neogene basins [1]. Over the years, this territory has witnessed several moderate to strong earthquakes. Oran, in particular, has experienced various moderate earthquakes, including the most significant one in 1790, with an intensity (EMS-98) of \(I_{0}=\) VIII [2], the 1949 earthquake with an intensity of \(I_{0}=\) V [3], and a 5.4 moment magnitude earthquake in 2008 [4]. The Oran region has been the object of numerous geological and geophysical studies. [PERSON] conducted the first geological study of the Oran region, establishing the stratigraphical units in the area, which led to the first geological map of the region [5]. Later, [PERSON] a geodynamic evolution model for the western part of the Lower-Chelif Basin using tectonic and sedimentological approaches [6]. [PERSON] focused on the geological and stratigraphic characteristics of the Great Sebkha located south of Oran [7]. In addition, [PERSON] studied the Murdjadjo fault and concluded that it was responsible for the 1790 earthquake [8]. Different soils and structures respond differently depending on the frequency content of an earthquake [9]. Hence, it is of importance to know their behavior at different frequencies. In the particular case of soil, ambient vibration three-component stations and array deployments are widely applied to investigate and characterize sedimentary basins (e.g., [10; 11; 12; 13; 14; 15; 16; 17]) and even some rock sites [18], due to the multiple advantages that these non-invasive techniques offer [19], with regard to other invasive techniques based on downholes, which allow for an accurate estimation of some rock properties but are more expensive and time-consuming [20; 21]. Concreting in the northern area of Algeria, these approaches based on ambient vibrations have been employed in the Lower and Middle-Chelif Basins, around 180 km east of our study area, in order to characterize the sedimentary cover (e.g., [22; 23; 24; 25; 26]). More recently, a study based on ambient vibration measurements and three-component stations was carried out in the city of Oran [27], as a starting point of our ongoing investigation in this area. In this work, the Horizontal-to-Vertical Spectral Ratio (HVSR) analysis, together with two of the most common array-based techniques, i.e., the frequency-wavenumber (f-k) and the spatial autocorrelation (SPAC) methods, have been applied in order to characterize the area under study by the uppermost 30 m of the soil column. Thus, one of the objectives of this study is to determine the shear-wave velocity structure in the shallowest sedimentary layers and to establish a variation map of \(V_{s30}\), where impedance contrasts with the sedimentary cover can result in ground-shaking amplification during strong seismic events. Figure 1: (**a**) Location of Oran region in Mleta plain in Lower-Chelif Basin. The rectangle (1) corresponds to Upper, (2) Middle, and (3) Lower-Chelif Basin, which is comprised between the Northern and Southern Tellian Atlas Mountains. The numbered green stars, 1, 2, and 3, are the 1790, 1949, and 2008, respectively, main seismic events in the region. The study area (Oran region) is indicated by the dashed frame and zoomed in (**b**).