## 2 Geological Framework  
The Middle-Chelif is an intra-mountainous basin structured during the Neogene [5; 10; 12], and located within the Tellian Atlas mountain belts (Figure 1). The depression is filled with a thick cover of Mo-Plio-Quaternary sediments. The basement is composed of hard clays and marks of Cretaceous age [12; 16]. In its southern part, autochthonous formations of Jurassic to Silurian age outcrop [22], and form the Temoulga, Rouina, and Doui massifts (Figure 2). The Middle-Chelif plain is crossed from west to east by the Cheliff River, the longest in Algeria, which contributes to form the actual alluviums.  
The stratigraphical column is composed of a succession of marine, continental, and lacustrine deposits. Lateral variations in facies were also reported [10; 16], and further confirmed in [21], where important lateral variations in the shear-wave velocity were observed within the same formations. All these perturbations in the sedimentary cover reflect several instability periods with intense tectonic activities and different episodes of marine regression and transgression that conditioned the sedimentation process [10]. However, the geologists are not in agreement concerning the age of the first sedimentary deposits. Some authors attributed those sediments to the Lower Miocene [10; 12], while others assigned them to the Middle Miocene (Serravallo-Tortonian) [16; 23]. The sediments are affected by a series of normal and thrust faults, located in the margins of the Middle-Chelif Plain.  
In terms of lithology, the Miocene series occupies the major part of the sedimentary column. These formations were described and detailed in [12]. The first deposits are detritic continental series of conglomerates and marks. These sediments are overlayed by different intercalations of marks, clays, limestones, and sandstones [12; 16]. A thin layer of blue marls marks the transition between the Miocene and the Pliocene sediments [10; 12]. The Miocene series outcrop in succession on the hills that overlook the city of El-Abadia (Figure 2). The shear-wave velocity varies from 640 to 1450 m/s for the Miocene formations [21]. The Pliocene is divided in two stages, marine and continental, with alternations between sands, sandstones,  
Figure 2: Geological map of the study area. Compiled and modified from [22; 24].  
and conglomerates [10; 16]. The Quaternary deposits are continental and predominant in the Middle-Chelif Plain. They are represented by Holocene and Pleistocene alluviums.  
The three cities under study are built on Quaternary alluviums of different thicknesses. The city of Ain-Defla is built at the bottom of the northern flank of the Doui massif (Figure 2), where the topmost layer of the soil is composed of old Quaternary clays and gravels (Pleistocene), reaching a maximum thickness of 60 m [21]. The old alluviums lay directly over hard Jurassic limestone and Silurian schists and quartzite [22].  
The cities of El-Attaf and El-Abadia are built on stiff Quaternary soil, the topmost layer is thin (<20 m) [21], and composed of recent alluviums (Holocene), which lie over older alluviums (Pleistocene). The engineering bedrock in El-Abadia is composed of Pliocene sandstones, while in El-Attaf, it is composed of Miocene marls and sandstones [14; 21]. In these cases, the engineering bedrock has been characterized as the first layer of the soil column that contains a shear-wave velocity value above 750 m/s.  
The presence of important sandbanks at shallow depths (<10 m) in El-Attaf and El-Abadia may lead to liquefaction phenomena during ground shaking. The risk is weak in El-Abadia since the sandy layers are dense and the ground water level is between 15 and 30 m deep [15]. However, in El-Attaf the risk is significant since the sandy banks are loose and the ground water level is shallow (between 7 and 10 m) [15].