## 4 Results  
### Geophysical Data  
#### 4.1.1 MASW Survey  
In February 2014, [PERSON] et al. (2018) [53] completed thirteen MASW sections (labelled from M1 to M13, Figure 2). The MASW survey covered all the geological formations defined in Adra town. Basic data for all VS models were frequency in the 2.8-43.0 Hz range, phase velocity in the 259-1198 m s\({}^{-1}\) range, length from 40 m (section M7) to 810 m (section M1), and prospecting depth from 43 m (section M9) to 84 m (section M1). From active and passive MASW measurements, the 1D VS models were generated and interpolated to create 2D VS sections (called MASW sections) (Figure 3). The sections were topographically corrected. The vertical-equispaced VS values from all MASW sections were georeferenced and interpolated to create 2D VS layers at different elevations (called MASW maps) (Figure 4). Figure 3: MASW sections M10, M4, M5, and M3; location is in Figure 2. A geological interpretation of VS values after [PERSON] et al. (2002) [19] and the projected piezometric level after the flash campaign carried out in September 2014 are included; location of open well and piezometer W3, W4, and W10 is in Figure 2. Sections are topographically corrected and its vertical-to-horizontal scale ratio is 1:1. This paper uses the MASW sections and maps for geological definition following the interpretative criteria reported by [PERSON] et al. (2020) [62]. These authors propose that VS propagation in sediments is a site-specific steady property determined by effective compaction and as such it is dependent on the age and depth of each geological material piled on vertical [63; 64; 65; 66]. In Adra, the VS values obtained in previous studies [19; 53; 67] were \(<\)350 m s\({}^{-1}\) for Holocene sediments, 350-600 m s\({}^{-1}\) for Pleistocene sediments, 600-900 m s\({}^{-1}\) for  
Figure 4: Some MASW maps at different elevation regarding the sea level in Adra town, showing observed and inferred normal and strike-slip fault systems, and location of MASW sections M1 to M13. Pliocene sediments, 900-1200 m s\({}^{-1}\) for WB, and \(>\)1200 m s\({}^{-1}\) for bedrock. Some sedimentary and tectonic processes can modify these ranges, such as cementation of coarse-grained sediments increasing VS up to 800 m s\({}^{-1}\) or high fissuring and weathering of the bedrock decreasing VS up to 700 m s\({}^{-1}\)[19; 67]. For hydrogeological interpretation, the available geological and hydrogeological information was superimposed on sections M1 to M13. Four sections (M10, M4, M5, and M3) of special interest to deduce the hydrogeological functioning of the NSUS are displayed in Figure 3.  
Sections M10 (Figure 3a), M4 (Figure 3b), and M3 (Figure 3c) show the NSUS, whereas section M5 (Figure 3b) shows the SFUS. Due to their continuity, sections M4 and M5 were interpreted together into a single section. A succinct description of these MASW sections is below. The uppermost 1-8 m thick VS \(<\) 300 m s\({}^{-1}\) is attributed to anthropogenic filling (sections M10, M4, M5, and M3), the underlying 2-10 m thick VS \(>\) 350 m s\({}^{-1}\) to Holocene colluvial and present littoral facies (section M5), the underlying 10-15 m thick 300 \(<\) VS \(<\) 600 m s\({}^{-1}\) to clay-rich g1 and g2 and coarse grained t1 and t2 (t1 is not identified in section M3), the underlying 5-10 m thick 600 \(<\) VS \(<\) 900 m s\({}^{-1}\) to cemented levels at the bottom of t1 and t2, the underlying 15-20 m thick 900 \(<\) VS \(<\) 1200 m s\({}^{-1}\) to WB, and the deeper VS \(>\) 1200 m s\({}^{-1}\) to the bedrock. This overall vertical VS distribution correlates well with the geological information, but some disruptions are observed. In M10, punctual VS \(<\) 300 m s\({}^{-1}\) underlying t2 is due to a dig gallery built in the 19 th century to drain groundwater. In M10, M4, and M3, some patches embedded into g1 and g2 with VS \(>\) 1000 m s\({}^{-1}\) are attributed to isolated bedrock blocks.