Seismic site-effects assessment in a fluvial sedimentary environment: case of Oued Martil floodplain, Northern Morocco

In Northern Morocco, seismic site-effects in general and liquefaction hazard in particular can occur in the event of a major earthquake due to the thick sedimentary cover characterizing the peripheral Neogene basins of the Alboran Sea. An example is Martil plain, which was the subject of important economic development during the last two decades. In this regard, we present in this study an assessment of seismic site-effect hazard through the horizontal-to-vertical spectral ratio (HVSR) method and the vulnerability index (Kg). Multiple Analysis of Surface Waves (MASW) method and core-drilling data are also used to complete our analysis and interpret the spatial distribution of Kg maps. Our findings suggest more vulnerability to liquefaction in the Southern segment of the basin, which can be explained by the asymmetrical geometry of Quaternary sedimentation, due to tectonic uplift that influences also the surface and subsurface hydrology processes. However, in this study, the main difficulties and the challenges that we faced were related to data acquisition during windy days and in the areas near industrial zones or near high traffic roads, which have significant effects on the accuracy of the HVSR analysis. In this study, HVSR microtremor measurements are recorded at 197 stations in the Martil floodplain to generate a seismic vulnerability index (Kg) map. According to the analysis results, the predominant frequency (F0) values range from about 0.2 to 10.6 Hz and the peak amplitudes (A0) values are in the range of 0.39–10.4. As a result, some districts, especially those classified as economically disadvantaged, are found to be the most exposed to this hazard (with a Kg > 10), which must be taken into consideration in future risk reduction and mitigation plans. We conclude the existence of significant seismic effects potential despite the moderate seismicity of the area. Therefore, our research needs to be completed by scenario-based seismic hazard modeling to investigate the capacity of seismic events at the region to produce the above-suggested amplifications.