Seismic site effects investigation in the urban area of Nador (NE Morocco) using ambient noise measurements

Seismic site effects are related to the local lithology, which modulates seismic waves and can enhance their amplification. For this reason, the horizontal-to-vertical ratio (H/V technique) was used to characterize them in the urban area of Nador. Ambient noise was measured at 190 different sites in order to characterize the sedimentary deposits from a seismic point of view, with further reference to 53 available geotechnical boreholes. The first investigation on site effects in Nador city, presented in this study, gives fundamental frequency values between 2.1 and 5 Hz and an amplification factor range between 2.5 and 11.4. No amplification is found in rock soils except for some small areas where weathered soils exist. The obtained results are compared with the H/V of a ground motion recorded in two stations. In general, results are in agreement and show that some areas display the phenomenon of amplification during a seismic action. The seismic vulnerability indices were also estimated, and values between 1 and 47 were obtained, indicating that the zones where the sediments overlie basaltic rocks are highly vulnerable, given the high impedance contrast. This microzonation study is a case study for the seismic risk assessment of Nador, ultimately contributing to urban planning and risk mitigation by defining suitable areas for building.