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Chlef City, Algeria, which is located in the Lower Cheliff Basin, is vulnerable to seismic hazards. Since there is no constrained velocity model for the Lower Cheliff Basin, particularly at greater depths, we conducted an ambient vibration investigation to map the shear‐wave velocity structure beneath the city, with the primary goal of supplementing the existing microzonation studies. Here, we inverted the Rayleigh wave ellipticity measurement curve of ambient vibrations measurements from 120 sites in Chlef City to estimate the shear‐wave velocity structure. The study area was subdivided into six zones based on similarities between the observed horizontal‐to‐vertical spectral ratio of the ambient vibrations curves. Our resultant shear‐wave velocity models show that the observed fundamental frequencies (0.3–1.6 Hz) are related to a thick layer (~800 m) of upper Miocene deposits, where Vs=1000 m/s on average. Generally, the Mesozoic basement shows Vs>2000 m/s. Moreover, the clear peaks observed at higher frequencies and only in the northwest part of the city are related to a thin layer of Quaternary deposits (Vs= ∼2000 m/s).

The city of Oued-Fodda is located in north-central Algeria on the margins of the Middle-Cheliff Basin. This region has suffered several destructive earthquakes. The strongest was the 1980 El-Asnam earthquake (Ms7.3), whose causative fault was located about 1 km north of the city of Oued-Fodda. Therefore, a good knowledge of the soil characteristics in this city may allow a better evaluation of the seismic risk and help to minimize damages in the future. With this objective, a detailed microzonation study of Oued-Fodda has been carried out in this study. For that, the horizontal-to-vertical spectral ratio (HVSR) method has been applied on 102 sites along the city, estimating the soil fundamental frequencies and their corresponding amplitudes. Besides, the Rayleigh wave ellipticity inversion has been accomplished in order to estimate the corresponding Vs profiles and provide two cross-sections of the geology under the city. In the central part of the city, high-frequency peaks are observed, between 12.5 and 15 Hz, which correspond to impedance contrasts at shallow depth (<20 m). In the surrounding plain, two clear peaks are identified in the ranges 1.8-3.5 Hz (fundamental frequencies) and 6.5-15 Hz (secondary peaks).

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