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The Middle-Chelif basin, in northwestern Algeria, is located in a seismically active region. In its western part lies the El-Asnam fault, a thrust fault responsible for several strong earthquakes. The most important being the El-Asnam earthquake (Ms = 7.3) of 1980. In the present study, ambient vibration data with single-station and array techniques were used to investigate the dynamic properties of the ground and to estimate the V s 30 structure in the main cities of the basin. Soil resonance frequencies vary from 1.2 to 8.3 Hz with a maximum amplitude of 8.7 in. Collapsing behavior has also been demonstrated west of the city of El-Attaf, reflecting a strong potential for liquefaction. A V s 30 variation map and a soil classification for each city were obtained mainly by inversion of the HVSR and Rayleigh wave dispersion curves. Finally, an empirical prediction law of V s 30 for the Middle-Chelif basin was proposed. [ABSTRACT FROM AUTHOR]

The ambient vibration horizontal-to-vertical spectrum method (HVSR) is widely used in microzonation and structural studies, assuming stability and reproducibility of HVSR frequency peaks. A 35-day continuous HVSR monitoring performed in the suburbs of Algiers (Algeria) reveals that HVSR peaks are not always stable and reproducible both in frequency and amplitude. HVSR curves show a first peak, which varies from 0.9 to 1.8 Hz with amplitudes from 0.9 to 3.3, and a second peak from 5 to 7.7 Hz with amplitudes from 1.4 to 4. These results show that HVSR peak frequencies and amplitudes are highly sensitive to nearby anthropogenic sources. [ABSTRACT FROM AUTHOR]

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). [ABSTRACT FROM AUTHOR]

The city of Oran experienced several earthquakes. Therefore, the characterization of its subsurface is crucial for a better evaluation of the seismic hazard. Using ambient vibration single-station and array measurements at 193 sites, the HVSR curves showed a variation of the fundamental frequency peak between 0.3 and 7.4 Hz, growing towards the west, with maximum amplitude of ∼6. The shallow structure has been explored by means of F–K analysis. Joint inversion of the HVSR and Rayleigh wave dispersion curves showed 3 layers of soft and stiff sediments of varying thicknesses overlying the bedrock. The shear-wave velocity (Vs) of the sediments varies between 280 and 1300 m/s and that of the bedrock can reach 2500 m/s. Cross sections show a decrease in sediments thickness from east to west where bedrock outcrops. The maximum depth of bedrock is 1050 m northeast of the city. The obtained Vs models support a soil classification ranging from stiff soil to hard rock. • Ambient vibrations methods are applied for the first time in Oran city. • Joint-inversion of HVSR and dispersion curves led to a model of the 3D Vs structure. • Boreholes data constrained the Vs profiles resulting from the joint-inversion. • This study contributes to the estimation of the seismic hazard in Oran. [ABSTRACT FROM AUTHOR]