Resonance, Directionality, and the Limits of Depth: Basin-Wide Ambient-Noise Characterization of the Mitidja Basin (Northern Algeria)
Abstract
Abstract The Mitidja Basin, Northern Algeria, is a structurally complex, densely urbanized coastal basin filled with Neogene and Quaternary sediments, extending ENE–WSW over approximately 90 km in length and 20 km in width. Despite its high seismic risk, sedimentary thickness and basement geometry remain unconstrained basin-wide, as previous ambient-noise studies focused on individual cities. This study presents the first systematic, basin-wide characterization of seismic ambient noise in the basin, based on 132 broadband measurement points on a regular grid. The Horizontal-to-Vertical Spectral Ratio (HVSR) technique was used to estimate the fundamental resonance frequency (f0), and Rayleigh-wave ellipticity inversion (successful at 72 sites) to derive 1D shear-wave velocity (Vs) profiles and basin-wide directionality. Results show f0 below 1 Hz across most of the basin, with localized high-frequency zones associated with shallow bedrock. Azimuth-dependent site response occurs basin-wide, oriented mainly N–S along the margins and E–W centrally, reflecting structural heterogeneity. The inversion resolves a two-layer sedimentary model, a Quaternary layer over a merged Neogene unit, overlying a mechanically stiff interface at depths of up to 950 m. Cross-validation against independent gravimetric modeling shows that this interface coincides with the true crystalline basement at only a minority of sites; elsewhere it marks an intermediate stiff horizon, reflecting the…
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