Billel Melouk

The city of Aïn Témouchent, located in northwest Algeria at the westernmost part of the Lower-Cheliff Basin, has experienced several moderate earthquakes in the past. The most significant event occurred on December 22, 1999, with a magnitude of Mw 5.7, resulting in 25 fatalities and severe damage to the town center. In this study, ambient noise measurements from 62 sites have been analyzed using the horizontal-to-vertical spectral ratio (HVSR) method to estimate the fundamental ground frequencies (f₀) and their corresponding amplitudes (A₀). The results reveal that f₀ varies between 0.8 and 4.8 Hz, with amplitudes ranging from 2 to 5.6. The inversion of the HVSR curves enabled the estimation of the sedimentary layer profiles, with the corresponding thickness variation and shear–wave velocity for each layer. Additionally, four array measurement points across the city center were carried out using the spatial autocorrelation (SPAC) method to derive Rayleigh wave dispersion curves. The inversion of these curves provided more precise layer parameters and improved the estimation of Vs profiles, particularly at shallow depths. The results indicate shallow layer velocities varying between 150 and 1350 m/s, with maximum sediment thickness reaching 390 m in the industrial zone, northeast of the city. Furthermore, Vs30 and vulnerability index maps have been developed to classify soil types and assess liquefaction potential within the city.

SUMMARY: In this work, the Moho depth and the velocity structure of the crust and upper mantle beneath broad-band seismic stations of the Algerian broad-band seismic network are investigated. Teleseismic P-wave receiver functions jointly inverted with Rayleigh wave dispersion curves obtained from local earthquakes have been used. The seismic stations are located in different geological settings including the Tell Atlas, High Plateaus and the Saharan Atlas. The crustal thickness and the Vp/Vs ratio are first derived by the H–κ stacking method of receiver functions. The inversion results show the variation in Moho depth in the different geological contexts. The shallowest depths of the Moho (∼20–30 km) are estimated along the Algerian continental margin and Tell Atlas. In the High Plateaus region, the Moho depths vary from 30–36 km, whereas the deepest Moho depths are found in the Saharan Atlas (36–44 km). Two-layer crust is observed in the whole study area. In the upper crust, ∼8–14 km thick, the average shear wave velocity is ∼3.0 km s−1. The lower crust of about 12–30 km thick has an average shear wave velocity that ranges between 3.4 and 3.8 km s−1. The lower crust is thicker than the upper crust particularly in the Saharan Atlas. The upper mantle shear wave velocity varies from 4.1 to 4.5 km s−1 maximum and is stable, generally, below ∼60 km depth. Two low-velocity zones are clearly observed particularly in the eastern part of the Tell Atlas and the High Plateaus. The first one about 10 km thick is in the lower part of the lower crust and the other one is in the upper mantle between 40 and 60 km depth. The obtained results are in accordance with the previous results found in the region, particularly those using land gravity and seismic data. As the first estimate of the Moho depth from earthquake data in northern Algeria, using the receiver function method, this study sheds new light on the crustal structure and the Moho depth in this region of the world.