Characterization of the subsoil structure in the Middle-Chelif Basin (Algeria) using ambient vibration data

Resumen

The northern part of Algeria is located in the border zone between the African and Eurasian plates. The collision between the two plates is expressed by a moderate to high seismicity, generally localized at the margins of the Neogene basins. The Middle-Chelif Basin is located in the northwestern part of Algeria, between the northern and southern Tellian Atlas mountain belts. The seismic activity is mainly generated by the El-Asnam fault, a 40 km long reverse fault located on the western edge of the basin. The 1980 El-Asnam earthquake caused significant damage in the cities of the basin. In particular, the cities of Oued-Fodda, El-Attaf and El-Abadia were heavily affected. In the western part of the alluvial plain of the Middle-Chelif, phenomena of cracks, settlements, landslides and liquefaction, have also occurred following the earthquake. This research aims to quantify dynamic properties of the soils of the Middle-Chelif Basin in terms of shear-wave velocity (Vs), fundamental frequency or vulnerability index (Kg) for the estimation of liquefaction potential. The calculation of dynamic soil properties allows a better assessment of the seismic hazard in the region. We have focused more on the characterization of the Vs structure of the superficial sedimentary layers in the entire Middle-Chelif Plain because of the role it plays in the amplification of the seismic waves during an earthquake. Secondly, these same soil parameters allow the creation of microzonation maps classifying the surface soil according to the criteria of NEHRP (National Earthquake Hazard Reduction Program). For this purpose, techniques based on single-station and array ambient vibration measurements are applied. Ambient vibrations were recorded at 323 sites using single-station, and at 18 sites using array measurements. The measurements were densified within urban areas. This thesis is divided into three main parts; the first one consists in a seismic microzonation of the city of Oued-Fodda, located at 1-2 km from the El-Asnam fault. The Horizontal-to-Vertical Spectral Ratio (HVSR) method was applied on ambient vibration records measured at 103 sites in the city and its surroundings. Maps of the variation of soil resonance frequencies, as well as their amplitudes, were provided. Inversion of the HVSR curves allowed obtaining 1D Vs models at each site. The 2D velocity profiles were used to image the shape of the sedimentary layers and the bedrock outcrop in the central part of the city. The second part aims to characterize the sedimentary deposits in the basin. The HVSR method was applied on ambient noise records measured at 164 sites and aligned on 20 NW-SE profiles. The Frequency-Wavenumber (F-K) technique was applied on array measurements at 7 sites. The 2D velocity profiles imaged the synclinal shape of the sedimentary deposits. A bedrock model was also provided. The third and last part consists of a more complete seismic microzonation in the three other main cities of the basin; Ain-Defla, El-Attaf and El-Abadia. Ambient vibrations were measured using a single-station at 56 sites and using arrays at 11 sites. As a result, maps of resonance frequency variation, Vs variation over the first 30 meters of the soil (Vs30) and soil classification were proposed in addition to a prediction equation for Vs30 in the region.