S-Wave Velocity Structure in the City of Oued Fodda, Northern Algeria, by Inversion of Rayleigh Wave Ellipticity

The city of Oued Fodda is located in north-central Algeria in the margins of the middle Cheliff basin. This region has suffered several destructive earthquakes, and the strongest was the 1980 El-Asnam earthquake (Ms 7.3). The causative fault of this earthquake is located at about 1 km north of the city of Oued Fodda. It is well known that a good knowledge of the soil characteristics in a city may lead to better evaluate the seismic risk and therefore help to minimize damages in the future. For this purpose, we used ambient vibration data with the application of the horizontal-to-vertical spectral ratio (HVSR) method to estimate the soil’s fundamental frequencies (f0) and the corresponding amplitudes (A0). We also applied the random decrement technique method (RayDec) to calculate the ellipticity of the Rayleigh waves. Two measurement campaigns were carried out in the city of Oued Fodda with a set of 102 measurement points. From the visual analysis of the calculated HVSR curves, we divided the study area into four different zones. Thirty Rayleigh wave ellipticity curves were then inverted to characterize the S-wave velocity structures in Oued Fodda city. In the central part of the city, the frequency values (12.5–15 Hz) correspond to the impedance contrast at shallow depth (0–20 m), between the Miocene blue marls (950–1350 m/s) and the ante-Neogene basement (2000–2600 m/s). In the surrounding plain, the frequencies vary between 1.8 and 3.5 Hz, and the existence of a second frequency peak (7.5–11 Hz) is due to the contrast between the Holocene (280–330 m/s) and the Pleistocene (615–710 m/s) deposits.