Topographic effects on seismic responses of steep sloped superficially weathered rock: the case of ‘Rocher Noir’ at Boumerdes city in Algeria

Rocks often showed an alteration in their first meters of depth, by the presence of weathered portion through variable thickness. Concentration of building damage near the edge of hard or dense cliff has been observed during a number of earthquakes. The numerical analysis of topographic effects of rocky steep slopes knowingly neglects the stratigraphic effect. In previous studies, it was evoked the fracturing of the upper parts of the site of ‘Rocher Noir’ in Boumerdes City, and the obtained HVSR curves were flat or presenting a peak over 10 Hz, explained by the fact that the metamorphic outcropping is overlaid by sediments. In this paper, interaction between the effects of stratigraphic and topographic amplification of this weathered rocky site is investigated through 1D and 2D numerical models. The goal of these investigations is to provide a better understanding of the potential hazard that is represented by amplified ground motion due to the coupling effect (1D amplification and topographic amplification) and to quantify the weathering effects on topographic amplification. Seismic responses (amplification functions and response spectra) are calculated at representative points located in the upper surface of the rocky model, assumed either hard or weathered. The obtained results show that the presence of a weathered portion on the surface can modify the ground response in the sense of higher amplification, larger bandwidth (modifying of the mode vibration), multiple response spectra peaks and inducing a considerable vertical acceleration component. Ambient noise measurements were conducted, the HVSR results agree well with those obtained by modelling.