Analysis of Site-Effects in the Urban Bay of Algiers: Integrated Multi-Method Approaches for Microzonation and Seismic Hazard Assessment

Abstract

The Bay of Algiers, an urban area exposed to high seismic hazard due to nearby active faults, has a complex geological and topographical framework. This study aims to characterize site-effects using three complementary methods: standard spectral ratio (SSR), horizontal/vertical spectral ratio from earthquakes (HVSRE) and ambient vibration (HVSRN), applied to a temporary network of nine stations covering the Bouzaréah Massif and the eastern part of the Mitidja Basin. Analyses of 76 local earthquakes and ambient vibration reveal site-effects for all stations except the one installed on the flat bedrock. Inter-comparison of the three methods reveals a strong convergence of the identified fundamental frequencies, , at five stations regardless of the origin of the site-effects, whereas stations located in more complex and heterogeneous subsurface conditions show significant deviations. The divergences between SSR and HVSRE results, highlight the sensitivity of the estimated site-response curves to the selected reference condition, while the SSR technique remains the most reliable for evaluating amplification at . Differences between HVSRE and HVSRN at several stations are mainly related to the wavefield composition of earthquake data (body waves) and ambient vibration (surface waves). An analysis of polarization highlights directional effects related to underground irregularities and relief. Inversion of Rayleigh wave ellipticity made it possible to estimate shear-wave velocity (V_S) of the bedrock between 1030 and 2140 m/s, consistent with the nature of the rocks. The multi-method approach provides a robust framework for site-response characteristics, and the obtained results contribute to better seismic risk management in the region.