Probabilistic seismic hazard maps and uniform hazard spectra with site effect integration for northern of Algiers

This study presents probabilistic surface seismic hazard maps for Algiers city in terms of PGA and spectral acceleration for a return period of 475 years (corresponding to 10% in 50 years). The site effect is integrated through a fully probabilistic approach. Seismic hazard analysis for reference rock sites (Vs30 = 800 m/s) is conducted first based on an updated seismic source model, including area sources and active faults, and three selected ground motion prediction equations, Epistemic uncertainties related to source model (slip rate for faults, Gutenberg Richter's parameters for area source) and ground motion equation selection are considered through logic tree approach. Then, 1D ground response analysis is performed to estimate soil amplification factors and their standard deviations considering uncertainties related to input motion variability. Finally, probabilistic site specific-PSHA using modified_GMPE with respect to amplification factors, is conducted for more than 4000 soil profiles. Surface seismic hazard maps in terms of PGA, and PSA(T) are shown for the study area, they highlight an important site to site variability within the same soil class (S2 and S3) according to the Algerian seismic design code (RPA). In addition, surface uniform hazard spectra (surface_UHS) for several selected sites belonging to different geotechnical units are compared with the current design spectra prescribed in seismic design codes (RPA, EC8). These findings emphasize the value of conducting a site-specific PSHA study, given the considerable discrepancy that can exist between a site-specific spectrum and a seismic design spectrum. While design spectra tend to be more conservative for longer periods (>0.3 s), the specific spectrum yields greater accelerations for shorter periods, particularly for soils of class S3.