Probabilistic Site Response Analysis: Methodology and Applicability-Paper

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The paper presents a fully probabilistic method for incorporating soil response into seismic hazard analysis. Conventional practice applies deterministic site-response results to a rock or stiff-soil uniform hazard spectrum (UHS), but this does not combine uncertainty in seismic hazard and soil response, leaving the surface spectrum’s actual return period uncertain.<br /><br />The proposed method first calculates rock/stiff-soil hazard curves and selects compatible ground-motion records. Site-specific soil profiles are then analyzed using nonlinear one-dimensional site-response models. The resulting amplification factors are fitted to a nonlinear amplification function, and uncertainty in amplification is estimated. These functions adjust the ground-motion prediction equations (GMPEs), after which the probabilistic seismic hazard analysis is repeated to produce surface hazard curves and UHS with defined exceedance rates.<br /><br />The method was demonstrated for a site on the Nile Delta continental shelf. Because geotechnical data were limited, best-estimate, lower-bound, and upper-bound soil profiles were developed. Mudline spectra for 475- and 2,475-year return periods were calculated using both deterministic and probabilistic analyses. The results were broadly comparable: probabilistic spectra were generally at or below the deterministic envelope at short and intermediate periods, although some cases produced slightly higher values at short periods or beyond the site period.<br /><br />The probabilistic approach provides a systematic way to include geotechnical uncertainty and supports risk-based assessments, including evaluations such as earthquake-induced liquefaction. It also allows spectra to be generated for different return periods without repeating the site-amplification study. Additional computational work is required, but automation can reduce the effort. The authors note that further research is needed on variability in site response, soil degradation curves, and the method’s performance in highly nonlinear, high-seismicity settings.

Keywords

probabilistic seismic hazard analysis

site response

soil amplification

ground-motion prediction equations

uniform hazard spectrum

nonlinear site-response model

geotechnical uncertainty

seismic hazard curves

Nile Delta

liquefaction