A Probabilistic Seismic Microzonation Framework for Rapidly Urbanising Nigerian Cities: Methodology Development and Application in Abuja and Port Harcourt

Nigerian cities are urbanising at rates that outpace the development of seismic hazard regulatory frameworks, and the absence of validated probabilistic seismic microzonation methodologies tailored to the geological diversity and geotechnical data constraints typical of Nigerian urban settings limits the application of site-specific seismic design provisions. This research develops and validates a probabilistic seismic microzonation framework for rapidly urbanising Nigerian cities, using Abuja in the North Central zone and Port Harcourt in the South South zone as contrasting test cases representing crystalline basement and soft sediment geological settings respectively. The framework combines probabilistic seismic hazard analysis at the reference rock level with three-dimensional site amplification models derived from a geostatistical integration of Vs30 measurements from HVSR, Multichannel Analysis of Surface Waves surveys, and geotechnical borehole data. Non-linear site response simulation using the DEEPSOIL programme propagates reference rock ground motions through the site-specific soil column models for a representative ensemble of input motions. Spatial interpolation of non-linear site amplification factors using machine learning-assisted kriging produces continuous maps of spectral acceleration at structural design periods. Uncertainty in the final microzonation maps is quantified by propagating measurement and model uncertainties through all stages of the framework. Application in Abuja shows that non-linear effects reduce amplification by 20 to 35 percent relative to linear site response estimates for frequent low-amplitude ground motions in the alluvial sites. The framework provides a replicable template for microzonation of any Nigerian city with minimum 40 Vs30 measurements. Keywords: seismic microzonation, probabilistic, Abuja, Port Harcourt, site amplification