Application of Seismic Hazard Analysis to Urban Planning in Ibadan, Oyo State

This study examines the application of probabilistic seismic hazard analysis to urban planning decisions in Ibadan, Oyo State, South West Nigeria. Ibadan is Nigeria's second largest city and is experiencing rapid physical expansion into areas where subsurface geology and potential seismic site effects have not been characterised. Although Nigeria is generally a low seismicity region, increasing urbanisation and construction of vulnerable unreinforced masonry structures in areas with thick soft sediment cover creates localised seismic risk that merits attention in urban planning. This study compiles available seismicity records for Southwest Nigeria from the ISC and NEISM catalogues, conducts HVSR ambient noise measurements at 35 stations distributed across Ibadan metropolitan area, and reviews the existing Ibadan Structure Plan for seismic hazard provisions. Probabilistic seismic hazard analysis is performed using the OpenQuake engine with regional seismic source characterisation and ground motion prediction equations calibrated for West Africa. Site amplification factors from HVSR measurements are incorporated as site corrections to the median hazard curves. The produced peak ground acceleration map at the 475-year return period is overlaid with the Ibadan land use plan. Findings reveal that the current Ibadan Structure Plan contains no provision for seismic risk in building regulations or zone designation. Peak ground acceleration exceeds 0.07g in thick sediment zones in the Bodija and Agodi areas for the 475-year return period event. These areas currently allow unrestricted high-density residential development without geotechnical or seismic assessment requirements. The study recommends integration of the produced seismic hazard map into the Ibadan Structure Plan review currently underway.

Keywords: probabilistic seismic hazard, urban planning, Ibadan, site amplification, building regulation.