Seismic Noise Analysis for Microzonation of Ibadan Metropolis, Oyo State

This study analyses ambient seismic noise to produce a seismic microzonation map for parts of Ibadan metropolis, Oyo State, South West Nigeria. Although Nigeria is generally considered a low-to-moderate seismic hazard region, urban growth and the proliferation of high-rise buildings in Ibadan make seismic site effect characterisation important for modern building code compliance and risk-informed urban planning. This study measures horizontal-to-vertical spectral ratios using the Nakamura method at 48 stations distributed across the Ibadan metropolitan area, including zones underlain by fresh basement rock, weathered basement, and sedimentary overburden. Three-component seismometers are deployed for 30-minute ambient noise recordings at each station. HVSR curves are computed and the fundamental resonance frequency and corresponding amplification factor are extracted at each station. Spatial interpolation produces site amplification and resonance frequency maps. Findings reveal resonance frequencies ranging from 1.2 to 8.4 hertz across the study area, with the lowest frequencies corresponding to thick overburden deposits in the Bodija and Agodi zones, while bedrock outcrops in Ring Road and Dugbe areas show high resonance frequencies above 6 hertz. Amplification factors reach 6.2 in soft sediment zones, indicating potential for significant site amplification during regional seismic events. The resonance frequency map shows strong correlation with available geological and geotechnical data. The study recommends the produced microzonation map for integration into the Ibadan Structure Plan and urban building permit evaluation process.

Keywords: seismic microzonation, ambient noise, HVSR, Ibadan, site amplification.