Seismic Ground Response Analysis for Urban Infrastructure Vulnerability Assessment in Abakaliki

Infrastructure vulnerability to seismic loading in Abakaliki is a growing concern given the increasing number of multi-story buildings and the poorly constrained local seismic ground response. This study performs a seismic ground response analysis for infrastructure vulnerability assessment in Abakaliki using a combination of microtremor HVSR measurements and geotechnical borehole data. Microtremor data were recorded at thirty sites distributed across the city using a Tromino three-component sensor. HVSR curves were computed for each site following the standard SESAME guidelines, and fundamental resonance frequencies and amplification factors were extracted. Borehole data from four reference sites were used to calibrate shear wave velocity profiles derived from HVSR inversion. Fundamental resonance frequencies across Abakaliki range from 0.8 Hz to 7.5 Hz, reflecting a strong geological control linked to the Abakaliki Anticlinorium and associated sediment thickness variations. Amplification factors between 2.3 and 6.1 are identified in low-lying areas underlain by alluvial fill, posing elevated risk to buildings with natural frequencies close to the site resonance. A vulnerability index map computed from the ratio of amplification factor squared to resonance frequency reveals high-vulnerability zones in central and eastern parts of the city. Building stock in these zones is assessed against the site response characteristics, and high-rise structures in the 3 to 8-story range are flagged for structural reassessment. The findings provide a practical framework for earthquake risk mitigation in Abakaliki and inform future seismic building code discussions for Ebonyi State.

Keywords: HVSR, seismic ground response, vulnerability index, Abakaliki, infrastructure assessment