Seismic Hazard Assessment and Ground Motion Characterization in Enugu Metropolis Using Ambient Noise Analysis

Rapid urbanization and infrastructure expansion in Enugu metropolis have raised concerns about earthquake hazard in the region, particularly given the poorly constrained seismicity of southeastern Nigeria. This study assesses seismic hazard and characterizes ground motion in Enugu metropolis using passive seismic methods, specifically ambient noise analysis. The primary objectives are to estimate the fundamental resonance frequency of soil sites, determine site amplification factors, and classify the subsurface according to National Earthquake Hazards Reduction Program (NEHRP) standards. Ambient noise data were recorded at twenty-two measurement points across selected areas of Enugu using three-component broadband seismometers. The horizontal-to-vertical spectral ratio (HVSR) technique was applied to derive resonance frequencies at each site. Results indicate fundamental resonance frequencies ranging from 1.2 Hz to 6.8 Hz, with lower frequencies corresponding to areas underlain by thick unconsolidated sediments and higher frequencies associated with competent bedrock outcrops. Amplification factors between 2.1 and 5.4 were recorded across the study area, implying differential site responses that carry significant engineering implications. Ground shear wave velocity profiles derived from the HVSR inversion suggest that portions of the Enugu urban core overlie soft sediment sequences susceptible to amplification during seismic events. These findings underscore the importance of microzonation studies in guiding urban planning and construction standards in Enugu State. The results provide a foundation for seismic risk mapping in the region and contribute to the broader geohazard assessment framework for the Anambra Basin.

Keywords: seismic hazard, ambient noise, HVSR, site amplification, Enugu