Microtremor Survey and Seismic Microzonation of Nsukka-Enugu Expressway Corridor for Road Infrastructure Planning

The Nsukka-Enugu expressway corridor traverses diverse geological units and is subjected to increasing vehicular loads, warranting a detailed assessment of site-specific seismic response characteristics. This study conducts a microtremor survey and seismic microzonation of the expressway corridor to characterize site amplification, estimate shear wave velocity profiles, and delineate zones of differential settlement or amplification risk. Microtremor measurements were carried out at forty stations spaced approximately 2 km apart along the corridor using three-component seismometers. The HVSR method was applied to determine resonance frequencies and amplification factors at each site. Shear wave velocity profiles were inverted using a Monte Carlo approach guided by available borehole data along the route. Fundamental resonance frequencies along the corridor range from 2.4 Hz to 9.6 Hz, reflecting variations in overburden thickness and sediment stiffness. Amplification factors are highest in segments underlain by colluvial deposits in the valleys between Ninth Mile Corner and Nike Lake, with values reaching 4.8. Average shear wave velocity to 30 m depth (Vs30) values range from 185 to 520 m/s, corresponding to site classes C and D under NEHRP classification. Differential settlement hazard zones are identified in three segments of the corridor, particularly where transitions between competent and soft ground conditions occur abruptly. The microzonation map generated from this study provides critical input for the design of road foundations and embankments along the corridor.

Keywords: microtremor, HVSR, seismic microzonation, Vs30, expressway corridor