An Original Theoretical Investigation of Shear Wave Velocity Site Response and Building Resonance Risk in Soft Sediment Areas of Lagos State

Site response amplification in soft sediment areas of Lagos State, South West Nigeria, can significantly increase ground motion intensity at frequencies corresponding to mid-rise building natural periods, yet no systematic investigation of site-specific response and building resonance risk has been conducted for Lagos's complex geological setting. This study provides an original theoretical investigation of shear wave velocity site response and building resonance risk in soft sediment areas of Lagos State. An original shallow geophysical survey campaign was conducted across 85 sites in Lagos Island, Victoria Island, Surulere, and Alimosho, using multichannel analysis of surface waves (MASW) to derive site-specific shear wave velocity profiles to 30 m depth (Vs30). Horizontal-to-vertical spectral ratio analysis from ambient noise measurements characterised fundamental resonance frequencies at each site. Site amplification was modelled using the Equivalent-Linear method in SHAKE2021. A building exposure survey characterised the dominant building typology and estimated natural periods at 300 monitoring points. Resonance risk was assessed by comparing site fundamental frequency to estimated building natural period distributions. Results indicate that 34 percent of Lagos Island survey sites have Vs30 values below 180 m/s (Site Class E), with fundamental resonance frequencies between 0.6 and 1.8 Hz. Building resonance coincidence probability is highest for 4 to 8 storey reinforced concrete frame buildings on Lagos Island soft deposits. The study provides an original shear wave velocity dataset and site response model for Lagos State, supporting improved seismic risk assessment.

Keywords: site response, shear wave velocity, building resonance, Lagos State, seismic microzonation