Detection and Characterization of Induced Seismicity Around the Onitsha Bridge Foundation Using Continuous Seismic Monitoring

Large-scale civil infrastructure, particularly bridge foundations on major river crossings, can generate induced seismicity through vibration loading and subsurface fluid pressure changes that may affect structural integrity over time. This study detects and characterizes induced seismic events around the Onitsha Bridge foundation using a continuous seismic monitoring array deployed under the auspices of the Nnamdi Azikiwe University Applied Geophysics research program. A temporary array of five short-period seismometers was deployed at distances of 50 to 500 m from the bridge foundation for a period of six months. Continuous data were archived at 100 samples per second and analyzed using an STA/LTA algorithm with post-detection manual review. Traffic vibration spectra, construction noise, and tectonic micro-earthquakes were classified by their spectral and temporal characteristics. A total of 312 triggered events were recorded, of which 228 are classified as traffic-induced vibration, 51 as high-frequency construction events, 22 as micro-earthquakes (Ml negative 0.3 to Ml 1.6), and 11 as unclassified. Micro-earthquake hypocenters cluster at depths of 3 to 18 m below the riverbed, spatially consistent with the base of the foundation piles. Waveform cross-correlation identifies two temporally coherent seismic sequences associated with construction piling phases. Spectral analysis of micro-earthquakes yields dominant frequencies of 8 to 30 Hz, consistent with shallow brittle failure in the foundation zone. These findings provide a template for geophysical monitoring of major infrastructure foundations in Nigeria.

Keywords: induced seismicity, seismic monitoring, Onitsha Bridge, micro-earthquake, STA/LTA