Development of Locally Calibrated Seismic Design Spectra for North-Eastern Nigeria Using Broadband Seismographic Data

The absence of locally calibrated seismic design spectra for North-Eastern Nigeria reflects both a historical gap in seismographic instrumentation coverage and an underestimation of seismic risk in a region that has experienced moderate-magnitude earthquakes associated with the Biu Plateau volcanic field and the Lake Chad Basin fault system. This study develops locally calibrated seismic design spectra for selected cities in the North East geopolitical zone including Maiduguri, Gombe, and Yola, using broadband seismographic records collected from a newly established 12-station network deployed across the region. Ground motion records from 47 earthquakes with moment magnitudes between 3.2 and 5.6 recorded over a 3-year monitoring period were processed to derive site-specific Fourier amplitude spectra and response spectra at 5 percent damping. Ground motion prediction equations were evaluated for their applicability to the North-Eastern Nigerian tectonic environment through residual analysis, and a locally adapted equation was derived by regression on the regional dataset. Site amplification functions were developed incorporating VS30 measurements from 38 site locations. Design spectra at 10 percent and 2 percent probability of exceedance in 50 years were constructed for each city using the locally calibrated ground motion prediction equation and probabilistic seismic hazard analysis. Results confirm that the current NNBC seismic zone maps underestimate spectral accelerations at short periods in Gombe and Maiduguri by 35 to 60 percent, with significant implications for the seismic design of healthcare and educational facilities. The study provides an original, data-driven foundation for seismic design code revision in North-Eastern Nigeria. Keywords: seismic design spectra, North-Eastern Nigeria, broadband seismography, ground motion prediction, probabilistic seismic hazard.