Probabilistic Seismic Hazard and Risk Assessments for Critical Facilities in Nigeria and Neighbouring Regions

Abstract

The frequency of occurrence of earthquakes in Nigeria, and its neighbouring regions of Cameroon, and the Gulf of Guinea is on the rise. This work aims to assess the potential seismic hazards in these regions and compute earthquake recurrence parameters (b-value, activity rates, and the expected earthquake maximum magnitudes), as well as assess possible impacts on critical facilities, especially in Nigeria. The earthquake data (which spanned 1901-2024) were acquired from the Nigerian National Network of Seismographic Stations (NNNSS), the Cameroonian seismic network, and the International Seismological Commission (ISC). The methodology involves data sorted using Seisan Software and declustering to remove unwanted events. The magnitude scales from the different data sources were harmonized into the moment magnitude (Mw). The Probabilistic Seismic Hazard Assessment (PSHA) in a Bayesian approach was used to compute the seismic hazard parameters, and the risk assessment was done at specific sites of the facilities by quantifying the probability of ground motion exceeding a certain threshold, using an empirical technique. Results show that the b-value, activity rates, and the expected maximum earthquake magnitudes (Mmax) for minimum magnitude (Mmin) of 3.0, for Nigeria, Cameroon and the Gulf of Guinea were 0.55 +- 0.50, 0.73 +- 0.13, 0.82 ± 0.67; 0.436 +- 0.129, 2.848 +- 0.234, 6.823 ± 1.350; 6.09 +- 0.31, 6.26 +- 0.30, 7.10 ± 0.27 respectively. The PGA values obtained in this study range from 0.08g to 0.24g. It was established that some critical facilities, such as major dams and bridges, etc., were vulnerable to earthquake hazards. The earthquake occurrence prediction models developed for the three regions confirmed that earthquakes with low magnitudes (3.0-3.5) have a high probability of occurrence within a short time interval. While the probability of annual occurrence of large-magnitude earthquakes in the regions is low, the probability of the regions experiencing large-magnitude events in the future (100, 500, and 1000 years) is high. It is therefore recommended that the routine incorporation of site response studies be enforced before the siting of critical infrastructure in Nigeria and its neighbours. The results have a direct positive impact on infrastructure planning and development, and seismic risk mitigation.