Seismicity and seismic hazard assessment in West Africa

No in-depth seismic hazard study for West Africa (WA) has ever been conducted, as the regional earthquake catalogues are incomplete. Such lack of comprehensive seismic hazard study for the region has negatively affected the planning and development of critical infrastructure and disaster risk management. Therefore, this study aims to bridge the knowledge gap by applying modern techniques to updating the existing catalogues and assessing the seismic hazards for the region. We updated the current earthquake catalogue for WA using information from the International Seismological Centre, published information, and data from seismic stations in the region. The seismotectonic setting of WA is considered stable continental crust despite several recorded earthquakes occurring in the region, the largest being the June 22, 1939 earthquake of magnitude M6.8 with epicentre in Ghana. However, WA has also been classified as a region of shallow crustal seismicity. Therefore, we investigated both these research schools of thought and compared their results. For each scenario, three different ground-motion models (GMMs) were applied and combined to produce each hazard map using logic tree formalism with equal weights. The region was divided into five seismic source zones for computation of the earthquake recurrence parameters, with the same parameters computed for the entire WA region. The computed Gutenberg–Richter b-value, activity rates λ, and regional maximum possible magnitudes mmax for the five zones ranged from 0.84 to 1.0, 0.3–2.1, and 5.2–7.0, respectively. The calculated b-value, λ, and mmax for the entire region were 0.77, 4.1, and 7.2, respectively. The estimated b-value of 0.77 falls within the generally accepted range for tectonic seismicity. The seismic hazard predicted by GMMs for stable continental areas was higher than that predicted for shallow crustal seismicity in the investigated region. Therefore, our results confirmed that WA is characterised by stable continental crust. The highest hazard levels were observed in parts of Ghana, Togo, Guinea-Bissau, Guinea, and the Cameroon Volcanic Line region (CVL), ranging between 0.02 g and 0.03 g.