Tectonic Activities of the Mid-Atlantic Ridge and Implication of Seismicity in West African Region

Abstract

The aim of this research is to contribute to insights on how the tectonic activities in the MAR may have been influencing observed seismic activities in the region of West Africa. The seismic activities in parts of Mid-Atlantic Ridge (MAR) have been investigated with respect to seismicity in the West African region, using Statistical and Probabilistic Seismic Hazard Assessment (PSHA) techniques. Earthquake data were extracted from different catalogues in West Africa and international agencies for the study. The catalogues were declustered to remove foreshocks, aftershocks and repeated earthquakes. The different magnitude scales were also harmonized to moment magnitude (Mw). The statistical technique involved Regression Analysis, Focal Depth and Frequency Distribution analyses of observed events to establish distinct trend, using a decade-long of earthquake sub-catalogues at different completion levels. The results, especially from the statistical technique, clearly showed a trend of an increasing seismicity along the MAR in the West African region, with respect to number of earthquakes occurrence and magnitudes from 1963 to 2014. This could be as a result of progressively increasing seismic activities in response to tectonic stresses along the ridge. The results from PSHA, the b-values, activity rates and maximum magnitude for the investigated region are 0.82, 168 and 7.1 respectively. The b-value falls well within the acceptable tectonic seismicity of 0.72 to 0.82. The results also show that magnitudes 6.00 and 7.0 earthquakes are likely to occur approximately every 2 and 10 years along MAR; the probability of these events occurring annually is high at 98% and 92%; and the probability of their occurrence once in 50, 100 and 1000 years is 100%, 100%, 100% (for surface waves magnitude, Ms 6.0) and 98%, 99% and 100% (for Ms 7.0) respectively. The increase in seismic activities along the ridge may be responsible for observed increase of seismic activities especially in the West African countries along the coast of the Atlantic Ocean. This calls for further research in future. Although, West Africa is not an active region compared to regions of high seismicity like East African Rift, Japan, etc., a trend that showed an increasing earthquake occurrence and appearance of larger magnitudes has been revealed in this study.