# Seismotectonic setting

The relationship between earthquake epicentral locations and active tectonic processes is known as seismotectonics. For a given region, it is controlled by the relative motions of the mosaic of crustal and oceanic plates that comprise the Earth’s lithosphere. The crustal plates can be further subdivided into the older, more stable cratons and younger, less stable mobile belts. The relative motions of the plates are the driving force behind the regional stress fields that cause tectonic earthquakes.

The main objective of this section is to outline the tectonic framework of the study area to gain an understanding of which fault systems may be seismically active.

## Tectonic framework

The West African region lies predominantly on Precambrian crust, which has been folded and fractured over hundreds of millions of years.1 These Precambrian rocks and their eroded surfaces provided a fairly levelled floor for the advance and retreat of shallow Palaeozoic seas forming the sedimentary rocks that overlay the ancient Precambrian floor.2 Most of West Africa’s mountain chains and plateaus originated as Precambrian folds.3 Following this period, volcanic activity in many of these plateaus deposited additional layers of igneous rock. Volcanic outpourings have occurred throughout West Africa’s geologic history, with major activity as recent as the Pliocene, and even the present. In the late Quaternary, intensive weathering of sandstone formations produced much of the present-day sand sheets that cover vast areas to the north.

The Leo uplift (Guinea Rise, Figure 1) contains an intensely eroded granitised root, that forms the West African craton.4 The western part of the craton includes elements of a Hercynian fold belt in thrust contact with an unfolded Paleozoic cover and a broad fold belt of upper Precambrian sediments is found to the east.5 There is a correlation between earthquake-epicentral locations and the Akwapin fault zone and the coastal boundary fault a few kilometers offshore as shown in Figure 2.6 It was shown that large-scale tectonics led to the superposition of the Pan-African and Hercynian provinces and the West African craton.7 The available earthquake source zone data, although limited and compounded by inadequate knowledge on the contributing tectonic forces, indicates that the remains of plate boundaries in the area are the likely earthquake source zones.8 Despite efforts to improve seismic monitoring coverage over the past few years, the network of seismographic stations in Western Africa can be described as sparse and inadequate for robust monitoring of regional seismic activities.9

## Neotectonics

In the West African region, major (with Modified Mercalli (MM) intensity of VI and above) and minor earthquakes have been observed over the last century.11 The larger magnitude events are connected to fractures and rifts as shown in Figure 2.12

The Pelusium megashear system extends from the eastern Mediterranean and across Africa to the Gulf of Guinea and continues along the Atlantic equatorial fracture zones, into the Amazon Basin.14 The Pelusium megashear has functioned as a system of en echelon left-lateral megashears since Precambrian times. At least four other geosutures paralleling the Pelusium across Africa also continue into the Atlantic as fracture zones.15 The compressional troughs that follow the shear zone are a result of an oblique collision between a northwestern African plate and a central plate consisting of southern and eastern Africa, the Arabian Peninsula, and the Levant.

A first-order left-lateral megashear system is controlling the active tectonics through 50$$^\circ$$-80$$^\circ$$ dipping second-order ductile-brittle shear zones in the areas between the megashears.16 The northwest striking faults are strike-slip faults attributed to transcurrent tectonics (Figure 2). The area requires regional structural analyses that classifies the fault populations into strike-slip, normal, thrust, Riedel and conjugate Riedel faults etc. and tests whether these features could fall into a transpressive or transtensional regime.

Transpression and transtension17 occur on all scales, from the microscopic to regional, as a consequence of deformation of the Earth’s lithosphere. On the regional scale, these are an inevitable consequence of relative plate motion on a spherical surface. The spreading centres at the mid-oceanic ridges cause the plates to move apart, resulting in strike-slip faulting on a regional scale that accommodates the strain within the plate. Within the plate, the strain will be focussed into zones of displacement (e.g. the megashears) that bound units of less deformed material. This results in fault and shear zone bounded blocks within which populations of smaller scale structures arise in response to the far-field plate tectonic stresses and large-scale body forces.18

The first documented earthquake in the Western African region occurred on June 30, 1615, with a moment magnitude of 5.9.19 The most prominent earthquake event, due to its devastating impact and surface-wave magnitude of 6.5, occurred on June 22, 1939, with an epicenter in Ghana.20 On December 22, 1983, a $$M_w$$ 6.3 earthquake occurred in northwestern Guinea, near the border with neighboring Guinea-Bissau. The communities of Gaoual and Koumbia experienced moderate damage and villages extending in a line between 5 and 15 km north of Koumbia were damaged extensively. Unconfirmed casualties include at least 275 people and 1000 people were injured. The shock was felt in Guinea-Bissau, Senegal, Gambia, Sierra Leone, and Liberia. Main-shock focal mechanism solutions derived from teleseismic data show a strong component of normal faulting motion that was not observed in the ground ruptures. Surface faulting occurred on a preexisting fault whose field characteristics suggest a low-slip rate with very infrequent earthquakes. Evidence of right-lateral displacements, such as left-stepping en echelon fractures, pressure ridges, and small thrusts could be seen at several places in the central reach of the fault, The largest vertical displacement observed was about 5-7 cm with a surface rupture length of 9.4 km.21

Smaller occurrences of earthquakes exist such as the $$M_w$$ 4.3 event on July 28, 1984, in Nigeria and the $$M_w$$ 4.4 event on September 11, 2009, at the border town between the Benin Republic and Nigeria. Recently smaller earthquakes of 3.0 and 3.1 magnitudes have occurred on 11 and 12 September 2016, with epicenters at Kwoi in Kaduna State, Nigeria.

There is a lack of consensus in the seismotectonic classification of the region, as a result of limited seismic hazard studies in the Western African region. Some researchers consider the Western African region as a stable continental region while others believe that that the region has active faulting or shallow crustal seismicity.22 Nonetheless it is important to note that stable continental regions can have strong earthquakes that could cause extensive damage.23

## Seismic scenarios

From the identified neotectonics features, the impact of the megashear fault MS (Figure 3) in the seismic hazard is analysed as an earthquake scenario. The maximum movement is expected to be constrained along it since the megashears are the connection with the mid oceanic ridges where the stresses are being generated. The other faults in between (A and B) would move in sympathetic response.

The maximum recorded event in the area was the $$M_w=6.3$$ magnitude earthquake of 22 December 1983, with epicentre 630 km from the mine.24 Adopting a conservative increment of 0.5 for the maximum magnitude for this area source in the probabilistic hazard input,25 an event with $$M_w=$$ 6.8 has been adopted for this scenario

The rupture plane was obtained using empirical relationships for a strike-slip fault26 and the rupture length and width considering an aspect ratio of L/W=1, which is increased if the top of rupture depth ($$Z_{TOR}$$) reaches the surface. From this rupture plane, the closest source-to-site epicentral distance for this controlling scenario resulted in $$R\approx$$ 38 $$\,\mathrm{km}$$

1. F. B. Fatoye and Y. B. Gideon, “Geology and Mineral Resources of the Lower Benue Trough, Nigeria.” Advances in Applied Science Research, 2013, 21–28.↩︎

2. P. Michel, “The Basins of the Senegal and Gambia Rivers: A Geomorphological Study.” 1973.↩︎

3. R. J. Church, “West Africa: A Study of the Environment and of Man’s Use of It” (Longman’s, Green; Co., Ltd., 1966).↩︎

4. S. Freeth et al., Natural Hazards in West and Central Africa. (Berlin, Heidelberg: Springer, 1992).↩︎

5. A. U. Kadiri and A. Kijko, “Seismicity and Seismic Hazard Assessment in West Africa,” Journal of African Earth Sciences, 2021, 104305.↩︎

6. K. Burke, “Seismic Areas of the Guinea Coast Where Atlantic Fracture Zones Reach Africa,” Nature 222 (5194), 1969, 655–57.↩︎

7. C. Dorbath et al., “Seismotectonics of the Guinea Earthquake of 22 December 1983.” Geophysical Research Letters, 1984, 971–74.↩︎

8. Kadiri and Kijko, “Seismicity and Seismic Hazard Assessment in West Africa.”↩︎

10. J. J. Peucat et al., “The Eglab Massif in the West African Craton (Algeria), an Original Segment of the Eburnean Orogenic Belt: Petrology, Geochemistry and Geochronology.” Precambrian Research, 2005, 309–52.↩︎

11. N. N. Ambraseys and R. D. Adams, “Seismicity of West Africa,” Annales of Geophysicae, 1986; K. M. Onuoha and T. S. D. Reidel, Natural and Man-Made Hazards (Dordrecht, 1988); K. U. Afegbua et al., “Towards an Integrated Seismic Hazard Monitoring in Nigeria Using Geodetic and Geophysical Techniques.” International Journal of Physical Sciences, 2011, 6385–93.↩︎

12. D. Neev, J. K. Hall, and J. M. Saul, “The Pelusium Megashear System Across African and Associated Lineament Swarms,” Journal of Geophysical Research, 1982, 1015–30; Onuoha and Reidel, Natural and Man-Made Hazards.↩︎

13. Kadiri and Kijko, “Seismicity and Seismic Hazard Assessment in West Africa.”↩︎

14. Neev, Hall, and Saul, “The Pelusium Megashear System Across African and Associated Lineament Swarms.”↩︎

15. Neev, Hall, and Saul.↩︎

16. M. E. Allialy, F. J. L. H. Kouadio, and A. Gnanzou, “Structural Control of Auriferous Mineralization in the Birimian: Case of the Agbahou Deposit in the Region of Divo, Côte d’ivoire.” International Journal of Geosciences, 2017, 189–204.↩︎

17. W. B. Harland, “Tectonic Transpression in Caledonian Spitzbergen.” Geological Magazine, 108, 1971, 27–42.↩︎

18. J. F. Dewey et al., Collision Tectonics (Geological Society, London, Special Publications, 1986).↩︎

19. Ambraseys and Adams, “Seismicity of West Africa.”↩︎

20. N. R. Junner, “The Accra Earthquake of June 1939, Gold Coast,” Geological Survey, Bulletin, 13, 1941, 3–41.↩︎

21. C. J. Langer, M. G. Bonilla, and G. A. Bollinger, “The Guinea, West Africa, Earthquake of December 22, 1983; Reconnaissance Geologic and Seismologic Field Studies” (US Geological Survey, 1985).↩︎

22. Kadiri and Kijko, “Seismicity and Seismic Hazard Assessment in West Africa.”↩︎

23. M. Greene, C. Godavitarne, and F. Krimgold, “Overview of the Maharashtra, India Emergency Earthquake Rehabilitation Program.” (12th World Conference on Earthquake Engineering., 2000).↩︎

24. “OpenQuake-Engine PSHA Input Model for the Western Africa Region,” 2018, https://hazard.openquake.org/gem/models/WAF/.↩︎

25. Donald L Wells and Kevin J Coppersmith, “New Empirical Relationships Among Magnitude, Rupture Length, Rupture Width, Rupture Area, and Surface Displacement,” Bulletin of the Seismological Society of America 84, no. 4 (1994): 974–1002.↩︎