Seismic and Tectonic Correspondence of Major Earthquake Regions in Southern Ghana with Mid-Atlantic Transform-Fracture Zones ()
1. Introduction
Earthquakes are natural crustal stabilization phenomena caused by lithospheric faulting and adjustments that help maintain the crustal stability and spheroidal structure of the Earth. Southern Ghana in West Africa is far away from any major present-day tectonic inter-plate boundaries, active tectonoseismic or volcanogenic regions of the world, yet for centuries the region is seismically active. A number of significant earthquakes have struck southern Ghana and West Africa in the past, some with destructive effects [1-7]. Seismic activities, including major and minor ones, have been going on quite regularly over the centuries to the present [7]. Previous recorded damaging and destructive earthquakes had been reported in places wide apart in southern Ghana, notably from the southwest in the districts of Axim and Elmina to the southeast in the Accra through to the Ho districts [4,6,7].
Recent seismicity in southern Ghana has heightened the need to find explanation for the causes of the earthquake activities in Ghana. Until now, there is no clear geological explanation for the previous and current ongoing seismicity of the Accra region and southern Ghana. Most of the discussions in the literature of the seismicity of Ghana have focused predominantly on earthquakes of the Accra-Ho seismic region at southeastern Ghana, and seem to generalize that for the whole country [e.g. 6-8], inherently suggesting a common cause for the seismicity of the entire southern Ghana. This generalization oversimplifies the nature and understanding of the phenomenon in Ghana.
The objectives of this paper are to establish and explain the nature and causes of the earthquake phenomenon and seismicity of southern Ghana, and then differentiate and correlate seismic activities in southern Ghana to causative tectonic fault systems.
2. Methods
The study gathered and evaluated new tectonic and geological information from the field, and reassessed historical record of seismic activities and data in southern Ghana. Re-interpretation based on new tectonic and structural data and information on the local and regional geology of the region was done, using also the re-interpreted data of the well documented 1939 Accra Earthquake. Field work results on the geology and tectonics were used with existing 1939 Earthquake parameters in constraining baseline characteristics and causes of the seismicity of the region. My hypothesis is that the epicentres of earthquakes in Ghana are located both onshore along the coastal regions of Ghana and her neighbouring countries and offshore in the Gulf of Guinea, hence, the causative forces and faults must be acting from that whole region. Major faults in southern Ghana were traced offshore and vice versa onshore. Historical earthquake events were correlated with the transform and fracture zones to deduce the zones of seismic activity and the tectonic relations.
3. Geological Setting
Figure 1 shows the study area of southern Ghana as part of continental West Africa in the equatorial Atlantic setting of Mid-Atlantic ridge and transform-fracture systems. Geologically, Ghana consists of the Proterozoic Birimian province of the West African Craton, at its west part, and the Dahomeyide Orogen at the southeast of mainland Ghana, both partially covered by the Neoproterozoic Voltaian cover sequence [1,9-12]. These two terranes are separated by a major NE-SW tectonic boundary, along which the Dahomeyide Orogen with its metasedimentary Akwapimian cover sequence is thrust westward onto the eastern margin of the West African Craton. The boundary zone is characterized by three boundaryparallel tectonic units, namely, the eastern margin of the crystalline Birimian crust as the western block, the narrow middle meta-sedimentary Akwapimian block, and the external crystalline granulite Dahomeyide terrane block to the east, all separated by clear lithotectonic boundaries [1,9,10,13,14].
Offshore in the Atlantic Gulf of Guinea, three main WSW-ENE trending structures are recognized, namely, St. Paul’s, Romanche and Chain transform faults and fracture zones (Figure 1; [15]), which are stable tectonic flow lines [16].
A transform fault is an active tectonic intra-plate boundary where a fault or set of parallel strike-slip faults has cut across a mid-ocean ridge and offset the ridge during seafloor spreading, in which tectonic plate fault-bounda-
Figure 1. Showing Google Earth satellite image of the Study Area in southern Ghana (West Africa), the mid-Atlantic Ridge and its transform faults and fracture systems, including St. Paul’s (P), Romanche (R) and Chain (C) transform faults and fracture zones (Modified after Source: [15]).
ries slide past each other in opposite direction and uniquely define the direction and sense of motion between offset ridges of two bounding plates [17]. According to Condie (1997) [17], there are three types of transform faults: the ridge-ridge, ridge-trench, and trench-trench types. The ridge-ridge type is most common globally, and such types tend to maintain their length constant over time. The other two types decrease or increase their length as they evolve. The St. Paul’s, Romanche and Chain transform faults are the ridge-ridge type. Transform faults may evolve and develop extensions beyond the zone of the offset ridges, where they produce large structural discontinuities in the seafloor crust and topographic breaks and highs known as “fracture zones” [17]. The fracture zones mark locations of progressed extensions of transform faults, where plate boundaries move in the same direction. Fracture zones occur across and often extend away from transform faults or mid-ocean ridges usually towards continents (Figure 1) or tectonic trenches. A fracture zone, therefore, is an elongate zone of prominent irregular topographic faults, breaks or mountain range that cuts across the ocean floor and major features thereon, and separates regions of different crustal depths developed during the construction and migration of the crust in seafloor spreading. A typical fracture zone is about 60 kilometres wide and several hundred kilometres long, and is characterized by high-angle strike-slip faults [18]. Series of such ridge-ridge type transform faults and fracture zones characterize the Atlantic seafloor of West Africa and Ghana, where they form straight to arcuate lineaments from the Mid-Atlantic Ridge to the African continent (Figure 1). The Ghana part of this region is influenced in its evolution by the St. Paul’s, Romanche and Chain transforms and fracture zones (Figure 1). The Romanche Transform and Fracture Zone and two boundary fault-splays are known to reach onshore Ghana [11, 19]. The question is: “What is the cause and nature of the seismicity of southern Ghana?”
4. Results
The results of the study, presented in Tables 1 and 2 and Figures 2-6, show that the ridge-ridge type transform faults and fracture zones of the Atlantic seafloor of West Africa reach Ghana and play a major role in earthquake activity in Ghana and West Africa. Table 1 outlines the major earthquakes so far experienced in Ghana, their locations, maximum intensities and magnitudes. In 1636 a strong magnitude 5.7 earthquake hit Axim. But before then in 1615, Elmina had experienced the first documented earthquake in Ghana [4]. In 1862 and1872 major earthquakes hit the same Accra region. In 1906 a magnitude 6.2 earthquake struck Ho and Accra. Another major earthquake struck Accra on 22nd June 1939, some 74 years ago, and became known as the 1939 Accra Earthquake [1]. The Accra events of 1862 and 1939 were the severest reported, and had been assigned maximum seismic intensity IX and magnitude 6.5 [1,3,6]. Table 2 shows key historical earthquake data for only western Ghana and Cote D’Ivoire separately, as basis for comparison with the greater majority at southeastern Ghana.
Over these last 400 years of seismicity, the greater majority of epicentral earthquakes have occurred in southeastern Ghana, none in central mid-belt and northern Ghana, and only a few earthquakes have struck the southwestern part of Ghana and southern Cote D’Ivoire (Tables 1 and 2). And for almost two and half centuries now, since the last major event in Cote D’Ivoire in 1879, the southwestern part of Ghana has been seismically quiet, while the southeast region is active to the present day.
The results in Figure 2, reconstructed from Figure 1, show the present tectonic and structural configuration of the Atlantic crust, Mid-Atlantic Ridge, and the equatorial transform and fractures zones, obtained from tracing of southern Ghana faults into offshore fracture zones and vice versa.
The Romanche transform fault and its zone constitute the most prominent of these tectonic features. It is a typical long ridge-ridge transform offset with a longer fracture zone. The present-day transform offset is roughly 888 kilometres long and the adjoining fracture zone is 1890 kilometres (Figure 2). The main eastern end of the Romanche fracture zone fault reaches onshore near GoiSege some 100 km east of Accra. Adjacent to the Romanche system is the St. Paul’s system to the north, and to the south is the Chain system (Figure 2; [21]). Figure 3 shows the map of the trends and patterns of the St. Paul’s and Romanche faults systems, their splays, and cross-cutting relations in southern Ghana. Figure 4 shows field evidence of faults traced from sea into the Accra Boundary fault on land, while Figure 5 presents ground evidence of the cross-fault at Kokrobite (Accra Coastal Boundary Fault-ACBF and Akwapimian Fault-AKF), located west of Accra. Two generations of faults, characterized by isoclinal shear fractures over 100 metres wide, cross cut at Kokrobite, the earlier Akwapimian Fault (AKF) having fault plane attitudes ranging from 280˚/ 90˚ to 326˚/76˚E, and the later generation Accra Coastal Boundary Fault (ACBF) having fault plane attitudes ranging from 060˚/80˚E to 090˚/85˚E (Figures 4 and 5).
The St. Paul’s fracture zone is traced onshore into the Ivory Coast Fault which continues into southwest Ghana and passes north of Axim (Figure 3). In southwest Ghana, the fault has been identified in gravity surveys [10]. This major fault has carved out a large fault-crafted basin south of it just along the Atlantic Gulf of Guinea coast, where the larger portion occurs in Cote d’Ivoire as the Abidjan Basin (Figure 3), and the tapered east end forms the southwest corner of Ghana, referred to as the Apollonian Basin. This constitutes the onshore Tano Basin, with its ~3500-metre thick Upper Cretaceous (100 - 65 Ma) marine sedimentary sequence, including the fossiliferous Nauli limestone, which sequence is confirmed by drilling [10,22]. The larger basin, which deepens westward into Cote d’Ivoire, was sunken in its formation, and later upthrown; this may explain the onshore formation of the limestone deposit (located some 50 kilometres west of Axim) and the general stratigraphic sequence. Structurally, the shelly limestone formation and the host strata dip generally 2˚ - 5˚ south to southwest [10]. This basin and the Axim-Elmina region have experienced earthquakes in the past (Table 2).
Table 1. Key Historical Earthquake Data and Time Intervals of Southern Ghana.
Table 2. Key Historical Earthquake Data of western Ghana and Cote D’Ivoire.
Figure 2. Map showing Western Africa, the mid-Atlantic Ridge and St. Paul’s, Romanche and Chain transform faults and fracture zones (Sources: Redrawn after Tout sur Google Earth, 2008, Figure 1 [15]; and Flashearth, 2009 [20]).
Figure 3. Map of West Africa showing the regional tectonic framework of the St. Paul’s, Romanche and Chain transform faults and fracture zones.
Figure 6 is map of southern Ghana showing the composite plot of epicentres of the historical earthquakes in Table 1 in relation to the causative faults linked to Atlantic fracture zones of the St. Paul’s and Romanche, respectively, which define two seismic regions, A and B, in Ghana, together with the seismic hazard zones of southern Ghana based on the matching of historical seismic intensities and magnitudes in relation to fracture zone fault.