Need for the Determination of the Nigerian Fault Lines

Earthquakes and Tremors are Tectonic activities traced to Crustal Motions with roots in the Continental Drift Theory. These activities occur mostly at Plate Boundaries and along or close to Fault Lines. Nigeria was erroneously considered aseismic in the past, but studies have shown that Nigeria was never aseismic as information on different Tremors and Earthquakes abound. The recent occurrences of Tremors in Abuja, the Nation’s Capital, call for a serious attention towards the definition of the Nigerian Fault Lines. An attempt towards the determination of the Fault Lines is made, using the Non-Geodetic and Geodetic Methods. The Non-Geodetic Method Seeks to connect the Epicenters of different Earthquakes and Tremors on record, in order to present a possible set of Fault Lines. Three Fault Lines, The Western Nigeria Fault Line, the Mid Nigeria Fault Line and the Eastern Nigeria Fault Lines were proposed. The Geodetic Method adopted the use of archived GNSS data from the Nigerian CORS stations between 2012 and 2014. Fourteen sites, ABUZ, BKFP, GCCT, CLBR, FPNO, FUTA, FUTY, GEMB, HUKP, MDGR, OSGF, RUST, ULAG and UNEC, were used for this experiment, and ten IGS stations, DEAR, ABPO, RAMO, DARK, MELI, ASCG, MOIU, ZAMB, NOT1, BHR4, were used for Network Stabilization in the experiment conducted with the GAMIT/GLOBK Software. The Mean Horizontal Velocity of Nigeria for the period was 22.5625±0.32583mm/yr East, 18.93±0.23417mm/yr North, found to agree with Bawa, Ojigi and Dodo (2018) and Altamimi et al (2011). The Analysis of the Position Solution, showed that stations along a particular line, share similar direction of motion, which agrees with the Non-Geodetic Proposal. It was recommended that in order to adequately and accurately determine the Fault System in Nigeria, about three times the existing number of CORS Stations must be established, and their data made readily available to researchers. a map of Nigeria with the locations of previous earthquakes and tremors used to identify the possible fault lines. From the spread and cluster of the previous tremors, three possible fault lines are proposed.


Introduction
Earthquakes and tremors are some of the most frequent natural disasters that threaten the lives and property of man in the earth today. The cause of these disasters have their roots explained in the continental drift theory (Ingolfsson, 2008) Researches have been found to agree that they are known to have their epicenters along plate boundaries and fault lines (Akkar et al, 2010, Reilinger and McClusky 2011, Cetin et al, 2014.The human and economic loss caused by these activities is enormous. According to Hackl, Malservisi and Wdowinski(2009), the 1994, Mw=6.7 Northridge and the 1995, Mw=6.8 Kobe earthquakes caused unprecedented damage of more than US $40 billion and US $100 billion, respectively. Also Akkar et al (2010) recorded that forty-two people lost their lives and 137 were injured during the MW = 6.1 earthquake that occurred in the Elazığ region of Eastern Turkey on 8 March 2010 at 02:32:34 UTC. Furthermore, Nwankwoala and Orji (2018) indicated the extent of damages felt in the events of different occurrences in Nigeria.
Nigeria lies on the eastern flank of the Atlantic Ocean, and according to Akpan and Yakubu(2010), the Atlantic Ocean margins have been opening consistently since Jurassic times. Unlike the Pacific Ocean margins which are characterized by subduction tectonics and occurrence of devastating earthquakes (convergent boundary), the Atlantic margins are generally thought to be quiet (divergent boundary) and as a result, there was little consciousness and preparedness for earthquake occurrences and mitigation in Nigeria. Against the backdrop of being perceived as aseismic, there have been several earthquakes and tremors in Nigeria (Akpan and Yakubu, 2010, Tsalha et al, 2015, Afegbua in Orakpo, 2017, Nwankwoala and Orji, 2018. Table 1 illustrates the reported cases of earthquakes and tremors in Nigeria, locations they were felt, the magnitude or intensity picked/felt and their probable epicenters.
With the attendant loss of lives and property involved in the events of these natural disasters, it is highly important and should be paramount in the priority list of any disaster mitigation and control agency that the fault lines be defined, monitored and be considered in the planning and siting of major developmental projects. The definition of these fault lines, therefore, cannot be over emphasized. It is only the first step towards the reduction of the high risks of loss of lives and properties on the event of these disasters.

Materials and Methods.
In order to define the tectonic fault lines, one can adopt two major approaches, the non-geodetic approach, and the geodetic approach.

Non-geodetic Approach
The non-geodetic approach involves the connection of the epicenters of previous earthquakes and tremors. Akpan and Yakubu (2010) adopted this method in their presentation of the Ifewara-Zungeru fault (figure 1). Eze et al (2011) further adopted this method in proposing possible fault lines in figure 2. To define the fault lines using this approach, the epicenters recorded in Table 1 were plotted on a map of Nigeria using ArcGIS 10.2 and lines of possible faults drawn.
-This method is not particularly accurate because it cannot be used for effective planning of projects, since it is only hypothetical. A fault line that is not hypothetical should possess the ability to be geographically located and possibly marked off. This would make it possible to factor it in land use planning.  Figure 3 shows a map of Nigeria with the locations of previous earthquakes and tremors used to identify the possible fault lines. From the spread and cluster of the previous tremors, three possible fault lines are proposed. i.

Result and Analysis
The Western Nigeria Fault Line: Thislineruns from the Atlantic Ocean, through Lagos, Ogun, Oyo, Kwara, Niger, Kaduna, Kano, and Jigawa states. It is akin to the Ifewara-Zungeru fault proposed by Akpan and Yakubu (2010). However, figure 3 and the records in Table 1 show that neither earthquake nor tremor was recorded in Osun State where Ifewara is located. It is in fact improper to insinuate that the fault line runs from Ijebu Ode through Ifewara as posited by Akpan and Yakubu (2010) when there is no epicenter in Osun rather close to Ibadan in Oyo State (1905 Tremor) and what was felt in Ile-Ife in 1936 the Ghanaian 6.5ML Earthquake with its epicenter in Akwapim Fault in Ghana. ii.
The Mid Nigeria Fault Line: This line also runs from the Atlantic Ocean, through Ondo (close to Okitipupa), Edo, Kogi, Abuja, Kaduna (Southern Kaduna), Bauchi, and Yobe states. This fault line is feared to be active, because the most recent tremors, as seen in Table 1, lie along this line. This is perceived to be the most active fault, owing to the recent Abuja Tremors. iii.
The Eastern Nigeria Fault Line:This fault line runs along the Abakaliki and Benue Troughs regarded by Nwankwoala and Orji (2018) as an example of a failed rift arm following the opening of the Southern Atlantic. They opined that extensional stress due to upwelling of magma beneath the region must have deformed the fault and created faults. After the extensional sets ceased, the weakened crust was covered with sediments over millions of years. This fault line run through Bayelsa (Igbogene), Rivers, Abia (Ohafia), Enugu (Ede-Oballa), Nassarawa (Obi) and Gombe States. The other Tremor sites in Adamawa and Taraba states were as a result of their proximity to the Cameronian Volcanic line as shown in Table 1 for the Gembu and Jalingo tremors of 1982 and Yola tremor of 1984 and 2005.
Journal of Environment and Earth Science www.iiste.org ISSN 2224-3216 (Paper) ISSN 2225-0948 (Online) Vol.11, No.10, 2021 From the Geodetic analysis, the Mean Horizontal Velocity of Nigerian for the period was 22.5625±0.32583mm/yr East, 18.93±0.23417mm/yr North. This result was found to agree with Bawa, Ojigi and Dodo (2018) and Altamimi et al (2011). From the preliminary results, it was discovered that the MDGR site was giving outrageous values and it was regarded as an outlier and excluded from the experiment.
The Position Solution from the globk_vel.orgfile was extracted and the Bearing and Distance covered by the NigNet Sites were computed from the change in Eastings (dE) and Change in Northings (dN) and shown in Table  2. Figure 4 is the Velocity Plot for the experiment from 2012 to 2014. It is evident from Table 2and Figure 4 that although the direction of all the stations used for the Velocity plot are in the North East direction, the bearings of the motions are not the same. The stations with the most deviating bearings from the majority are the CGGT and the RUST stations at 67 o 5'37.8492" and 83 o 15'56.7288" respectively whose accuracies were also found to be poor as a result of verry few GNSS logs. However, it is interesting to note that the ten degrees (10 o ) range of change in bearing of the different displacements is wide enough to suggest possible presence of fault lines.

Conclusion
The evaluation of the Geodetic method of fault determination used in this experiment agrees with the Non-Geodetic Method. It can then be concluded that there are indeed more than one fault lines in Nigeria. The Mid Nigerian Fault Line appears to be the most active fault line, given the recent tectonic activates around Abuja. This is closely followed by the Western Nigerian Fault Line given the date of the most recent tectonic activity along the line to be the Abeokuta 4.4M in 2011. The Eastern Nigeria Fault Line is adjudged the most inactive fault line. There is however a dire need to establish more than five times the number of COR Stations in Nigeria. The Data from those stations should be made readily available to researchers, as data availability was the greatest challenge for this experiment. New stations to be established, should concentrate around the three proposed fault lines to help in further investigations of tectonic activities within the country.