people underneath. - CHAPTER ONE
1.1 Introduction
Nigeria is not situated on any known belt, yet Nigeria had experienced earth tremors in the past. Most of the events that occurred in Nigeria were not instrumentally recorded because there was no such equipment in the country at that time.
Unlike the east African region, West Africa was not known to be seismogenic in the past and as a result of this, most people tend to believe that seismic activities are confined to North Africa and the surrounding area of the rift valley system in East Africa (Onuoha, 1988). But recent findings have shown that Nigeria may not be completely free from earthquakes (Adepelumi, 2009; Akpan and Yakubu 2010). Since there is evidence of tremors in Nigeria in the past, any future occurrence of earth tremors in the country are likely to occur along these fault Zones. Possible mechanisms for these intra-plate tremors have been examined to include regional stresses created by Nigeria’s position between two Cratons and Zone of weakness resulting from magnetic intrusions and other tectonic activities in the sediments (Eze et al, 2011).
Historical and recent seismicity data have indicated that disastrous earthquakes have occurred in other parts of Africa far away from the Atlas mountain region and also in the area far from the rift valley system (Onuoha, 1988). This development indicates that Nigeria and some West Africa countries are likely to witness devastating earthquake in future. This is in line with recent review of earthquake. Occurrences and observations in Nigeria which shows several minor tremors had been experienced in some parts of the country in 1933, 1939, 1964, 1984,1990, 1994, 1997, 2000 and 2006 (Akpan and Yakubu, 2006).
Adepelumi (2009) has predicted the probabilistic occurrence of earthquakes in the South western town of Ijebu Ode and its environs between the year 2008 and 2028. Aside the statistical prediction for earthquake occurrence in Ijebu Ode in future, remote sensing, geological and geophysical studies had earlier revealed the presence of a NNE-SSW trending Ifewara Zungeru fault zone which has been shown to be linked with the Atlantic fraction system Olujide and Udoh, (1989).
Despite the result from various studies that indicate that Nigeria could experience major earthquakes in the future, much attention has not given to seismicity study in the country.
It is against this background that this research is carried out to review earthquake monitoring and sensing in Nigeria.
1.2 Global Seismicity
Seismicity is the global distribution of earthquake source (epicenters). Earthquake epicenters are not uniformly distributed over the earth’s surface, but occur predominantly along narrow zones of interplate seismic activity.
The major earthquake Zone in the world are the Circum-pacific zone, which accounts for about 75- 80% of the annual release of seismic energy, the Mediterranean-transasistic zone which account for 15-20% of the annual seismic energy and the Oceanic ridges annually released seismic energy (Lowrie, 2007). Each of these zones is characterized by active volcanism.
The remaining part of the earth is considered to be aseismic (i.e not seismically active). However, no region of the earth can be completely free from earthquakes because about 1% of the global seismicity is due to intraplate earthquake which occur from the major seismic zones (Lowrie, 2007). Earthquake also known as earth tremor is as a result of sudden release of energy in the earth’s crust that creates seismic waves.
Most earthquakes are minor tremors. Large earthquakes usually begin with slight tremor but rapidly take the form of one or more violent shocks, and end vibrations of gradually diminishing force called after shock. The subterranean points of origin of an earthquake is Called the focus; the point on the surface directly above the focus is the epicenter.
Earthquakes fall into the broad categories;
i. Natural earthquake, caused by natural force and
ii. Artificial earthquake, which occur due to such human activities which cause disturbance in the earth’s crust producing vibrations of various magnitudes.
Natural Earthquakes are classified as;
1. Tectonic earthquakes
2. Volcanic earthquakes
Earthquakes can be classified based on their focal depths. Earthquake with focal depths less than 70km are called shallow earthquake and occur in all the seismically active zones. About 85% of earthquakes are shallow earthquake and occur only in the oceanic ridge system. Earthquakes with focal depths between 70-300km are called intermediate earthquake and account for about 12% and deep earthquake have focal depth greater than 300km and account for about 3%. These occur only in the circum-pacific and Mediterranean-transasiatic seismic zones and accompany the process of plate subduction (Lowrie, 2007).
Earthquake can also be classified as tectonic and volcanic earthquakes.
Tectonic Earthquakes: Are caused by the sudden earth movements, generally along faults, usually at depths varying from about 4.5km to 24km below the earth surface. Majority of natural earthquakes including the most powerful and damaging ones belong to this category.
The term ‘tectonic’ refers to any structural change in rocks due to their deformation or displacement.
Volcanic Earthquake: Are associated with explosives eruption of volcanoes. Naturally they occur in volcanic zones of the world the Circum-pacific Belt is a typical example where volcanic earthquakes are common. Their intensity and magnitudes depend on the explosive power of volcanic eruption.
Earthquakes can also be classified based on focal depth the depth of focus refers to the depth at which an earthquake occurs. Earthquakes occurring at a depth of less than 70km are classified as “Shallow-focus” earthquakes, while those with a focal-depth between 70 and 300km are commonly termed “mid-focus” or intermediate-depth earthquakes. Deep -focus earthquakes may occur at much greater depths in the monthly (Ranging from 300km to 700kilometers.) (Greene 11, H. W and Burnley, P. C. 1989)
Figure1.Map of Nigeria Showing Seismic Activity.
Map of Nigeria, Areas with seismic activity are shaded.
Alaska is the most earthquake prone state and one of the most seismically active regions in the world.
Earthquakes occur namely in three main areas;
The ring of fire also called the Circum-pacific Belt, is the zone of earthquake surrounding the pacific Ocean-about 90% of the world earthquake occur there. The ring of fire is the most seismically and volcanically active zone in the world. (Bakar, and Francis, 1978). The circum-pacific seismic belt, or the Ring of fire which loops around the pacific Ocean taking in the south African Coast, Central America, the west coast of America and on to Japan, the Philippines. The south pacific Islands and New Zealand. 81% of the world’s earthquakes occur here due to the number of converging plates and young growing mountain and deep Ocean trenches.
The Alpide belt Which is the next most seismically active region in the world (5-6% of earthquake) is the Alpide belt which extends / stretches from Java to Sumatra through the Himalayas, the Mediterranean region, eastward through, Turkey, Iran , Northern India and out into the Atlantic. This area accounts for 17% of the world’s earthquakes and also some of the most destructive in history.
The Mid-Atlantic ridge: the third most prominent earthquake belt which is a fracture zone along the bottom of the Atlantic Ocean where moltery mantle comes to the surface and creates new crust. (Redfern,2001).Earthquakes do occur outside these areas though in the U.S.A or Central Europe for example.
The mid-Ocean ridge, a divergent tectonic plates boundary located along the floor of the Atlantic Ocean, and part of the longest mountains in the world. It separates the European plate and North American plate in North Atlantic and the African plate from the South American plate in south Atlantic. (Edgar, 1977).
1.3 Geology/Tectonic Setting of Nigeria.
Apart from being located within the interpolate area, Nigeria’s land mass is made of Precambrian to Early Paleozoic crystalline basement rocks about half of which is covered by sedimentary rocks of cretaceous to recent age. About two-thirds of the country’s land mass is underlain by the Precambrian basement complex consisting of gneisses, pigmatites, schist, various metamorphic rocks and granites as shown in geologic map (Fig 1) (Eze et al, 2011). These are in places intruded and interspersed by the “older granites” which originated in the pan-African orogeny (Ohejide and Udoh, 1989), basement complex rocks outcrop in four main areas of the country: North of Rivers and Benue, covering parts of kaduna, plateau, Bauchi, Kanu and Sokoto States; Southern Nigeria, covering the greater parts of Kwara, Oyo, Ogun; and Ondo States; Southeast Nigeria, Spanning the Northern parts of Cross Rivers State and as far North as Yola; and North of Benue River in Gongola State (Eze C. L et al, 2011). These crystalline basement rocks have subjected to deformation of different intensities throughout the geological period. Consequently, North-South (N-S). Northeast Southwest (NE-SW), Northwest-Southeast (NW-SE), North Northeast-South Southwest NNE-SSW), North Northwest-South Southeast (NNW-SSE) and to a lesser extent, East-West (E-W) fractures have developed (Ohejide and Udoh, 1989). The basement rocks are overlain by Cretaceous and Tertiary sediment of the seven major sedimentary basins; the Calabar Flank, the Benue trough, the Chad Basin, Lullemmenden (Sokoto) Basin, the Dahomey Basin and the Niger Delta Basin. Sedimentary successions in recent age (Kogbe, 1989). The Niger Valley is possibly a graben and the Benue a sinistral shear zone that may be part of a fracture zone within the pelsiummegashear system (Sheidegger and Ajakiaye, 1985). The Benue trough is also considered as a “failed” rift since it contains no ophiolites (Nwachukwu, 1972). Some of the important fault systems in Nigeria are the Ifewara Zungeru, Anka and Kalangai fault system. They are interpolate to have a result from transcurrent movements (Garbe, 2003).
1.4 Aims and Objectives of the Study
The aim of this study is to investigate earthquake monitoring and sensing in Nigeria. Specially, the study sought to;
i. Give an overview of earthquake monitoring and sensing in Nigeria.
ii. Identify active and non-active Zones in Nigeria
iii. Identify earthquakes monitoring techniques in Nigeria.
1.5 Significance of the Study
Considering the damage caused by earthquake disaster, the impact of this research endeavor will be enormous. The research will be immense benefits to researchers, government & policy maker and the general public.
To the researchers, it will create room for more research in the different areas of earthquake science. Also it will help them to know the active and non-active earthquake zone in Nigeria.
• To the government, it would guide them in accessing the competency of an area to accommodate heavy projects.
• To the policy makers, it would help them in making laws guiding the use of building materials (which areas would require more quality materials than other areas).
• It would guide the investors who intend to raise structures in earthquake prone areas.
