An earthquake is a violent tremor in the earth crust which send out a series of vibrating waves in all directions.
These are sudden movement or vibrations of in the earth crust. They occur as a result of disturbances within and below the earth crust
CAUSES OF EARTHQUAKES
Most earthquakes one caused by sudden movement within the earth crust particularly where there are fractures of faults, stress is built up within the earth crust when a certain point of tension or intensity is reached, cracked or ships occurs within the earth crust. This produce the shock effect that sands but vibrations.
The point at which the shock originates is caved in a nut shell earthquakes are caused by the development of faults or cracks in the crust which results from the collision between the tectonic plains.
The movement of molten rock below or within crust or the sudden release or stress which are slowly built up along the plain. The point at which the shuck originates is called Origin or Focus and the point on the earth surface directly above the focus or where the shock waves first meet the surface with Intensity of earthquake is called seismograph. This instrument records the vibration produced by an earthquake. The effect produced by the earthquakes is referred to as Intensity. The magnitude of shock of the earthquake which refers to the total amount of energy released is measured according to Richter’s scale.
When earthquakes occur on the seafloor for example, it sets up huge waves called tsunamis. This waves at times more than 10m high invade adjacent costal lands drowning people and livestock and pull down structure. For example on December 27, 1939 an earthquake and accompanying waves killed 50,000 people and destroy 100,000 homes in Turkey.
Regions of earthquake occurrence
POPULAR EXAMPLES OF DISASTROUS EARTHQUAKES
Africa is generally free from earthquake except along the edge (Morocco, Algeria and Egypt) where some earthquake have been recorded.
In East Africa, some major earthquakes have occurred in the rift valley zone e.g. Tanzania, near the shore of Tanganyika in the December 1910 and in Kenya in January 1928. The Accra region of Ghana is the only part of West Africa that has not experienced tremors since the second half of the 18th century.
Major tremors occurs in 1867, 1906 and 1937. In the last few years, minor earth tremors have been recorded in the basement complex areas in Nigeria particularly in Oyo and Ogun state.
In 1964, earthquake measuring 8.5am struck Alaska, capital anchored was severally damaged even though the epicentre was 120km away.
THE EARTHQUAKE PROCESS
The point in the interior of the earth from where the vibration or earthquake waves originates is called the focus of the earthquake. The waves radiate upwards incomes of waves known as underground body waves. They arrive the surface in concentric circles where they are celled the surface waves. The vertically above the focus which is the same as the centre of the earthquakes. The epicentre is the point in the point on the earth surface directly above the focus or where the shock waves first hit the surface.
The instrument used in measuring the intensity of earthquake is caked seismography. The instrument records the vibration by an earthquake.
TYPES OF SHOCK WAVES
There are two type of waves which are: (i) Body waves (ii) Surface waves
1. BODY WAVES: this waves travel through the crust. There are two types of body wave.
a. Primary Waves: this causes the coastal rocks to move backwards and forward in the direction of the wave movement.
b. Secondary waves: this cause the coastal make to move from side to side i.e. at right angles to the direction of the wave movement.
2. SURFACE WAVES: this travel through the surface rocks. They are of two types
a. Love waves: this causes the surface to move from side to side at right angel to the direction of the wave movement
b. Rayleigh waves: this causes the surface rocks to have a vertical circular movement.