SS3 First Term Geography Lesson Note – Earthquakes

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WEEK:  1       

TOPIC:  EARTHQUAKES

CONTENT:

1. DEFINITION, ORIGIN AND FOCUS
2. Tremor
3. Epicentre

iii. Shock waves

1. CAUSES OF EARTHQUAKE
2. Faults from collision of tectonic plates
3. Sudden release of stress

SUB-TOPIC 1: EARTHQUAKES

DEFINITION: These are sudden movement or vibrations in the earth crust. They occur as a result of disturbance within, and below the earth crust. The transmission of the resultant vibrations outward to the surface, leads to a series of shocks waves that give rise to earthquakes.

The point at which the shock originates is called the Origin or focus and the point on the earth surface directly above the origin or where the shock wave first hit the surface is called epicenter. The instrument used in measuring the intensity of earthquakes is called seismograph.

Earthquakes could be disastrous where there are densely populated areas.

TREMOR: Earth tremors are vibrations within the earth’s crust produced by gentle waves. However, earth tremors are not severe as earthquakes. Its only occurs every few minutes.

EPICENTRE: This is the point on the earth’s surface where the shock waves first hit the surface. It is directly above the origin. The effect produced by the earthquake is referred to as the INTENSITY. The magnitude of the shock of the earthquake which refers to the total amount of energy released is measured according to the RICHTER’S SCALE.

In 1935, C. F. Richter devised a type of scale for measuring the magnitudes of earthquakes. It ranges from 0 to 9 and the largest yet recorded is 8.6 on this scale. Earthquakes measuring 0 to 4 are minor, from 4 to 7 are moderate and over 7 is severe.

There are two types of shock waves.

1. Body waves
2. Surface waves

BODY WAVES: These waves travel through the crust and are of two ways

1. Primary waves: These cause the crustal rocks to move backward and forward in the direction of the wave movement.

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