An earthquake is defined as a violent tremor in the earth’s crust which sends out a series of vibrating waves in all directions.
Causes of Earthquakes
Earthquakes occur as a result of disturbances or movements within and below the earth’s crust, especially where there are fault or fractures. For some reasons, stresses are built up within the earth’s crust. When a certain point of tension or intensity is reached or attained, cracks or slips occur within the earth’s crust. This produces the shock effects that send out the vibrations.
The transmission of the resultant vibrations outward to the surface leads to a series of shock waves. It is these shock waves that give rise to an earthquake.
The intensity of an earthquake depends on the following factors:
- The amount of shift, that is, crustal displacement or movement along a fault.
- The size of the rock mass involved in the movement or displacement.
Measurement of Earthquake
A scale for measuring the power of earthquake was first devised in 1935 by C.F Richter. This scale is called The Richter Scale. The scale is graded from 0 – 9. Earthquake measuring 0 – 4 are minor, from 4 – 7 are moderate and over 7 are severe.
Some examples of earthquake
- In 1960, an earthquake measuring 8.5 on Richter scale struck Alaska, and the capital city, Anchorage, was severely damaged even though the epicenter was 120 kilometers away. If the epicenter had been directly under Anchorage, the entire city would have been flattened.
- On the 10th of October, 1986, an earthquake measuring 8.8 on the Richter scale struck El Salvador in South America, causing 600 deaths and vast material damage.
- On the 8th of October, 2005, an earthquake measuring 7.6 on the Richter scale hit the Pakistan- controlled area of Kashmir region. About 70,000 people were confirmed dead, and about three million others rendered homeless.
Other Notable Examples of Disastrous Earthquakes include
- The great Lisbon earthquake of 1755.
- The Tokyo and Yokohama earthquake of 1923.
- The San Francisco earthquake of 1906.
- The Kansu (China) earthquake of 1920.
- The Agadir (Morocco) earthquake of 1906.
- The Accra (Ghana) Earth tremor of 1939.
The point at which the shock originates is called the focus or origin. The point on the earth’s surface directly on top of the origin is called Epicenter. Epicenter is the point where the effect of the shock is greatest. The shock waves travel from the focus to the epicenter and from various other parts of the surface. The waves also travel laterally on the surface as long wave (Fig. 38.3). The instrument used for measuring the intensity of an earthquake is called Seismograph.
Types of Shockwaves
There are two types of shock waves. These are: (a) Body waves (b) Surface waves.
(a) Body waves: These waves travel through the crust and are of two types:
- Primary Waves: These cause the crustal rocks to move backward and forward in the direction of the wave movement.
- Secondary Waves: These cause the crustal rocks to move from side to side, that is, at right angles to the direction of wave movement.
(b) Surface Waves: These travel through the surface rocks. They are of two types:
- Long Waves: These cause the surface rocks to move from side to side, at right angles to the direction of wave movement.
- Rayleigh Waves: These cause the surface rocks to have a vertical circular movement. Earth Tremors are vibrations within the earth’s crust produced by gentle waves. However, earth tremors are not as severe as earthquakes. It only occurs every few minutes.
When earthquake occurs on the sea floor, for example, it sets up huge waves called tsunamis. These waves at times travel more than 10 metres high, invading adjacent coastlands, drowning people and livestock, and pulling down structures. For example, in 2011, an earthquake accompanied by wave killed over 50,000 people and destroyed over 80 houses in Japan.
Effects of Earthquakes
- Earthquakes cause the death of people and animals in places where they occur.
- They also cause the destruction of buildings, roads, bridges, railways and telecommunications.
- They also displace parts of the earth’s crust, either vertically or laterally.
- They can also raise or lower coastal rocks.
- They can also raise or lower parts of the ocean floor.
- They can cause landslides and open up deep cracks in surface rocks.