Causes Of Earthquake, Effects, Measurement, Shockwaves

Hi, welcome to this hall on the topic: Effects Of Earthquake, Causes, Measurement, Shockwaves.

Table Of Contents

1. Definition of earthquake
2. Causes of earthquake
3. Measurement of earthquake
4. Types of shock waves
5. Effects of earthquake
6. Revision Questions.

Definition Of Earthquake

An earthquake is defined as a violent tremor in the earth crust which send out a series of vibrating waves in all directions.

Causes Of Earthquakes

Earthquakes occur as a result of disturbance or movement 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:

1. The amount of shift, that is, crustal displacement or movement along the fault.
2. The size of the rock mass involved in the movement or displacement.

Measurement Of Earthquakes

A scale for measuring the power of earthquake was first devised in 1935 by C.F Ritcher. This scale is called the Ritcher scale. This scale is graded from 0 to 9. Earthquake measuring 0 – 4 are minor, from 4 – 7 are severe.

Examples Of Earthquake

1. in 1960, an earthquake measuring 8.5 on Richer scale struck Alaska, and the capital city, Anchorage, was severely damaged even though the epicenter was 120km away. If the epicenter has been directly under Anchorage, the entire City would have been flattened.
2. On the 10th of October 1986, and earthquake measuring 8.8 on the Ritcher scale struck El Salvador in South America, causing 600 deaths and vast material damage.
3. On the 8th of October 2005 an earthquake measuring 7.6 on the Ritcher scale hit the Pakistan-controlled area of Kashmir region. About 70,000 people were confirmed dead, and about 3 million others rendered homeless.

Other notable examples of disastrous earthquake include

1. The great Lisbon earthquake of 1755.
2. The Tokyo and yokohoma earthquake of 1923.
3. The San Francisco earthquake of 1906.
4. The Kansu (China) earthquake of 1920.
5. The Agadir (Morocco) earthquake of 1906.
6. The Accra (Ghana) earthquake 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 part of the surface. The waves also travel laterally on the surface as long wave. 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 and (b) Surface waves.

1. Body waves: This 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 crustal rocks to move from side to side, that is, at right angles to the direction of wave movement.

2. 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 set up huge waves called tsunamis. This waves at times travel more than 10m high, invading adjacent coast lands, drowning people and livestock and pulling down structures. For example, in 2011, an earthquake accompanied by wave killed over 50,000 people and destroy over 80 houses in Japan.

Effects Of Earthquakes

1. Earthquakes cause the death of people and animals in places where they occur.
2. They also cause the destruction of buildings, roads, Bridges, railways and telecommunications.
3. They also displays parts of the earths crust, either vertically or laterally.
4. They can also raise or lower coastal rocks.
5. They can also raise a lower parts of the ocean floor.
6. They can cause landslides and open up deep cracks in surface rocks.

Revision Questions

1. What is earthquake? (b) Write short note on (i) Focus (ii) Epicenter (iii) Seismograph (iv) Earth tremor (v) Tsunamis.
2. (a) Discuss the major types of shock waves. (b) List four effects of earthquakes and two popular examples of earthquakes. (c) State two major causes of earthquake.