Earthquake

An earthquake is a sudden shaking (or trembling) of the earth which lasts for a very short time. An earthquake is caused by the violent movements of rocks deep inside the earth's crust. Earthquakes occur all the time all over the earth. Most of these earthquakes are so mild that they are not even noticed (or felt) by us. Major earthquakes arc much less frequent but they are very dangerous. Earthquakes can cause immense damage to houses, other buildings, bridges, dams and people, etc. Earthquakes can also cause floods, landslides, and tsunamis.

A major earthquake can cause damage to life and property on a large scale. A major earthquake occurred in India on 26th January 2001 in Bhuj district of Gujarat. Another major earthquake occurred on 8thOctober 2005 in Uri and Tangdhar towns of North Kashmir. 

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Structure of Earth

The inside of earth is made up of three main layers: Core, Mantle and Crust. 

1. Core: The innermost part of the earth is called its core. The core of earth is made up mostly of molten iron. The core of earth is extremely hot. 
2. Mantle: The central region of earth (between the core and crust) is called mantle. Mantle is the middle layer of the earth. Mantle is mostly made of dense, solid rocks. Some of the mantle is., however, a mixture of solid rocks and hot molten rocks (liquid rocks)like the lava from a volcano. Heat coming from the core of earth warms the mantle. This heating sets up huge convection currents in the mantle. The giant convection currents occurring in the mantle can make the mantle move very, very slowly.

The structure of earth

3. Crust: The outermost layer of earth is called crust. The crust of earth is made of comparatively lighter rocks than that of mantle. The crust of earth is thicker where there is land (or continents). The crust of earth under the oceans is thinner. The whole crust of earth is not in one piece. The crust of earth is made of many huge pieces of flat rocks (rather like tiles on a bathroom floor). Each piece of the earth's crust is called a plate. Thus, the crust of earth is divided into many plates. The plates of earth on which continents exist are called continental plates whereas those plates of earth on which oceans exist are called oceanic plates. The plates of crust are in fact very, very large fragments of earth's crust. The solid plates which make up the earth's crust are floating on the partially molten rocks of mantle beneath (see Figure 12)

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Due to convection currents taking place in the mantle, the plates of earth's crust are moving around very, very slowly. The reason the earthquakes occur is that the earth's crust is made of a number of plates which are able to move. Earthquakes occur mostly at the edges of moving plates of the crust (or boundaries of the moving plates of the earth).

Why Do Earthquakes Occur

The earthquakes occur when the moving plates of the earth's crust:

• Slide past one another, and
• Collide with one another.

We will describe both these cases in detail, one by one

1. The plates of earth's crust are made up of huge rocks having rough edges. Due to their highly rough edges, the movement of two crust plates relative to one another is not smooth. When the two huge plates of earth’s crust slide past one another, they rub against one another ferociously and the rocks on their edges get the two crust plates stop moving for some time. During this time, the plates are still pushing against one another and trying to move but they are not moving (due to entanglement of their rocky edges). This builds up pressure between the two plates of crust. When sufficient pressure has been built up between the two crust plates, the entangled rocks of two plates break open with a big jolt. When the entangled rocks of the two crust plates break open suddenly with a big jolt, the earthquake occurs releasing a tremendous amount of energy. This sudden release to tremendous amount of energy produces shock waves (or seismic waves) which make the earth shake. After the earthquake, the plates of earth’s crust start moving again and continue to move until they get entangled again.

2. When the two plates of the earth's crust moving in opposite directions collide with each other head on, then the ends of these crust plates buckle and fold forming new mountains and causing earthquakes. A tremendous amount of energy is released when the two huge plates of the earth's crust collide with each other. This collision of an oceanic plate and a continental plate also causes an earthquake. Tremors on the earth can also be caused when a volcano erupts. 

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Seismic Zones (or Fault Zones)

Since earthquakes are caused by the movements of earth's plates, the boundaries of the plates are the weak zones where earthquakes are most likely to occur. In India the area most threatened by earthquakes are Kashmir, Western and Central Himalayas, the whole of North-East, Rann of Kutch, Rajasthan and Indo-Gangetic Plane. Some areas of South India also fall in earthquake danger zone.

The encircling belt (shown in orange colour) in this figure indicates  the seismic zones of the world where most of the earthquakes occur.

Richter Scale

The magnitude (or intensity) of an earthquake is expressed on the Richter Scale. The Richter Scale is a series of numbers from 1 to 12 used to express the magnitude (or size) of an earthquake. An earthquake of magnitude 2 is ten times as strong as an earthquake of magnitude 1.Similarly, an earthquake of magnitude 3 on Richter scale is 100 times as strong as an earthquake of magnitude 1 on the same scale. 

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Measure to Protection against Earthquake

Protection against Earthquakes is highly destructive and they cannot be predicted. So, it is necessary that we take precautions to protect ourselves all the time (especially if we live in a seismic zone of the country). Some of the important precautions which can be taken by the people living in seismic zones (or earthquake prone areas) for protection against earthquakes are as follows:

1. All the houses and other buildings in seismic /ones should be designed and constructed in such a way that they can with  and major earthquake tremors. It is possible to do so by using modern buildingtechnology1 which is available these days.
2. In highly seismic areas, the use of mud and timber (wood) for building houses is better than using heavy construction materials. The roofs of houses in such areas should be kept as light as possible so that in case a roof falls dung an earthquake, the damage will not be too much.
3. The cupboards and shelves should be fixed to the walls so that they do not fall easily when shaking occurs during an earthquake Glass bottles should not be placed on high shelves and heavy objects should be placed low to the ground. 
4. The objects such as heavy mirrors, photo frames, wall clocks and water-heaters, etc., should be mounted securely on the walls at such places in the house that they do not fall on the people in the house in the event of an earthquake.
5. All buildings (especially tall buildings) should have fire-fighting equipment in working order because some buildings may catch fire during an earthquake (due to electric short circuits).

In case an earthquake occurs, we should take the following precautions to protect ourselves depending on whether we are at home or outdoors at that moment.

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If we are at home when an earthquake occurs, then:

1. We should take shelter under a sturdy table or a kitchen counter and stay there till the shaking due to earthquake stops. All this while, we should cover our head with hands.
2. We should stay away from tall and heavy objects (like steel admirals. cabinets, book racks and refrigerators, etc.) so that if they topple, they may not fall on us. We should also stay away from glass windows that may shatter due to vibrations.
3. If we are in bed, we should just move on the bed to be close to a wall but not get up from the bed we should protect our head with a pillow.

If we are outdoors when an earthquake occurs, then:

1. We should stay at a clear spot away from buildings, trees and over-head power lines, etc. We should also sit on the ground (so that we may not fall down due to shaking of ground).
2. If we are in a car (or bus), we should not come out of it. The car (or bus) should be driven slowly to a clear spot away from buildings, trees and over-head electric wires, etc. We should not come out of the vehicle till the tremors stop.

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