Many modern technologies, such as computers and smartphones, are built on the principles of Physics Topics such as quantum mechanics and information theory.
Earthquake : Definition, Causes, Effects & Measurement
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 are much less frequent but they are very dangerous. Earthquakes can cause immense damage to houses, other buildings, bridges, dams and people, etc. A lot of people get killed when they get buried under the debris of collapsed houses and other buildings during an earthquake. Earthquakes can also cause floods, landslides, and tsunamis (read as sunamis).
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 (see Figure). Another major earthquake occurred on 8th October 2005 in Uri and Tangdhar towns of North Kashmir. A great loss of human life and property (houses and other buildings, etc.) occurred in both these earthquakes. A major tsunami (caused by an earthquake) occurred in the Indian Ocean on 26th December 2004. All the coastal areas around the Indian Ocean suffered huge loss of life and property during this tsunami.
Earthquakes are a destructive natural phenomena. The other destructive natural phenomena such as cyclones and lightning can be predicted in advance to some extent so that we get some time to take measures to protect ourselves and minimise the damage to life and property. This is not so in the case of earthquakes. Earthquake is a destructive natural phenomenon which cannot be predicted in advance. Nobody can tell when and where an earthquake will occur. This unpredictable nature of earthquakes makes them even more dangerous. In order to understand why earthquakes occur, we should first understand the inner structure of the earth. This is described below.
Structure of Earth
The inside of earth is made up of three main layers : Core, Mantle and Crust (see Figure).
(i) CORE. The innermost part of the earth is called its core (see Figure). The core of earth is made up mostly of iron. The core of earth is extremely hot. Most of the earth’s core (called outer core) is liquid (molten iron) whereas the inner part of core (called inner core) is under such high pressure that it is solid (solid iron).
(ii) MANTLE. The central region of earth (between the core and crust) is called mantle (see Figure). 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
(iii) CRUST. The outermost layer of earth is called crust (see Figure). 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). Due to convection currents taking place in the mantle, the plates of earth’s crust are moving around very, very slowly (see Figure). 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
(i) slide past one another, and
(ii) 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 entangled. Due to entanglement of the rocks at their edges, 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 (see Figure). 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 of 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 (see Figure). A tremendous amount of energy is released when the two huge plates of the earth’s crust collide with each other. This tremendous amount of energy sends shock waves (called seismic waves) throughout the earth. When these shock waves reach the surface of earth, the ground starts shaking violently.
This shaking of the ground is felt as an earthquake. In this way, the collision of two moving plates of earth’s crust also causes earthquake. In fact, the earth’s plates in the Himalayan region push against one another (or collide with one another) raising mountains and causing earthquakes. Actually, what we have described above happens when two continental plates (or land plates) collide head on with each other.
If, however, a moving oceanic plate collides head on with a continental plate (land plate), then the oceanic plate goes under the continental plate (into the mantle), and the continental plate buckles and folds to form a new mountain. 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 or a big meteorite hits the earth or an underground nuclear explosion is carried out. Most of the earthquakes are, however, caused by the movements of earths plates.
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. The weak zones of earth’s crust (which are more prone to earthquakes) are called ‘seismic zones’ or ‘fault zones’. Most major earthquakes occur in the well defined ‘seismic zones’ called earthquake belts. The seismic zones mark the edges of the huge mobile pieces of the earth’s crust called plates.
The seismic zones (or earthquake belt) of the earth are shown in the world map given in Figure. In India, the areas 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.
Earthquake tremors produce shock waves (called seismic waves). The shock waves travel in all directions through the Earth and also reach the surface of the earth. Seismograph is an instrument which measures and records the magnitude of an earthquake in terms of the shock waves it produces. A seismograph detects the shock waves produced by an earthquake and also records the shock waves on the paper in the form of a graph. Seismograph is also known as seismometer. A seismograph consists of a heavy weight (like a heavy metal ball) suspended from a support with the help of a strong wire (see Figure).
The support is attached to the base of seismograph. And the base of seismograph is fixed rigidly to a solid rock on the surface of earth. A pen is attached to the lower end of the hanging heavy weight. This pen can trace lines on a graph paper wound around a rotating drum which lies beneath it (as shown in Figure). Please note that the purpose of using a freely suspended heavy weight is that, being heavy, it has a lot of inertia and hence it remains stationary during an earthquake when the rest of seismograph (including its support) fixed to the earth shakes (or vibrates) during an earthquake.
We will now describe the working of a seismograph. When an earthquake occurs, the earth starts shaking due to which the base of seismograph fixed to the earth also starts shaking. But the freely suspended heavy weight (and the pen attached to it) do not shake during the earthquake, they remain stationary. Since the graph paper on the rotating drum shakes with the shaking earth, the pen attached to suspended weight records the vibrations produced by earthquake on the graph paper which moves under it (see Figure).
Actually, the pen traces the relative movement between the shaking earth and the stationary, suspended heavy weight. This trace on the graph paper records the earthquake. A trace produced by a seismograph during an earthquake is shown in Figure. The seismograph record is also known as seismogram. A seismograph record (or seismogram) shows seismic waves or earthquake waves recorded on a graph paper by a seismograph. It also shows the duration (or time) for which the earthquake lasts, in seconds.
Earthquake recording laboratories (called seismic stations) have been established all around the world. By studying the seismograph records of an earthquake produced at various seismic stations, scientists can construct a complete map of an earthquake giving its focus, epicentre, magnitude and duration, etc. (see Figure). The place inside the earth’s crust where the earthquake is generated, is called ‘focus’ of the earthquake (see Figure). The focus of an earthquake is deep underground. The seismic waves (or earthquake waves) spread from the focus and travel through the earth in all directions. Focus is actually the ‘source’ of an earthquake. The point on earth’s surface directly above the focus is called epicentre (see Figure).
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. Just like the Decibel Scale for measuring the loudness of sound, the Richter Scale is not a linear scale. This means that an earthquake of magnitude 2 on Richter Scale is not two times as strong as an earthquake of magnitude 1 on this scale. In fact, 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. And an earthquake of magnitude 6 on the Richter Scale is 1000 times stronger than an earthquake of magnitude 4. In other words, an earthquake of magnitude 6 on Richter Scale has 1000 times more destructive energy than an earthquake of magnitude 4.
An earthquake of magnitude 1.5 on the Richter Scale is the smallest earthquake that can be felt by us. An earthquake of magnitude 4.5 on the Richter Scale causes some damage but not much damage. The earthquakes having magnitudes higher than 7 on the Richter Scale are really destructive earthquakes. Both, Bhuj and Kashmir earthquakes had magnitudes greater than 7.5 on the Richter Scale. The earthquakes which measure 8.5 or more on Richter Scale are devastating. During these earthquakes, the affected part of earth’s surface shakes violently. Such earthquakes can destroy entire cities and villages causing a great loss of life and property.
Protection Against Earthquakes
Earthquakes are 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 :
(i) All the houses and other buildings in seismic zones should be designed and constructed in such a way that they can withstand major earthquake tremors. It is possible to do so by using modern building technology which is available these days. In our country, the Central Building Research Institute at Roorkee has developed know how to make quake-proof houses. The people should consult qualified architects and structural engineers while constructing houses and other buildings in earthquake prone areas so that they can be made ‘quake safe’ as far as possible.
(ii) 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 during an earthquake, the damage will not be too much.
(iii) 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.
(iv) 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.
(v) 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.
If we are at home when an earthquake occurs, then :
- 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.
- We should stay away from tall and heavy objects (like steel almirahs, 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.
- 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 :
- 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).
- 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.