Types of Earthquakes, What is Earthquakes, Seismic Waves, Depth of Earthquakes, Measurement of Earthquakes, Classification of Earthquakes, Effect of Earthquakes
What is Earthquakes
An earthquake in simple words is shaking of the earth. It causes due to release of energy, which generates waves that travel in all directions. The release of energy occurs along a fault. A fault is a sharp break in the crustal rocks. Rocks along a fault tend to move in opposite directions. As the overlying rock strata press them, the friction locks them together. However, their tendency to move a part at some point of time overcomes the friction.
As a result, the blocks get deform and eventually, they slide past one another abruptly. This causes dissipation of energy, and the energy waves travel in all directions. The point where the energy is released is called the focus of an earthquake, alternatively, it is called the hypocentre. The energy waves travelling in different direction reach the surface. The point on the surface, near to the focus, is epicentre.
It is the first one to experience the waves. It is a point directly above the focus.
Types of Earthquakes
1. Tectonic Earthquakes
This type of earthquake is generates due to sliding of rocks along a fault plane. This movement causes imbalance in the crustal rocks which result in earthquakes of varying magnitude, depending upon the nature of dislocation in the rock strata.
2. Volcanic Earthquakes
Volcanic activity is one of the main causes of earthquakes. In fact, volcanic activity and seismic events are so intimately related to each other that they become cause and effect for each other. Each volcanic eruption follows by an earthquake and many of the severe earthquakes can cause volcanic eruptions. The explosive violent gases during the process of volcanic activity try to escape upward and hence they push the crustal surface from below with grade force. This leads to severe tremors of high magnitude, which depend upon the intensity of volcanic eruptions.
3. Collapse Earthquakes
In areas of intense mining activity, sometimes the roofs of underground mines collapse causing minor tremors.
4. Explosion Earthquakes
Ground shaking may also occur due to explosion of chemical or nuclear devices.
5. Reservoir induced Earthquakes
The earthquakes which occur in the areas of large reservoirs are referr as reservoir induced earthquakes. Above may also be referred as various causes of earthquakes with one and two being the natural causes of earthquakes while 3, 4 and 5 represent anthropogenic or man made cause of earthquakes.
The waves generate by an earthquake are called the ‘seismic waves’ or ‘earthquake waves’. Which are record by an instrument called a seismograph or the seismometer. For further understanding of earthquake waves, refer to the portion of the notes on interior of earth.
Depth of Earthquakes
Earthquakes focus depth is an important factor in shaping the characteristics of the waves and the damage they inflict. The focal depth can be deep, intermediate or shallow. Deep focus earthquakes are rare destructive because the wave amplitude is greatly attenuated by the time it reaches the surface. Shallow focus earthquakes are more common and extremely damaging because of their closed proximity to the surface.
Measurement of Earthquakes
Its events are scaled either according to the magnitude or intensity of the shock.
1. Magnitude Scale
Magnitude is the amount of energy release and is based on the direct measurement of size of seismic waves. The magnitude scale is call as the Richter Scale. The Richter Magnitude Scale was developed in 1935 by Charles F. Richter as a mathematical device to compare the size of earthquakes.
The magnitude of an earthquake is determined from the logarithm of the amplitude of waves recorded by seismographs. Because of the logarithmic basis of the scale, each whole number increase in magnitude represent at 10 fold increase in measured amplitude, as an estimate of energy, each whole number step in the magnitude scale correspond to the release of about 31 times more energy than the amount associated with the preceding world number value.
2. Intensity Scale
Intensity of an earthquake is measures in term of its effect on human life. The intensity of an earthquake at a specific location depends on a number of factors. Some of them are:
– total amount of energy released,
– distance from the epicentre,
– types of rocks and the degree of consolidation.
The Mercalli intensity scale is a scale for measuring the intensity of an earthquake. The scale quantifies the effects of an earthquake on the earth’s surface, human, objects of nature and man made structure on a scale of I through XII, with I denoting ‘not felt’ and XII ‘total destruction’. Data gather from individual who have experience the quack, and an intensity value will be given to their location.
Distribution of Earthquakes
Most earthquakes in the world associates with the following:
– zones of young fold mountains,
– zones of faulting and fracturing,
– the zones representing the junctions of continental and oceanic margins,
– the zones of active volcanoes,
– along the different plate boundaries.
Seismic Belts of the world
1. Circum-Pacific Belt
The Belt includes the coastal margins of North America, South America and East Asia. These are as represent the eastern and western margins of the Pacific Ocean respectively. Which account for about 65% of the total earthquakes of the world. The Western marginal zones are represented by the Rockies and Andes mountain chains. These are also zones of convergent plate boundaries where the Pacific oceanic plate is subducted below the American plates. The Eastern marginal zones represents the island are of Kamchatka, Sakhalin, Japan and Philippines. The earthquakes are used to collision of the Pacific and the Asiatic plates and the consequent volcanic activity. Japan records about 1500 seismic shocks every year.
2. Mid-Continental Belt
The Mid-Continental Belt includes the Alpine mountains and their of shoots in Europe, Mediterranean Sea, Northern Africa, Eastern Africa and the Himalayas. The mid-continental belt extent through Sulaiman and Kirthar zones in the west. The Himalayas in the north and Myanmar in the east. The belt represents the weaker zone of Fold Mountains. About 21% of the total seismic events are record in this belt.
3. Mid-Atlantic Ridge Belt
The Mid- Atlantic Ridge Belt includes the Mid-Atlantic Ridge and several islands near the Ridge which helps in recording moderate earthquakes which occur due to the moving of plates in the opposite directions. Thus the sea floor spreading and the fissure type of volcanic eruptions cause earthquakes of moderate intensity in this region.
Seismic Zones of India
The Indian sub -continent is highly prone to multiple natural disasters including earthquakes, which is one of the most destructive natural hazards with the potentiality of inflicting huge loss to lives and property. Earthquakes pose a real threat to India with 59% of its geographical area vulnerable to seismic disturbance of varying intensity including the capital city of the country. The varying geology at a different locations in the country implies that the lively hood of damaging earthquakes taking place at different location is different. That’s why, a seismic zones map is in need so that buildings and other structures located in different regions can be design to withstand different level of ground shaking.
Effects of Earthquakes
1. Deformed Ground Surface
The earthquake tremors and the resultant vibrations, result in the deformation of the ground surface and Due to the rise and subsidence of the ground surface and faulting activity. The alluvium fill areas of the flood planes may develop facture at several places.
2. Damage to man-made structures
Man-made structure such as buildings, roads, rails, factories, dams, bridges etc gets severely damage.
3. Damage to towns and cities
The towns and cities affect worst as due to a high density of buildings and population. Under the impact of tremors, large buildings collapse. Men and women get trap under the debris. Ground water pipes become useless and thus water supply is totally comes to halt.
4. Loss of human and animal life
The destructive power of an earthquake depends upon the loss it can cause in term of loss of life arid property. The Bhuj earthquake of India in 2001 caused over one lakh human casualties.
5. Flash Floods
Very strong seismic events result in the collapse of dams and cause severe flash floods. Floods also causes when tremors blocks the flow of water in the rivers. Sometimes the main course of the river get changes due to the blockage.