EARTHQUAKES IN INDIA
Earthquakes are vibrations of the Earth caused by ruptures and sudden movements of rocks that have been strained beyond their elastic limits. In other words, earthquakes are movements within the earth caused by natural or man made stresses. Earthquakes are casued by (i) volcanic eruptions, (ii) ruptures and sudden movements of rocks (folding and faulting), (iii) movement of plates (plate tectonics), and (iv) anthropogenic factors.
The structure of the subcontinent oflndia is characterised with plate-boundaries in which folding, and faultings are a regular feature. Man has constructed numerous dams and reservoirs across rivers. All these factors, individually or collectively lead to earthquakes in the country. Consequently,
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Fig. 2.23 Lakshadweep Islands
every year numerous earthquakes are recorded in the Himalayan region, the Great Plains of India, Purvanchal and the Islands of Andaman and Nicobar. Some of the important earthquakes of great magnitude have been plotted in Fig. 2.24. It may be seen from this figure that most of earthquakes of high magnitude occurred in the folded mountains of Himalayas. Their frequency is, however, low in the relatively more stable Peninsular India. On the basis of seismic intensity and frequency, a committee of experts comprising seismologists and geologists have divided India into six seismic zones (Fig. 2.25).
About 60 per cent of the total area of the country is under moderate and high seismic zones. A belt of about 3500 km length and about 500 km width in north India is the main earthquake prone zone of India, characterised with devastating earthquakes of high magnitude. Some of the important earthquakes recorded in India during the last one hundred years have been plotted in Fig- 2.24.
