Earthquakes are by far the most unpredictable and highly destructive of all the natural disasters. Earthquakes that are of tectonic origin have proved to be the most devastating and their influence is also quite large. These earthquakes results from a series of earth movements brought about by a sudden release of energy during the tectonic activities in the earth’s crust.
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As compared to these the earthquakes associated with volcanic eruption, rock fall, landslides, subsidence particularly in the mining areas, impounding of dams and reservoirs etc. have limited area of influence and the scale of damage. The Indian plate is moving at a speed of one centimeter per year towards the north and north eastern direction and this movement of plates is being constantly obstructed by the Eurasian plate from the north.
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As a result of this both the plates are said to be locked with each other resulting in accumulation of energy at different points of time. Excessive accumulation of energy results in building up of stress, which ultimately leads to the breaking up of the rock and the sudden release of energy causing earthquakes along the Himalayan arch.
Distribution Pattern of Earthquakes in India:
India falls quite prominently on the ‘Alpine – Himalayan Belt’. This belt is the line along which the Indian plate meets the Eurasian plate. This being a convergent plate, the Indian plate is thrusting underneath the Eurasian plate at a speed of 5 cm per year.
The movement gives rise to tremendous stress which keeps accumulating in the rocks and is released from time to time in the form of earthquakes. The seismic zoning map of India is divided into four zones namely Zone II, III, IV and V, with zone V being most vulnerable to earthquakes. Much of India lies in zone III. New Delhi the capital city of India lies in zone IV where as big cities like Mumbai and Chennai are in zone III.
Some of the most vulnerable states are Jammu and Kashmir, Himachal Pradesh, Uttranchal, Sikkim, and the Darjeeling and subdivision of West Bengal and all the Seven States of North-east. Apart from these regions, the Central-Western parts of India, particularly Gujarat and Maharashtra have also experienced some severe earthquakes.
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Recently, some earth scientists have come up with a theory of emergence of a fault line and energy build-up along the fault line represented by the River Bhima (Krishan) near Latur and Osmanabad (Maharashtra) and the possible breaking down of the Indian plate.
Geological Survey of India has divided India into five zones as show in fig.
(i) Very low damage risk zone
(ii) Low damage risk zone
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(iii) Moderate damage risk zone
(iv) High damage risk zone
(v) Very high damage risk zone
General Characteristics of Earthquake:
Earthquake vibrations occur in a variety of frequencies and velocities. The actual rupture process may last for a few seconds to as long as one minute for a major earthquake. The ground shaking is caused by ‘body waves’ and ‘surface wave’.
i. Deep: – 300 to 700 kms from the earth surface.
ii. Medium: – 60 to 300 kms
iii. Shallow: less than 60 kms
The deep focus earthquakes are rarely destructive because by the time the waves reach the surface the impact reduces. Shallow focus earthquakes are more common and are extremely damaging because of their proximity to the surface.
Effects of Earthquakes:
1. Loss of Life and Property:
There is devastating loss of life and property if the intensity of the earthquake is more that 5 on the Richter scale. Building, roads, railways, bridges, dams etc. suffer sever damage when an earthquakes Strikes them.
2. Topographical Changes:
The main effects of earthquakes on topographical features are seen in the form of offsets along faults, fissures, scarps, elevation and depressors of coasts, etc. Earthquake are often followed by landslides in hilly areas. Due to earthquake vibrations, the looser material at or near its Maximum Static Stable angle may become unstable and move along the slope of hill.
3. Liquefaction:
Soil liquefaction is a phenomenon where low density saturated sands of relatively uniform size inside the earth start behaving like a Jelly with no strength to hold a building up, and the building just sinks or gets tilted on one side.
The phenomenon of liquid faction is particularly important for dams, bridges, underground pipelines and building close to river bank, seashore or large lakes. Vast tracts of plains of Uttar Pradesh and Bihar where soil is generally soft and water table is high, offer favourable condition for such effects.
Earthquake Hazard Mitigation:
It is not possible to prevent the occurrence of an earthquake; hence the next best option is to emphasist on disaster preparedness and mitigation rather than curative measures.
(a) Establishing earthquake monitoring centers for regular monitoring and fast dissemination of information among the people to the vulnerable areas. Use of GPS can be of great help in monitoring the movement of tectonic plates.
(b) Preparing a vulnerability map of the country and dissemination of vulnerability risk information among the people and educating them about the ways and means for minimising the adverse impacts of disasters.
(c) Modifying the house type and building-design in the vulnerable areas and discouraging construction of high-rise buildings, large industrial establishments and big urban centers in such areas.
(d) Finally, making it mandatory to adopt earthquake resistant designs and use light materials in major construction activities in the vulnerable areas.