Paragraph on Hazardous Effects of Earthquakes !
The hazardous effects of earthquakes do not depend only upon their magnitude and intensity alone but on so many other factors such as population, nature of rocks, type of buildings, etc.
Even a weak earthquake can do great damage if it strikes a densely populated area, an area of weak structure or an area where buildings are weak.
The direct and indirect disastrous effects of earthquakes include deformation of ground surface, damage and destruction to human structures such as buildings, rails, roads, dams, bridges, factories, destruction to towns and cities, loss of human and animal lives, fires, floods, landslides, etc.
Major hazards of earthquakes are briefly described as under:
1. Loss of Life and Property:
There is devastating loss of life and property if the intensity of the earthquake is more than 5 on the Richter scale. Buildings, roads, railways, bridges, dams, etc., suffer severe damage when an earthquake strikes them. Several villages, towns and cities are completely ravaged.
The towns of Bhuj, Bhachau, Anjar, Gandhidham, and Ratnal were completely destroyed by the earthquake which struck Bhuj on 26.1.2001. Property worth Rs. 2,000 crore was destroyed. The maximum damage is noticed near the epicentre of the earthquake. This damage is reduced as we move away from the epicentre. An earthquake becomes hazard or disaster only when it strikes the populated area and causes unaccountable loss to life and property.
2. Topographical Changes:
The main effects of earthquakes on topographical features are seen in the form of offsets along known faults, fissures, scarps, elevation and depression of coasts, etc. Earthquakes 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.
Cracks and fissures which occur as a result of a severe earthquake facilitate landslides. Severe earthquake which struck Uttarkashi in Uttaranchal (1991) caused many cracks and fissures in Varunavat Parvat. This gave birth to landslides which caused heavy damage to Uttarkashi town in the year 2003.
Large masses of dry earth and rock, called earth avalanches, slide over considerable distances. If wet soil slips down the hill side, a slump occurs. Earthflows is another phenomenon which is primarily restricted to ground water. The earthquake is either accompanied or succeeded by a sudden burst of water from a locality where it normally appears in springs.
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 liquefaction is particularly important for dams, bridges, underground pipelines and buildings close to river banks, sea shore or large lakes.
Vast tracts of plains of Uttar Pradesh and Bihar where soil is generally soft and the water table is high, offer favourable conditions for such effects. The great Bihar-Nepal earthquake of 1934 produced a belt of slumping extending from Bettiah in north-west to Purnea in the south-east (a distance of nearly 320 km), surrounding the epicentral tract, in which all buildings were either tilted or sank in soft alluvium.
Widespread subsidence over large area was observed. There were innumerable fissures through which large quantities of sand and water were thrown up to the surface, thereby causing large scale destruction to standing crops. The soil became totally unfit for cultivation. Rann of Kachchh also covers a vast area where large scale liquefaction of ground was observed during the Bhuj earthquake of 2001.