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Earthquakes in Delhi: A capital problem


Introduction
As per the Indian seismic zone map, Delhi and its surroundings are placed in what is termed, seismic zone IV (zone V implies the highest and zone IV is reasonably expected to sustain a shaking of Modified Mercalli Intensity VIII (on a scale of I to XII) in the future. MM Intensity VIII has been defined as:

"Damage slight in specially designed structures; considerable in ordinary substantial buildings with partial collapse; very heavy in poorly built structures. Panel walls thrown out of framed structures. Fall of chimneys, factory stacks, columns, monuments, and walls. Heavy furniture overturned. Sand and mud ejected in small amounts. Changes in well water. Persons driving motor cars disturbed,"

Considering many other day-to-day problems of a mega city like Delhi (traffic, environmental pollution, water and power shortage, etc.), the issue of seismic risk for Delhi has not drawn the attention that it deserves. This article is an attempt to put the issue of earthquake problem for Delhi in perspective.

Most death and destruction in an earthquake are caused by collapse of man-made construction. Since the mankind is far from the possibility of being able to successfully predict earthquakes, the only option is to build or houses and other facilities to survive earthquake. Clearly, engineers have to play a major role by ensuring that (a) new constructions are earthquake resistant, and (b) the existing structures are strengthened appropriately to be able to withstand earthquake shaking.

Great earthquakes disasters
Housner (1998) has articulated the issue of great earthquake disasters in a very simple way. Recall that in the 1976 Tangshan (China) earthquake (magnitude 7.8), as per official estimates about 243,000 persons died out of 1,000,000 population while the unofficial estimates on casualty are much higher. This earthquake occurred in a place that was considered to have a rather low seismic hazard and therefore the constructions there were not made earthquake resistant. However , even smaller earthquakes can cause great disaster. For instance, in the 1960 Agadir (Morocco) earthquake resistant. However, even smaller earthquakes can cause great disasters. For instance , in the 1960 Agadir (Morocco) earthquake (M5.7) about 10,000 persons died out population of 30,000. Nearer home, during the 1993 Killari (Latur) earthquake (M6.5) in Maharashtra (India), more than 30% population was killed in several villages. Again, killari (Latur) region used to be considered safe from earthquake view point and placed in the lowest seismic zone (zone I)

As put by Professor Housner, a great disaster may occur if three conditions are met: (a) an earthquake of sufficiently large magnitude, (b) occurrence of the earthquake close enough to a population centre, and (c) the population centre having buildings which are not earthquake resistant. The question we must address is: is there a possibility of these conditions being met for Delhi at some time in the future? In view of huge population and the economic-cum-political significance of Delhi, occurrence of such conditions can cause not just a great disaster; but a mega disaster.


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