(Sample Material) UPSC Mains GS Online Coaching : Paper 1 - "Important Geophysical Phenomena"

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Subject: General Studies (Paper 1 - Indian Heritage and Culture, History & Geography of the World & Society)

Topic: Important Geophysical Phenomena


Natural Hazards and Disasters

Man has been facing natural hazards and disasters since the dawn of civilisation. The earth has been undergoing various changes; both slow and catastrophic from the very beginning. It is worth understanding some of the basic terms before we make a detailed study of the natural hazards and disasters and their effects on human beings.


Changes that affect humans adversely are called hazards. A hazard comprises a dangerous condition or event, natural or manmade, that could cause injury, loss of life or damage to property, livelihood or environment. In other words, hazards are defined as “phenomena that pose n threat to people, structures or economic assets and which may cause a disaster. They could be either manmade or naturally occurring in our environment.” (Disaster Preparedness Training Manual, Philippine National Red Cross 1954). A natural disaster pertains to a natural phenomenon which occur in proximity and pose a threat to people, structures and economic assets caused by biological, geological, seismic, hydrological or meteorological conditions or processes in the natural environment.

Types of Hazards

The list of hazards is very long and hazards can be classified in a number of ways. Generally hazards are classified depending upon their causes, mitigation strategies or their effects on societies. We shall confine ourselves to classification of disasters depending upon their causes only.

Types of Hazards depending upon their causes

Hazards could be caused by nature or man and are consequently called natural and man made. A third type of hazards are caused both by nature and man are called socio-natural hazards.

1. Natural Hazards. These are caused by the forces of nature and man has no role to play in such hazards, following are the main examples of natural hazards:
• Earthquakes
• Volcanic eruptions
• Cyclonic storms
• Tsunamis
• Floods
• Drought
• Landslides


Earthquake is a violent tremor in the earth’s crust, sending out a series of shock waves in all directions from its place of origin. If you throw a stone in a pond of still water, a series of concentric waves are produced on the surface of water. These waves spread out in all directions from the point where the stone strikes the water. Similarly, any sudden disturbance in the earth’s crust may produce vibrations in the crust which travel in all directions from the point of disturbances. Earthquakes constitute one of the worst natural hazards which often turn into disaster causing widespread destruction and loss to human lives.

Magnitude and Intensity of Earthquakes. Magnitude and intensity are two ways of measuring the strength of an earthquake. Earthquake’s magnitude is determined by the use of seismograph, an instrument which continuously records, ground vibration. It is measured on the Richter scale. This scale was developed by Charles Francis Richter in 1935 and is known after his name. It was modified in 1965 by Richter and his colleague Beno Gutenberg. It is an instrumentally measured scale and is a measure of the amount of force or energy released at the earthquake source. It is calculated from the amplitude measured by a standard Wood-Anderson seismograph and corrected fh terms of a seismograph supposed to be kept at a distance of 100 km from the epicentre. The number indicating magnitude or intensity on Richter Scale ranges between 0 and 9 but actually the scale has no upper limit of number because it is an open ended and logarithmic scale. Each whole number on this scale represents 10-fold increase in the measured wave amplitude. Translated into energy, each whole number demonstrates a 31.5 times increase in amount of energy released. Thus a 3 on the Richter Scale represents 31.5 times more energy than a 2 and 992 times more energy than a 1. The vibration of an earthquake with a magnitude of 2 are 10 times greater in amplitude than those of an earthquake with a magnitude of 1, and the vibrations of an earthquake with a magnitude of 8 are one million times greater in amplitude than those of an earthquake with a magnitude of 2. The largest earthquake ever recorded had a magnitude of 8.9 on the Richter Scale. It is worth noting that earthquakes larger than this magnitude are not likely to occur because rocks are not strong enough to accumulate more energy. A 2 point quake^ is barely perceptible, a 5 may cause damage to structures, a 7 is severe and an 8 is a violent quake.

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