CBSE Book Class 11 Disaster Management Natural Hazards

Chapter 2

NATURAL HAZARDS - CAUSES, DISTRIBUTION PATTERN, CONSEQUENCE, AND MITIGATION MEASURES

The discussion on various terminologies has helped us in having a basic understanding of disaster management. However, each hazard has its own characteristics. To understand the significance and implications of various types of hazards we must have a basic understanding about the nature, causes and effects of each hazard type and the mitigation measures that need to be taken up. In this chapter, we would discuss the following hazards namely earthquake, tsunami, landslide, flood, cyclone and drought that we normally face in our country. Earthquake is one of the most destructive natural hazard. They may occur at any time of the year, day or night, with sudden impact and little warning. They can destroy buildings and infrastructure in seconds, killing or injuring the inhabitants. Earthquakes not only destroy the entire habitation but may de-stabilize the government, economy and social structure of the country. But what is an earthquake? It is the sudden shaking of the earth crust. The impact of an earthquake is sudden and there is hardly any warning, making it impossible to predict.

Cause of Earthquake :

The earth’s crust is a rocky layer of varying thickness ranging from a depth of about 10 kilometers under the sea to 65 kilometers under the continents. The crust is not one piece but consists of portions called ‘plates’ which vary in size from a few hundred to thousands of kilometers (Fig 2.1.1). The ‘theory of plate tectonics’ holds that the plates ride up on the more mobile mantle, and are driven by some yet unconfirmed mechanisms, perhaps thermal convection currents. When these plates contact each other, stress arises in the crust (Fig 2.1.2). These stresses can be classified according to the type of movement along the plate’s boundaries:

a) pulling away from each other,

b) pushing against one another and

c) sliding sideways relative to each other.

All these movements are associated with earthquakes.

The areas of stress at plate boundaries which release accumulated energy by slipping or rupturing are known as 'faults'. The theory of 'elasticity' says that the crust is continuously stressed by the movement of the tectonic plates; it eventually reaches a point of maximum supportable strain. A rupture then occurs along the fault and the rock rebounds under its own elastic stresses until the strain is relieved. The fault rupture generates vibration called seismic (from the Greek 'seismos' meaning shock or earthquake) waves, which radiates from the focus in all directions.

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