This is one of the Natural Disasters. These are uncontrollable events that
cause sudden and massive destruction. For example, Earthquake in Gujarat. The
extent of damage to life and property is so extensive that normal
socio-economic mechanisms of relief are rendered inadequate. Special efforts
are required to handle the situation. Analysts point out that natural disaster
affecting the survival of citizens is as much an internal security concern as
external aggression, internal subversion or centrifugal violence which affect
our country. The ultimate benchmark for security in any civil society, in any country, is
human survival, particularly when faced with natural disasters, according to
analysts. The climatic, territorial and environmental diversity of a large
country like India are bound to be subject to some kind of natural calamity. In
recent times, India has suffered from cyclones, earthquakes, floods and tidals.
Definition: A Earthquake is "A series of shock waves in the earth's crust/upper mantle." They are the earth's natural means of releasing stress. There are several cause of Earthquakes, and they are:
1. Presence of Fault Planes and other Stressful Environments: Geological
studies have revealed that earthquakes are caused by stresses within the earth.
These stresses develop because of instability in the geological formations
below the earth due to the presence of fault planes and other stressful
environments like the internal heat of the earth. Under these conditions the
rock bodies shift releasing energy in the form of shock waves which can cause
earthquakes.
2. Plate Movements: Earthquakes are also caused by the movement of earth's
plates - large, relatively rigid segments of the lithosphere (the solid i.e.,
rocky outer part of the earth's crust). When this stress is increased beyond a
point, the earth's crust is forced to break. The stress is released as energy
which moves through the earth in the form of waves.
The shock waves which cause the earthquakes can be classified into two categories and they are:
Surface Waves: Travel through the surface of the earth.
Body Waves: Travel through the body of the earth getting reflected and
refracted in the process. The body waves can be further divided into the P
(primary) and S (secondary) waves.
1. Different Velocities of Propagation in Different Mediums: The velocities
of propagation differ from one medium to another as the earth is non-homogenous
and layered medium. Thus, P waves can travel through a solid as well as a
liquid where as the S waves can travel only through a liquid medium.
2. Different Velocities of Propagation within the Same Medium: The
propagation velocities of P and S waves are also different within a given
medium, with P waves travelling faster than S waves. This difference in
velocities is made use of by the earth scientists in locating the epicentre of
the earthquakes.
EARTHQUAKE TERMINOLOGY
1. Plate-Tectonics: According to this theory, the upper most 60-90 km thick
layer of the earth called lithosphere, and is divided into several large and
small plates. These plates, which are floating on the lower layer of the earth
called mantle, are constantly in motion and interact with each other. The
interacting margins of these plates are regions of severe deformation causing
structural dislocation and earthquakes.
2. Focus: The center of the earthquake, which is the source of seismic waves
produced during an earthquake.
3. Epicentre: The point on the earth's surface situated directly above the
focus of an earthquake.
4. Intensity: The measure of the ground shaking and damage to the surface
and the effects on humans.
5. Magnitude: The quantity to measure the size of an earthquake in terms of
its energy.
6.Seismograph & Seismogram: The instrument used for recording ground
motions as a function of time is called a "Seismograph" and the
records are known as "Seismograms". Using these Seismograms,
scientists estimate the magnitude, epicentre and focal depth of the earthquake.
7. Richter Scale: A logarithmic scale used for comparing the magnitude of
earthquakes. It was invented by an American Seismologist, Charles Richter in
1935.
Magnitude represents the amount of energy released by an earthquake as
determined by measurements on standardised instruments. The scale ranges from 0
to 10. On this scale, an earthquake of magnitude 8 represents seismograph
amplitudes ten times larger than those of Magnitude 7. The largest earthquakes
recorded were of magnitude of 8.9 (Lisbon, 1755) on the Richter scale and the
smallest about minus three.
Mercalli Scale: It is more subjective in assessing the effect of earthquakes. An earthquake registering 5 on the Mercalli scale is defined as having made furniture to shake and church bells ring, but triggering little or no damage. But an earthquake measuring 12 on the Mercalli scale would have destroyed all man-made objects; and created new topography by forming new lakes, huge falls of rock and major earth faults. Russia has a 12 point scale and Japan a seven-point system.
I. Aftershocks: Earthquakes triggered either on the main-shock fault- a fracture in the rock- or near it. Bigger earthquakes have more and larger aftershocks.
In 1993, in India (Latur), 9,748 people are affected. And, in 2001, in India ( Gujarat), 13,800 are affected.