Earthquakes

Sakshi Education

I. Introduction:

1. Natural Disasters :

These are uncontrollable events that cause sudden and massive destruction, like the 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.

2. Internal Security Concern :

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.

3. Human Survival :

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.

4. Earthquakes:

A series of shock waves in the earth's crust or upper mantle. They are the earth's natural means of releasing stress.

5.Cause of Earthquakes:

a. 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.

b. Plate Movements:
Earthquakes are also caused by the movement of earth's plates- large, relatively rigid segments of the lithosphere (the solid, 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.

6.Earthquake Waves:

The shock waves which cause the earthquakes can be classified into two broad categories.

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.

a. 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. The P waves can travel through a solid as well as a liquid where as the S waves can travel only through a liquid medium.

b. 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 earthscientists in locating the epicentre of the earthquakes.

7.Earthquake Terminology:

A.Plate Tectonics:

According to this theory the upper most 60-90 km thick layer of the earth called lithosphere, 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.

B.Focus:

The centre of the earthquake, which is the source of seismic waves produced during an earthquake.

C.Epicentre:

The point on the earth's surface situated directly above the focus of an earthquake.

D. Intensity:

The measure of the ground shaking and damage to the surface and the effects on humans.

E. Magnitude:
The quantity to measure the size of an earthquake in terms of its energy.

F.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.

G. 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.

H. 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 mainshock fault- a fracture in the rock- or near it. Bigger earthquakes have more and larger aftershocks.

J. Major Earthquakes since 1902:

DATE

LOCATION

DEATHS

1976

China

?xml:namespace>

255,000

1920

China

?xml:namespace>

200,000