An earthquake is a motion of the ground surface, ranging from a faint tremor to a wild motion capable of shaking buildings apart and causing gaping fissures to open up in the ground.

The earthquake is a form of energy of wave motion transmitted through the surface layer of the earth in widening circles from a point of sudden energy release-“the earthquake focus”. It is, how­ever, evident that no earthquake can possibly originate at a mere point alone.

I. Causes of Earthquakes:

1. Tectonic earthquakes:


Earthquakes are produced by sudden movements along faults, and are mostly, therefore of tectonic origin. The concept of possible mode of origin of tectonic earthquakes is known as ‘elastic-rebound theory’.

Such earthquakes generally result from sudden yielding to strain’, produced on the rocks by accumulating stresses. This causes the breaking of rocks and produces relative displacement of rocks.

Such faulting causes shaking because displacement of rocks can only be possible by overcoming frictional resistance against the walls of the fault-plane. The association of earthquakes with fault- lines is an established fact.

Elastic-rebound theory:


According to Prof H.F. Reid, mate­rials of the earth, being elastic, can withstand a certain amount of stress without deforming permanently, but if the stress is continued for a long period of time, or if it is increased in magnitude, the rocks will first take a permanent deformation or strain and eventually rupture.

A fault is a break or fracture in the materials of the earth along which there has been displacement. When the rupture occurs, rocks on either side of the fault tend to return to their original shape because of their elasticity and an elastic rebound occurs. It is this rebound that sets up the seismic waves.

Thus the energy stored in the system through decades, is releas­ed instantaneously causing underground dislocation of rocks and setting up vibrations which are feeble or strong, as the case may be.

2. Volcanic earthquakes:


Usually, earthquakes associated with volcanoes are more localized both in extent of damage and in intensity of the waves produced in comparison to those which are associated with faulting motions. A shock may be produced by any of the following mechanisms:

(a) Explosion of the volcano upon the release and expansion of gases and lavas,

(b) Faulting within the volcano resulting from pressures in the chamber of molten rock, and

(c) Collapse of the centre of the volcano into the space formed by the extrusion of gases and molten matter.


Besides the above, sometimes local reasons may cause feeble earthquakes, like failure of dams under the pressure of the impound­ing water etc.

II. Terminologies Associated with Earthquakes:

(a) Focus:

It is the exact spot underneath the earth’s surface at which an earthquake originates. It is also known as Hypocentre.


(b) Epicentre:

It is the point on the surface of the Earth, above the focus of an earthquake.

(c) Isoseismal line or isoseists:

It is a line joining all points at which the intensity of the earthquake is the same. It is, in fact, an isodiastrophic line of equal damage.


(d) Homoseismals or coseismals or homoseists:

These are lines joining the places where the shock arrives at the same time.

III. Types of Earthquakes:

Natural earthquakes are of three types; according to the depth of their origin. They are as follows:

(a) Shallow-focus earthquakes. In this case the seismic shocks originate at a depth of about 30 miles or less, below the earth’s sur­face.

(b) Intermediate-focus earthquakes. In this case the shock- waves originate at a depth between 30 to 150 miles.

(c) Deep-focus earthquake. Here the point of origin of the shock is at a depth between 150 to 450 miles.

According to the origin of the earthquakes, they are also of three types like-tectonic, volcanic and submarine earthquakes. The submarine shocks often generate very large waves on the surface of the seas and destroy the coastal tracts. These submarine earthquakes are known as ‘Tsunamis’.

Three main types of wave-motion are generated by an earth­quake:

(a) Primary or ‘P’ waves:

These are longitudinal waves similar to sound waves and travel in solid, liquid and gaseous media. They have short wavelength and high-frequency.

(b) Secondary or ‘S’ waves:

These are transverse waves, also known as sheer waves, travel only in solid media. In comparison to primary waves, they are slow in motion. They travel at varying velocities through the solid parts, proportional to the density of the materials. They are also having short wavelength and high fre­quency.

(c) ‘L’ waves:

These are transverse vibrations and are con­fined to the outer skin of the crust. They are also known as surface waves or Rayleigh (R) waves. They have low velocity, low frequency and long-wavelength. These are responsible for most of the destruc­tive force of the earthquake.

IV. Scale of Intensity:

Various scales have been proposed to estimate the intensity of earthquake from the amount of damage caused. These scales are:

(i) Rosi-Forrel scale,

(ii) Mercalli-scale, and

(iii) Richter scale of earthquake magnitude etc.

In the Rosi Forrel scale, the intensity has been classified into severe, catastrophic and disastrous.

The Mercalli intensity scale has devised twelve numbers with the increase of intensity. In this case-number 1 detected only by seismographs.

Gradually the number increases when the earthquake intensity becomes feeble, slight, moderate, strong etc. At number ‘8’ it is “destructive”. Similarly it becomes “disastrous” at number ’10’, and at number ’12’, the effect is totally catastrophic, where there is total destruction and objects thrown into air.

In the Richter-scale, the scale number ranges from ‘0’ to ‘9’. Here it is particularly important to notice that a magnitude-‘8’ earthquake is 10 times larger than a magnitude-7 earthquake, 100 times larger than a magnitude-6-earthquake, and 1000 times larger than a magnitude-5-earthquake.

V. Recording of Earthquakes:

The instrument used for recording of seismic shocks is known as ‘Seismograph’, and the records of seismic shocks prepared and presented by seismographs are known as ‘seismograms’.

VI. Distribution of Earthquakes:

The destructive earthquakes are concentrated in a ring surroun­ding the Pacific Ocean. This ring coincides with the Circum-Pacific Ring of Fire.

The second chain is termed as East-Indian, which extends over Indonesia, Andaman-Nicobar, Islands and Burma.

The third belt extends over Himalayas, Kun-Lun, Tien Shan and Altai Range up to the Lake Baikal.

Another belt extends from the Pamir Knot to Afghanistan, Iran, Turkey, Greece, Rumania, Atlas Mountains, Gibralatar and the Azores Islands.

A belt also extends from the Gulf of Aden, between Seychelles and Maladive Islands, turns to the West-South of Africa and goes up to the Falkland Islands.

Another belt also runs along the Great Rift Valley of East Africa.

It is noticed that the present earthquake regions are associated with the younger fold-mountain regions, and the present earthquake activity is a phase of the end of the Alpine-Orogeny.