Earthquakes originate due to various reasons which fall into two major categories viz. non-tectonic and tectonic.

(1) Non-Tectonic Causes :

The non-tectonic causes of earthquake in the diagram given below indicates the isoseismal lines and their relation to the epicentre and to the wave paths radiating from the focus of an earthquake.

elude those associated with the geological agents operating upon the surface of the earth, volcanic eruptions as well as with the collapse of subterranean cavities.


(a) Surface causes Perceptible vibrations may set up by :

(i) the dashing waves and crashing breakers along the sea­shores;

(ii) abrupt descending of running water from a higher altitude as in the case of water-falls in adjacent areas;

(iii) rock falls and avalanches in mountains, large landslides etc. Apart from the above, there are also artificial surface-causes which produce perceptible tremors, for example, underground explosion of bombs, passage of trains and tanks, working of heavy machinery in industrial areas, explosion in mines, failure of dams under the pressure of the impounding water etc.


(b) Volcanic-causes : Volcanic earthquakes occur around active volca­noes mainly due to explosive eruption and also due to the hydraulic shocks of magma that forcibly fills underground chambers and chan­nels. A shock may also be produced by any of the following reasons:

(i) explosion of the volcano upon the release and expansion of gases and lava,

(ii) faulting within the volcano resulting from pressures in the chamber of molten rock, and

(iii) collapse of the centre of the volcano into the space formed by extrusion of gases and molten magmatic materials.


Usually volcanic eruptions are preceded or accompanied by earthquake. Such earthquakes occur only occasionally and are more localised both in its intensity and damage.

(c) Collapse of Subterranean Cavities: Sometimes because of the removal of support from below, by the action of underground water, the ground surface subsides or collapses suddenly producing local tremors. This is usually noticed in the caverns of Karst areas.

(2) Tectonic Causes

About 95 per cent of all the earthquakes are due to sudden earth movements along existing or new faults. The association of earthquakes with fault-lines is an established fact. As such, earth­quakes caused by faulting or folding in the crust are known as tectonic earthquakes.


The term tectonic (Greek word tekton means a builder) refers to the structural changes of the crust due to deformation or displacement. Such earthquakes generally result from sudden yielding to strain produced on rocks by accumulating stresses.

Prof. H.F. Reid has proposed the Elastic Rebound Theory to explain the origin of tectonic earthquakes. According to this theory materials of the earth, being elastic, can withstand a certain amount of stress without undergoing a permanent deformation.

When the stress exceeds the elastic limit a crack or fracture is developed. Frictional resistance along the fracture prevents the fractured blocks from being slipped off from each other and thus promotes a build up of strain. With the superimposition of more stress the rock units on either side of the fracture are subjected to enormous strain and there comes a stage when the rocks can not bear more straining i.e. at the time when the frictional resistance is overcome.

At this stage, there occurs a sudden slipoff of the fractured blocks to position of no strain. Thus the energy stored in the system through decades is released instanteneousiy causing underground dislocation of rocks and waves of energy are sentout through the earth. These waves of energy are called seismic waves that set up vibrations on the earth’s surface.


The crack or fracture alongwhich the displacement of rocks occurs is known as a fault. Thus, according to the “Elastic Rebound Theory”, earthquakes are commonly associated with movement along a fallt.

The preceding diagram, indicates the elastic rebound theory. Most of the earthquakes probably take place due to tectonic reasons, which is further supported by the fact that the regions of intense and frequent earthquakes coincide with those characterised by new intense tectonic movements.