An Earthquake is a sudden vibration or oscillation in the Crust of the Earth. It is a form of energy of wave motion transmitted through the surface layer of the Earth. The place of origin of an Earthquake in the Earth’s crust is called ‘Focus’. The point on the Earth surface which is perpendicular to the focus, receiving seismic waves is called ‘Epicentre’.

Causes of Earthquake: On the basis of occurrence earthquakes can be classified into three types. They occur due to several causes such as tectonic forces, folding, faulting, volcanic eruptions landslides, avalanches, man-made factor.

Tectonic Earthquakes: These are caused due to folding, faulting and displacement of blocks of rocks in the Earth’s crust. They are highly intensive and destructive seismic activities.

Volcanic Earthquakes: These are associated with the volcanic activities and are usually caused at great depths. They are with low intensity and magnitude. The destruction and damage are slightly lesser than earthquakes caused by Tectonic forces.

c. Earthquakes caused by man-made factors: Over interaction of man underground nuclear explosion etc, are causing great change in the crustal formation leading to seismic activity. E.g. 

Koyna dam (India)-1967, Hoover dam (U.S.A)-l 935, Mangladam (Pakistan). Effects of Earthquakes: Earthquakes cause violent disturbances and widespread damage and destruction, especially on the epicentre region.

• Earthquakes cause changes in the earth’s surface. Cracks and fissures develop in the earth’s crust. 
• Vibrations, caused by earthquakes, lead to landslides in mountainous regions, damming the rivers. 
• vertical or horizontal cracks are formed, and the land is elevated or lowered, forming lakes. 
• Deep cracks may be developed in dams after the earthquake. For instance, deep cracks were developed in the Koyna dam after the earthquake. 
• The beds of rivers may be raised by the earthquakes. For example, the bed of the river Brahmaputra was raised during the Assam earthquakes. 
• here is the greatest danger of falling of houses, roads railway lines factories, bridges, dams, aerodromes, and other man-made structures, resulting in loss of life and property. 
• The underground water system is disturbed.