• Earthquakes are vibrations of the earth caused by ruptures and sudden movements of rocks that have been strained beyond their elastic limits. 
  • It can be considered as a form of energy of wave motion transmitted through the surface layer of the earth. 
  • The place of origin of earthquake inside the earth is known as Focus or Hypocenter. 
    Epicenter: It is the point on the earth's surface vertically above the focus, where shock waves reach first. It is the most affected area. 
    Seismograph: It is an instrument which records the waves generated by an earthquake. 
    Isoseismal line: The line joining places which experience equal seismic intensity. 

Causes of Earthquakes 

  • Volcanic Eruptions 
  • Faulting 
  • Plate Tectonics 
  • Anthropogenic Factors

Distribution of Earthquakes 

Major earthquake zones are: 

  • The Circum-Pacific Zone: This is a convergent plate boundary zone where most widespread intense earthquake occurs along subduction zone. Above 66 per cent of the total earthquakes of the world are recorded in this zone. 
  • The Mid-Atlantic Zone: This zone, characterised by the divergent plate boundary, spreads over the Mid-Atlantic ridges and several islands near it. 
  • Mid-Continental Zone: This zone extends along the Alpine mountain system.of Europe, through Asia Minor to the Himalayan mountain system including the mountains of China and Myanmar. 
  • Intraplate Earthquakes: As an exception, these earthquakes occur not along the plate boundaries but in the continental platforms. The earthquakes occurring in peninsular India is a noted example of this class.