- 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
- 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.