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Sansad TV: The Science of Earthquakes

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Earthquakes:

  • They are the result of sudden movement along faults within the Earth. The movement releases stored-up ‘elastic strain’ energy in the form of seismic waves, which propagate through the Earth and cause the ground surface to shake.
  • Such movement on the faults is generally a response to long-term deformation and the build-up of stress.

Factors that cause earthquakes

  • The Earth’s crust consists of seven large lithospheric plates and numerous smaller plates. These plates move towards each other (a convergent boundary), apart (a divergent boundary) or past each other (a transform boundary).
  • Earthquakes are caused by a sudden release of stress along faults in the earth’s crust.
  • The continuous motion of tectonic plates causes a steady build-up of pressure in the rock strata on both sides of a fault until the stress is sufficiently great that it is released in a sudden, jerky movement. Earthquakes caused by plate tectonics are called tectonic quakes.
  • Induced quakes are caused by human activity, like tunnel construction, filling reservoirs and implementing geothermal or fracking projects. The earthquakes that occur in the areas of large reservoirs are referred to as reservoir induced earthquakes.
  • Volcanic quakes are associated with active volcanism. They are generally not as powerful as tectonic quakes and often occur relatively near the surface. Consequently, they are usually only felt in the vicinity of the hypocenter.
  • Collapse quakes can be triggered by such phenomena as cave-ins, mostly in karst areas or close to mining facilities, as a result of subsidence.
  • Ground shaking may also occur due to the explosion of chemical or nuclear devices. Such tremors are called explosion earthquakes.

Why Earthquakes more common in certain parts of the world than others

  • Over 90% of earthquakes – including almost all of the largest and most destructive ones – happen at or near so-called plate boundaries, where the 15 or so major subdivisions (“plates”) of the Earth’s crust and uppermost mantle move towards, alongside, or away from each other.
  • Most of the plates’ movement is focused at these boundaries, so large earthquakes far away from these boundaries are much less common.
  • Earthquakes can strike any location at any time, but history shows they occur in the same general patterns year after year, principally in three large zones of the earth:
    • Circum-Pacific seismic belt: The world’s greatest earthquake belt is found along the rim of the Pacific Ocean, where about 81 per cent of our planet’s largest earthquakes occur. It is also known as “Ring of Fire”.
    • Alpine earthquake belt: It extends from Java to Sumatra through the Himalayas, the Mediterranean, and out into the Atlantic. This belt accounts for about 17 percent of the world’s largest earthquakes.
    • Submerged mid-Atlantic Ridge: The ridge marks where two tectonic plates are spreading apart (a divergent plate boundary).

Conclusion

  • Unlike other disasters, the damages caused by earthquakes are more devastating.
  • Since it also destroys most of the transport and communication links, providing timely relief to the victims becomes difficult.
  • It is not possible to prevent the occurrence of an earthquake; hence, the next best option is to emphasis on disaster preparedness and mitigation rather than curative measures.