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DAILY NEWS ANALYSIS

  • 02 May, 2021

  • 20 Min Read

Everything about: Earthquake

Everything about: Earthquake

What is an Earthquake?

  • A fault is a sharp break in the crustal rocks. When lithospheric plates move, the surface of the Earth vibrates (release of Energy and the Energy waves travel in all directions). An earthquake is the sudden release of the Energy in the Earth’s crust that creates seismic waves.
  • The energy accumulation site is identified w deformed rocks caused by tension or compression.
  • The subterranean spot at which rocks begin to shift/rupture is Focus or Hypocenter of Earthquake, whereas the point vertically over the Focus is Epicenter, which experiences the 1st waves and the greatest damage which decreases as we go outwards.
  • The waves generated by an Earthquake are called seismic waves recorded by an instrument Seismograph. The magnitude (Energy that is released) of the Earthquake is measured by Richter Scale whereas the intensity (Damage caused) is measured by Mercalli Scale.
  • During Earthquake, the rocks in the path of P waves get compressed/ expanded in the direction of propagation so it affects their volume rather than shape. In the case of S waves, it changes the shape and not volume.
  • Earthquakes are by far the most unpredictable and highly destructive of all-natural disasters due to their suddenness. Earthquakes that are of tectonic origin have proved to be the most devastating and their area of influence is also quite large than other causes.

Body Waves

Body waves are generated due to the focus of the Earth and move through the body/interior of Earth in all directions.

  • Primary (P) waves:
    1. They are longitudinal waves so can pass through both solids and liquids.
    2. They travel parallel to the direction of the wave thus it creates density difference in the material leading to stretching and squeezing of the material.
    3. Also as the density of the medium increases, their velocity also increases. But they travel slowly through liquids, so at the depth of 2900 km, they reach liquid molten core so their velocity reduces.
    4. As they reach the inner core (which is solid) their velocity increases again. They are similar to sound waves.
  • Secondary (S) waves:
    1. They are transverse waves so can't pass through liquids.
    2. They travel perpendicular to the direction of the wave thus it creates crests and troughs.
    3. They travel to a depth of 2900 km after which they get deflected since they reach the outer core which is liquid.

Surface Waves

  • The body waves interact with the surface rocks and generate a new set of waves which move along the surface, thus called Surface waves.

Love (L) waves

  • They are surface waves and don't go deeper into the earth. The travel is perpendicular to the direction of propagation.
  • L waves are the most destructive. In L waves movement of particles takes place in the horizontal plane only but @ 90º to the direction of propagation of the wave.
  • L waves move like a Snake. The surface waves get significantly amplified when they pass through soft ground like alluvial deposits.
  • There is compression and rolling over of soft alluvial deposits which is called liquefaction.

Raleigh (R) waves:

  • R waves are analogous to sea waves i.e. movement of particles takes place in the vertical plane. L waves are faster than R waves so the sequence of arrival is PSLR.

Causes of Earthquakes

  • Plate Movements: Ex. The Himalayan region has C-C convergence.
  • Faulting and Folding: Ex. Bhuj & Latur earthquake.
  • Volcanic Eruptions.
  • Gaseous Expansion and Contraction inside the Earth
  • Hydrostatic pressure (Ex. Reservoir induced) Ex. Koyna Dam Earthquakes in MH.
  • Anthropogenic Causes: Mining & drilling.

Types of Earthquakes

  • Tectonic EQs: Most common. Generated due to sliding of rocks along fault lines.
  • Volcanic EQs: A special class of Tectonic EQ confined to the areas of volcanoes.
  • Induced EQs: Occurs in the areas of large reservoirs.

Distribution of Earthquakes in the World

  • The World's distribution of Earthquakes coincides very closely with that of volcanoes.
  • ~70% of Earthquakes occur in the Circum Pacific belt. ~20% occur in the Meditteranean- Himalayan belt including Asia Minor, Himalayas and parts of Northwest China. Elsewhere Earth's crust is relatively stable and is less prone but not immune to earth tremors.

Earthquake-prone areas in India

  • The entire region covering fourteen states (located in western and central Himalayas, northeast, and parts of the Indo-Gangetic basin) is highly prone to earthquakes.
  • Some of the most vulnerable states are J&K, HP, UK, SK and Darjeeling, all NE states.
  • The hilly regions are also prone to earthquake-induced landslides. The other seismically active regions of the country include the Gulf of Khambhat and Rann of Kutch in Western Gujarat (1819, 1956, 2001), parts of peninsular India like MH (1967, 1993), the islands of LD and A&N.

Impacts of Earthquake

1) Impact on the ground:

  • Slope instability and Landslides or Avalanches (often cause obstructions in the flow of rivers and channels resulting in the formation of reservoirs).
  • Liquefaction, Fires in some areas, Deformation on ground surface.
  • Fissures on the upper layers of the earth’s crust through which water and other volatile materials gush out, inundating the neighbouring areas.
  • Sometimes, rivers also change their course causing floods and other calamities in the affected areas.

2) Impact on

  • manmade infrastructure: Damage to settlements, infra, industries.
  • On the water: Flash floods, Tsunamis (waves generated by tremors and not an EQ), Hydro-Dynamic Pressure.
  • On Biodiversity: Loss of human and animal lives, Robs the population of their material and socio-cultural gains that they have preserved over generations. It renders them homeless, and unemployed, increases poverty.

Earthquake Disaster Management:

  • In our present state of knowledge, earthquakes can neither be prevented nor predicted in terms of their magnitude, or place and time of occurrence. Also, unlike other disasters, the damages caused by it are more devastating.
  • So, the most effective measures of risk reduction are pre-disaster mitigation, preparedness & preventive measures. Since it also destroys transport and communication links, providing timely relief to the victims becomes difficult. Hence the above methods should be combined w expeditious & effective rescue and relief actions immediately after the occurrence of the earthquake.
  • Establishing earthquake monitoring centres for regular monitoring and fast dissemination of information among the people in the vulnerable areas. Use of GPS can be of great help in monitoring the movement of tectonic plates.
  • Preparing a vulnerability map of the country and dissemination of vulnerability risk information among the people.
  • Educating the people about the ways and means minimizing the adverse impacts of disasters.
  • Modifying the house types and building designs in the vulnerable areas and discouraging the construction of high-rise buildings, large industrial establishments and big urban centres in such areas.
  • Finally, making it mandatory to adopt earthquake-resistant designs and use light materials in major construction activities in vulnerable areas.

Source: Aspire IAS Notes, NCERTs


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