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Focus: GS-I Geography, GS-III Disaster Management

Why in news?

  • At least 14 people were killed in Turkey and Greece after a strong earthquake struck the Aegean Sea in October 2020, bringing buildings crashing down and setting off tidal waves which slammed into coastal areas and islands.
  • The Earthquake struck with a magnitude of up to 7.0.

Turkey and Earthquakes

  • Crisscrossed by major fault lines, Turkey is among the most earthquake-prone countries in the world.
  • More than 17,000 people were killed in 1999 when a 7.6 magnitude quake struck Izmit, a city southeast of Istanbul.
  • In 2011, a quake in the eastern city of Van killed more than 500.

Aegean Sea

  • The Aegean Sea is an elongated embayment of the Mediterranean Sea located between the Greek and Anatolian peninsulas.
  • In the north, the Aegean is connected to the Marmara Sea and the Black Sea by the straits of the Dardanelles and the Bosphorus.
  • The Aegean Islands are located within the sea and some bound it on its southern periphery, including Crete and Rhodes.
  • The Thracian Sea and the Myrtoan Sea are subdivisions of the Aegean Sea.
  • The Aegean Sea has been historically important, especially in regards to the civilization of Ancient Greece, who inhabited the area around the coast of the Aegean and the Aegean islands.
  • The rocks making up the floor of the Aegean are mainly limestone, though often greatly altered by volcanic activity that has convulsed the region in relatively recent geologic times.


  • An earthquake is shaking of the earth. It is a natural event. It is caused due to release of energy, which generates waves that travel in all directions.
  • The release of energy occurs along a fault. Rocks along a fault tend to move in opposite directions. This causes a release of energy, and the energy waves travel in all directions.
  • The point where the energy is released is called the focus of an earthquake, alternatively, it is called the hypocentre.
  • The point on the surface, nearest to the focus, is called epicentre. It is the first one to experience the waves. It is a point directly above the focus.

Earthquake Waves

All the natural earthquakes take place in the lithosphere.

Earthquake waves are basically of two types Body Waves and Surface Waves.

Body waves

  • Body Waves are generated due to the release of energy at the focus and move in all directions travelling through the body of the earth.
  • There are 2 types of body waves and they are, Primary waves [P] and Secondary [S] waves.
  • Primary waves are the first to appear on the surface and hence the name P waves.
  • P-waves vibrate parallel to the direction of the wave.
  • This exerts pressure on the material in the direction of the propagation.
  • P waves can travel through gaseous, liquid and solid materials.
  • Secondary waves (S waves) appear after P waves.
  • The direction of vibrations of S-waves is perpendicular to the wave direction in the vertical plane.
  • Hence, they create troughs and crests in the material through which they pass.

Surface waves

  • The body waves interact with the surface rocks and generate new set of waves called surface waves. These waves move along the surface.
  • The velocity of waves changes as they travel through materials with different densities. The denser the material, the higher is the velocity.
  • Their direction also changes as they reflect or refract when coming across materials with different densities.
  • Surface waves are considered to be the most damaging waves.

Measurement of earthquakes

The earthquake events are scaled either according to the magnitude or intensity of the shock.

  1. Richter scale – The magnitude scale is known as the Richter scale. The magnitude relates to the energy released during the quake. The magnitude is expressed in absolute numbers, 0-10.
  2. Mercalli scale – The intensity scale is named after Mercalli, an Italian seismologist. The intensity scale takes into account the visible damage caused by the event. The range of intensity scale is from 1-12.
  3. Medvedev–Sponheuer–Karnik scale – This is a macroseismic intensity scale used to evaluate the severity of ground shaking on the basis of observed effects in an area of the earthquake occurrence.

-Source: The Hindu, Hindustan Times

December 2023