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 Why do Earthquake Waves develop Shadow Zones? Also explain how earthquake waves help in studying interior of the earth. (10 marks)
Approach:
  • Introduce with quakes and shadow zones
  • Explain why shadow zones are formed - use separate diagrams from P-waves and S-waves
  • Explain how studying quake waves helps study interior of earth - like shadow zones, refraction of P-waves etc.
  • Conclude appropriately
Model Answer :
A seismic shadow zone is an area of the Earth's surface where seismographs can barely detect an earthquake after its seismic waves have passed through the Earth.
Development of Shadow Zones:
When earthquakes occur, they produce primary (P) and secondary (S) waves (called compression and shear waves sometimes). P waves can travel through both liquid and solid materials, while S waves cannot travel through liquid materials. Therefore, when these waves hit the liquid outer core of Earth, P-waves are refracted and S-waves are stopped, creating  shadow zones. There are 2 different types of shadow zones. The P-wave shadow zone is 105-142 (or 145) degrees and the S-wave shadow zone is 105-105 degrees. A zone between 105°  and 145°  from epicentre was identified as the shadow zone for both the types of waves.
Interior of the earth:
Earth quake waves help in study of interior of the earth, and the following characteristics of earth's interior are revealed by studying earthquake waves:
  • Earth has a layered structure : The seismic waves travel in curved paths, and get reflected and refracted when travelling through the earth. Analysis of the variation in the properties of various waves shows a layered structure in the interior of earth.
  • Earth's interior has varying material density: The study of the speed and direction of the earthquake waves reveals that earth's interior has layers of varying densities.
  • Core is liquid: S waves cannot pass through liquid. It is evident from the S-wave shadow zone that earth's core is a liquid which is located at a depth of 2900 km from surface.

While earthquakes are considered natural disasters due to the devastation they cause, measuring and calculating the behaviour of earthquake wave on seismograph has also helped us study the interior structure of earth.
Subjects : Geography