Geography for Kerala PSC: Earthquake Waves

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Kerala PSC Exams

Geography for Kerala PSC: Earthquake Waves

  • All-natural earthquakes take place in the lithosphere. An instrument called ‘seismograph’ records the waves reaching the surface.
  • A curve of earthquake waves recorded on the seismograph. The curve shows three distinct sections, each representing different types of wave patterns.
  • Earthquake waves are basically of two types. They are body waves and surface waves.
  • Body waves move in all directions travelling through the body of the earth.
  • The body waves interact with the surface rocks and generate new set of waves that waves move along the surface, hence called surface waves.
  • The velocity of waves changes according to the density of material through which they travel. As the density increases the velocity of seismic waves decreases.
  • When seismic waves pass through materials with different densities, their direction will change as a result of their reflection or refraction.
  • Reflection causes waves to rebound whereas refraction makes waves move in different directions.
  • These variations in the direction of waves are studied with the help of their record on seismograph.

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 two types of body waves. They are called P and S-waves.

P Wave

  • These are also called ‘primary waves’. The are short wave of high frequency. 
  • P-waves move faster than S wave. It is the first wave to arrive at the surface.
  • The P-waves are Longitudinal similar to sound waves so it can travel through gaseous, liquid and solid materials.
  • P wave has its highest velocity in Solid. But its velocity decreases when it passes through liquid or gas.
  • P-waves vibrate parallel to the direction of the wave.  When p waves pass through a material, it exerts pressure on the material in the direction of the propagation.
  • As a result, it creates density differences in the material leading to stretching and squeezing of the material.

S wave

  • S-waves are transverse wave which is also called secondary waves.
  • S-waves is that they can travel only through solid materials.
  • This characteristic of the S-waves helped scientists to understand the structure of the interior of the earth.
  • 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.
  • They arrive at the surface after P wave.

Surface Waves

  • The body waves interact with the surface rocks and generate new set of waves that waves move along the surface, hence called surface waves.
  • The surface waves are the last to report on seismograph.
  • These waves vibrate perpendicular to the direction of propagation.
  • They cause displacement of rocks, and hence, the collapse of structures occurs. Surface waves are considered to be the most damaging waves.
  • There are two type of Surface wave. They are Love Wave or L wave and Rayleigh Wave or R wave.
  • These are with of long wavelength and low frequency. They travel slower than Body Waves.

Shadow Zone

  • There exist some specific areas where the Earthquake waves are not reported. Such a zone is called the ‘shadow zone’.
  • The seismographs located at any distance within 105° from the epicentre, recorded the arrival of both P and S-waves.
  • The seismographs located beyond 145° from epicentre, record the arrival of P-waves, but not that of S-waves. A zone between 105° and 145° from epicentre was identified as the shadow zone for both P & S waves.
  • The entire zone beyond 105° does not receive S-waves. It is known as shadow zone of S-wave.
  • The shadow zone of S-wave is much larger than that of the P-waves.
  • The shadow zone of P-waves appears as a band around the earth between 105° and 145° away from the epicentre.

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