Earthquake Waves & Shadow Zones

Earthquake waves or seismic waves are vibrations generated by an earthquake, explosion, or similar energetic source and propagated within Earth or along its surface. Earthquakes generate four principal types of elastic waves; two, known as body waves, travel within Earth, whereas the other two, called surface waves, travel along its surface. Seismographs record amplitude and frequency of seismic waves and yield information about Earth and its subsurface structure.

BODY WAVES

  • Generated due to release of energy at the focus and travel in all directions within the body of the Earth.
  • The velocity as well as direction of waves changes as they travel through materials with different densities. The denser the material, the higher is the velocity. Reflection causes waves to rebound whereas refraction makes waves move in different directions.
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P- WAVES (PRIMARY WAVES)S-WAVES (SECONDARY WAVES)
Move faster and are the first to arrive at the surface.They arrive at the surface with some time lag. 
They travel through gaseous, liquid and solid materials.They can travel only through solid materials. 
They vibrate parallel to the direction of the wave (Longitudinal waves). This leads to density differences in the material because of stretching and squeezing of the material.The direction of vibrations is perpendicular to the wave direction in the vertical plane (Transverse Waves) Hence, they create troughs and crests in the material through which they pass. 
They are also called as ‘compressional waves’They are also called as ‘distortional waves’

SURFACE WAVES

  • The body waves when interact with the surface rocks, they generate new waves called as surface waves.
  • They are also called as long period waves.
  • The surface waves are the last to report on seismograph, but they cover longest distance of all the seismic waves. 
  • These waves are more destructive. They cause displacement of rocks, and hence, the collapse of structures occurs.
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CONCEPT OF SHADOW ZONES

Shadow zones are the angular areas from the given earthquake where the seismographs do not record any earthquake waves. These are different for P and S waves. Further they also vary with each earthquake. The angles are measured with respect to the Epicenter.

< 105 degrees = No shadow zone for P as well as S waves (Both the waves are recorded by seismograph)
105-145 degree = Shadow zones for both (Both P & S waves are not recorded)
> 145 degrees = Shadow zone for S waves (P waves recorded)
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Seismic waves help to know about Earth’s interior:

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  • Non-linear travel path confirms that the Earth’s structure in not homogeneous rather heterogeneous.
  • Curved path tells us that on an average there is an increase in density as one moves deeper into the earth.
  • Based on change of velocity of these waves it is proved that there are three locations where there are major velocity changes hence help us to infer that there are three zones/layers of varying densities inside the Earth.
  • Bouncing back of waves after hitting abrupt boundary between two layers help to determine the depth of the discontinuities and thickness of various layers. 
  • Absence of S-waves inside the core confirms the presence of liquid core at the depth of 2900 km. 
  • The velocity decreases in upper part of the upper mantle for a depth of 100-200 km which confirms the presence of lighter material in between.
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