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Seismic Waves

by Swati

Seismic Waves:

  • Seismic waves are the waves which are formed by sudden breaking of rock within the earth.
  • Earthquake produces two types of seismic waves which are as follows:
  1. Body waves
  2. Surface waves

1.Body Waves:

  • Body waves are seismic waves that travel through the body of the earth. 
  • Body waves are reflected and transmitted at interfaces where seismic velocity and/or density change, and they obey Snell’s law.
  • The two different types of body waves are:
  1. P-Waves:
  • P-Waves (P stands for primary or pressure or push pull). 
  • These waves are also called longitudinal waves or compressional waves due to particle compression during their transport.
  • These waves involve compression and rarefaction of the material as the wave passes through it but not rotation. 
  • P-wave is transmitted by particle movement back and forth along the direction of propagation of the wave.
  • The most correct description of P-waves is, it is a dilational or irrotational waves. 
  • P-wave has the greatest speed and appears first on seismograms.
  1. S-Waves:
  • S-Waves (S stands for secondary or shear or shake). 
  • Also known as transverse waves, because particle motions are transverse to the direction of movement of the wavefront, or perpendicular to the ray.
  • These waves involve shearing and rotation of the material as the wave passes through it, but not volume change. 
  • S-waves have speeds less than P-waves, and appear on seismograms after P-waves.

2. Surface Waves:

  • Surface waves are seismic waves that are guided along the surface of the Earth and the layers near the surface.
  • These waves do not penetrate the deep interior of the earth, and shallow earthquakes(nuclear explosions do not generate these surface waves) normally do not generate these waves.
  • Surface waves are larger in amplitude and longer in duration than body waves. 
  • These waves arrive at seismograph after the arrival of P- and S-waves because of their slower velocities. 
  • The two different surface waves are:
  1. Rayleigh waves:
  • Rayleigh waves or descriptively called “ground roll” in exploration seismology. 
  • The particle motion of this wave is confined to a vertical plane containing the direction of propagation and retrogrades elliptically. 
  • The particle displacements are greatest at the surface and decrease exponentially downward. 
  • Rayleigh waves show dispersion, and its velocity is not constant but varies with wavelength. 
  • This wave is similar to how ocean waves propagate.
  1. Love waves:
  • These waves (named after A.E.H. Love, who discovered them) travel by a transverse motion of particles that is parallel to the ground surface.
  • This wave is somewhat similar to S-waves. 
  • Love waves cannot exist in a uniform solid, and can only occur when there is a general increase of S-wave velocity with depth. 
  • Their existence is another proof the Earth’s vertical in homogeneity.
  • The particle motion is transverse and horizontal. 
  • Generally, Love wave velocities are greater than Rayleigh waves, so Love waves arrive before Rayleigh waves on seismograph.

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