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:
- Body waves
- 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:
- 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.
- 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:
- 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.
- 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.