Seismic Waves

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.

2. 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.

2. 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.