8.2 Types of seismic wave
P wave is primary compression type, characterized by alternative compressions
and dilations, with their direction same as the direction in which the wave is
propagating. Its is around 5-7 Km/s. among all the seismic waves, P wave travels
fastest in materials, so it obtained first on a seismogram.
The motion of S (secondary wave) is transverse in nature and is in the direction perpendicular to the direction of propagation. Typical velocity of S wave in earth crust is 3-4 Km/s. These do not travel through fluids, so do not exist in earth’s outer core (inferred to be primarily liquid iron) or in air or water or molten rock (magma). S waves travel slower than P waves.
The motion of L (love) or surface wave is transverse horizontal, perpendicular to the direction of propagation and generally parallel to the earth’s surface. This wave exists because of the earth’s surface. The amplitude is largest at the surface and decrease with depth. Love waves are dispersive, i.e. the wave velocity is dependent on frequency; with low frequencies propagating at higher velocity. Depth of penetration of the Love waves is also dependent on frequency, with lower frequencies penetrating to greater depth.
R (Rayleigh) wave is another surface wave which has its motion both in the direction of propagation and perpendicular and phased, so that the motion is generally elliptical either prograde or retrograde. Rayleigh waves are also dispersive and its amplitudes generally decreases with depth. Appearance and particle motion are similar to water waves. Depth of penetration of the Rayleigh waves is also dependent on frequency, with lower frequencies penetrating to greater depth.
The magnitude of an earthquake is related to the amount of energy released during the event. Seismic energy attenuates as it travels away from the zone of energy release and spreads out over a greater volume of material. Hence, intensity of the bedrock motion decreases as the distance of a particular site from the zone of energy release increases. Measurement of mechanical properties of soil and rock masses is, to a certain extent, possible with seismic methods. If P and S waves are measured, a so-called ‘seismic E modulus’ and ‘seismic Poisson’s ratio’ can be determined:
The relation between the frequency and the wavelength is:
where V is the velocity of seismic wave, f is its frequency, and λ is the wavelength.
The relation by deere et. al. relates Rock Quality Designation to seismic velocities measured in the field and in the laboratory:
Vfield = seismic velocity measured in the field
Vlaboratory = seismic velocity measured in the laboratory
Where, RQD is the Rock Quality Designation. The seismic velocity measured in the laboratory is done on intact rock. The velocity measured in the field is the velocity of a signal passing through intact rock but also through or around discontinuities and will hence result in a lower velocity. Inhomogeneities in soil or rock mass, such as boulders in a soil or joints in rock mass will not individually be detected if the wavelength of seismic wave is low compared to the dimensions of the inhomogeneities.