In solid mechanics, S waves, secondary waves, or shear waves (sometimes called elastic S waves) are a type of elastic wave and are one of the two main types of elastic body wavess, so named because they move through the body of an object, unlike surface waves.

S waves are transverse waves, meaning that the direction of particle movement of an S wave is perpendicular to the direction of wave propagation, and the main restoring force comes from shear stress. Therefore, S waves cannot propagate in liquids with zero (or very low) viscosity; however, they may propagate in liquids with high viscosity. Similarly, S waves cannot travel through gases.

The name secondary wave comes from the fact that they are the second type of wave to be detected by an earthquake seismograph, after the compressional primary wave, or P wave, because S waves travel more slowly in solids. Unlike P waves, S waves cannot travel through the molten outer core of the Earth, and this causes a shadow zone for S waves opposite to their origin. They can still propagate through the solid inner core: when a P wave strikes the boundary of molten and solid cores at an oblique angle, S waves will form and propagate in the solid medium. When these S waves hit the boundary again at an oblique angle, they will in turn create P waves that propagate through the liquid medium. This property allows seismologists to determine some physical properties of the Earth's inner core.

History

In 1830, the mathematician Siméon Denis Poisson presented to the French Academy of Sciences. At a sufficient distance from the source, when they can be considered plane waves in the region of interest, the first kind consists of expansions and compressions in the direction perpendicular to the wavefront (that is, parallel to the wave's direction of motion); while the second consists of stretching motions occurring in directions parallel to the front (perpendicular to the direction of motion).

Theory

Isotropic medium

For the purpose of this explanation, a solid medium is considered isotropic if its strain (deformation) in response to stress be the displacement vector due elastic vibrations, understood to be a function. The deformation of the medium at that point can be described by the strain tensor

. The strain tensor is related to the 3×3 stress tensor

is the Kronecker delta are the Lamé parameters being the material's shear modulus

From Newton's law of inertia is the density denotes partial derivative with respect to time. Combining the last two equations one gets the seismic wave equation in homogeneous media

Using the nabla operator notation of vector calculus

Taking the curl

This formula is the wave equation, which is the material's shear strain. Its solutions, the S waves, are linear combinations of sinusoidal plane waves of various wavelengths where ω is the angular frequency and .

Taking the divergence.

The steady state SH waves are defined by the Helmholtz equation where is the wave number.

S waves in viscoelastic materials

Similar to in an elastic medium, in a viscoelastic is the frequency dependent phase velocity. One common approach to describing the shear modulus in viscoelastic materials is through the Voigt Model is the viscosity.

S wave technology

Magnetic resonance elastography

Magnetic resonance elastography (MRE) is a method for studying the properties of biological materials in living organisms by propagating shear waves at desired frequencies throughout the desired organic tissue. This method uses a vibrator to send the shear waves into the tissue and magnetic resonance imaging to view the response in the tissue. The measured wave speed and wavelengths are then measured to determine elastic properties such as the shear modulus. MRE has seen use in studies of a variety of human tissues including liver, brain, and bone tissues.

See also

• Earthquake Early Warning (Japan)

• Lamb waves

• Longitudinal wave

• Love wave

• Rayleigh wave

• Shear wave splitting

References

Further reading