# S-wave

In seismology, S-waves, secondary waves, or shear waves (sometimes called an elastic S-wave) are a type of elastic wave and are one of the two main types of elastic body waves, so named because they move through the body of an object, unlike surface waves.[1]

Plane shear wave
Propagation of a spherical S-wave in a 2d grid (empirical model)

The S-wave is a transverse wave, meaning that, in the simplest situation, the oscillations of the particles of the medium are perpendicular to the direction of wave propagation, and the main restoring force comes from shear stress.[2] Therefore, S-waves can't propagate in liquids[3] with zero (or very low) viscosity; However, it may propagate in liquids with high viscocity.[4][5]

Its name, S for secondary, comes from the fact that it is the second direct arrival on an earthquake seismogram, after the compressional primary wave, or P-wave, because S-waves travel slower in rock. Unlike the P-wave, the S-wave cannot travel through the molten outer core of the Earth, and this causes a shadow zone for S-waves opposite to where they originate. They can still appear in 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 inner core.[6]

## History

In 1830, the mathematician Siméon Denis Poisson presented to the French Academy of Sciences an essay ("memoir") with a theory of the propagation of elastic waves in solids. In his memoir, he states that an earthquake would produce two different waves: one having a certain speed a and the other having a speed a/3. 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).[7]

## Theory

### Isotropic medium

For the purpose of this explanation, a solid medium is considered isotropic if its strain (deformation) in response to stress is the same in all directions. Let ${\displaystyle {\boldsymbol {u}}=(u_{1},u_{2},u_{3})}$ be the displacement vector of a particle of such a medium from its "resting" position ${\displaystyle {\boldsymbol {x}}=(x_{1},x_{2},x_{3})}$ due elastic vibrations, understood to be a function of the rest position ${\displaystyle {\boldsymbol {x}}}$ and time ${\displaystyle t}$. The deformation of the medium at that point can be described by the strain tensor ${\displaystyle {\boldsymbol {e}}}$, the 3×3 matrix whose elements are

${\displaystyle e_{ij}={\frac {1}{2}}(\partial _{i}u_{j}+\partial _{j}u_{i})}$

where ${\displaystyle \partial _{i}}$ denotes partial derivative with respect to position coordinate ${\displaystyle x_{i}}$. The strain tensor is related to the 3×3 stress tensor ${\displaystyle {\boldsymbol {\tau }}}$ by the equation

${\displaystyle \tau _{ij}=\lambda \delta _{ij}\sum _{k}e_{kk}+2\mu e_{ij}}$

Here ${\displaystyle \delta _{ij}}$ is the Kronecker delta (1 if ${\displaystyle i=j}$, 0 otherwise) and ${\displaystyle \lambda }$ and ${\displaystyle \mu }$ are the Lamé parameters (${\displaystyle \mu }$ being the material's shear modulus). It follows that

${\displaystyle \tau _{ij}=\lambda \delta _{ij}{\bigl (}\sum _{k}\partial _{k}u_{k}{\bigr )}+\mu (\partial _{i}u_{j}+\partial _{j}u_{i})}$

From Newton's law of inertia, one also gets

${\displaystyle \rho \partial _{t}^{2}u_{i}=\sum _{j}\partial _{j}\tau _{ij}}$

where ${\displaystyle \rho }$ is the density (mass per unit volume) of the medium at that point, and ${\displaystyle \partial _{t}}$ denotes partial derivative with respect to time. Combining the last two equations one gets the seismic wave equation in homogeneous media

${\displaystyle \rho \partial _{t}^{2}u_{i}=\lambda \partial _{i}{\bigl (}\sum _{k}\partial _{k}u_{k}{\bigr )}+\mu {\bigl (}\sum _{j}\partial _{i}\partial _{j}u_{j}+\mu \partial _{j}\partial _{j}u_{i}{\bigr )}}$

Using the nabla operator notation of vector calculus, ${\displaystyle \nabla =(\partial _{1},\partial _{2},\partial _{3})}$, with some approximations, this equation can be written as

${\displaystyle \rho \partial _{t}^{2}{\boldsymbol {u}}=\left(\lambda +2\mu \right)\nabla (\nabla \cdot {\boldsymbol {u}})-\mu \nabla \times (\nabla \times {\boldsymbol {u}})}$

Taking the curl of this equation and applying vector identities, one gets

${\displaystyle \partial _{t}^{2}(\nabla \times {\boldsymbol {u}})={\frac {\mu }{\rho }}\nabla ^{2}(\nabla \times {\boldsymbol {u}})}$

This formula is the wave equation applied to the vector quantity ${\displaystyle \nabla \times {\boldsymbol {u}}}$, which is the material's shear strain. Its solutions, the S-waves, are linear combinations of sinusoidal plane waves of various wavelengths and directions of propagation, but all with the same speed ${\displaystyle \beta =\textstyle {\sqrt {\mu /\rho }}}$

Taking the divergence of seismic wave equation in homogeneous media, instead of the curl, yields a wave equation describing propagation of the quantity ${\displaystyle \nabla \cdot {\boldsymbol {u}}}$, which is the material's compression strain. The solutions of this equation, the P-waves, travel at the speed ${\displaystyle \alpha =\textstyle {\sqrt {(\lambda +2\mu )/\rho }}}$ that is more than twice the speed ${\displaystyle \beta }$ of S-waves.

The steady-state SH waves are defined by the Helmholtz equation[8]

${\displaystyle (\nabla ^{2}+k^{2}){\boldsymbol {u}}=0}$

where k is the wave number.