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In seismology and other areas involving elastic waves, **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 waves, so named because they move through the body of an object, unlike surface waves.^{[1]}

S waves are transverse waves, meaning that the direction of particle motion of a S wave is perpendicular to the direction of wave propagation, and the main restoring force comes from shear stress.^{[2]} Therefore, S waves cannot propagate in liquids^{[3]} with zero (or very low) viscosity; however, they may propagate in liquids with high viscosity.^{[4]}^{[5]}

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.^{[6]}

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 and the other having a speed . 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]}

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 be the displacement vector of a particle of such a medium from its "resting" position due elastic vibrations, understood to be a function of the rest position and time . The deformation of the medium at that point can be described by the strain tensor , the 3×3 matrix whose elements are

where denotes partial derivative with respect to position coordinate . The strain tensor is related to the 3×3 stress tensor by the equation

Here is the Kronecker delta (1 if , 0 otherwise) and and are the Lamé parameters ( being the material's shear modulus). It follows that

From Newton's law of inertia, one also gets

Using the nabla operator notation of vector calculus, , with some approximations, this equation can be written as

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

This formula is the wave equation applied to the vector quantity , 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

Taking the divergence of seismic wave equation in homogeneous media, instead of the curl, yields a wave equation describing propagation of the quantity , which is the material's compression strain. The solutions of this equation, the P waves, travel at the speed that is more than twice the speed of S waves.

The steady state SH waves are defined by the Helmholtz equation^{[8]}

**^**What are seismic waves? UPSeis at Michigan Tech**^**S wave US Geological Survey**^**"Why can't S-waves travel through liquids?".*Earth Observatory of Singapore*. Retrieved 2019-12-06.**^**Greenwood, Margaret Stautberg; Bamberger, Judith Ann (August 2002). "Measurement of viscosity and shear wave velocity of a liquid or slurry for on-line process control".*Ultrasonics*.**39**(9): 623–630. doi:10.1016/s0041-624x(02)00372-4. ISSN 0041-624X. PMID 12206629.**^**"Do viscous fluids support shear waves propagation?".*ResearchGate*. Retrieved 2019-12-06.**^**University of Illinois at Chicago (17 July 1997). "Lecture 16 Seismographs and the earth's interior". Archived from the original on 7 May 2002. Retrieved 8 June 2010.**^**Poisson, S. D. (1831). "Mémoire sur la propagation du mouvement dans les milieux élastiques" [Memoir on the propagation of motion in elastic media].*Mémoires de l'Académie des Sciences de l'Institut de France*(in French).**10**: 549–605. From p.595: "*On verra aisément que cet ébranlement donnera naissance à deux ondes sphériques qui se propageront uniformément, l'une avec une vitesse*a*, l'autre avec une vitesse*b*ou*a*/ √3*" ... (One will easily see that this quake will give birth to two spherical waves that will be propagated uniformly, one with a speed*a*, the other with a speed*b*or*a*/√3 ... ) From p.602: ... "*à une grande distance de l'ébranlement primitif, et lorsque les ondes mobiles sont devenues sensiblement planes dans chaque partie très-petite par rapport à leurs surfaces entières, il ne subsiste plus que des vitesses propres des molécules, normales ou parallèles à ces surfaces ; les vitesses normal ayant lieu dans les ondes de la première espèce, où elles sont accompagnées de dilations qui leur sont proportionnelles, et les vitesses parallèles appartenant aux ondes de la seconde espèce, où elles ne sont accompagnées d'aucune dilatation ou condensation de volume, mais seulement de dilatations et de condensations linéaires.*" ( ... at a great distance from the original quake, and when the moving waves have become roughly planes in every tiny part in relation to their entire surface, there remain [in the elastic solid of the Earth] only the molecules' own speeds, normal or parallel to these surfaces ; the normal speeds occur in waves of the first type, where they are accompanied by expansions that are proportional to them, and the parallel speeds belonging to waves of the second type, where they are not accompanied by any expansion or contraction of volume, but only by linear stretchings and squeezings.)**^**Sheikhhassani, Ramtin (2013). "Scattering of a plane harmonic SH wave by multiple layered inclusions".*Wave Motion*.**51**(3): 517–532. doi:10.1016/j.wavemoti.2013.12.002.

- Shearer, Peter (1999).
*Introduction to Seismology*(1st ed.). Cambridge University Press. ISBN 0-521-66023-8. - Aki, Keiiti; Richards, Paul G. (2002).
*Quantitative Seismology*(2nd ed.). University Science Books. ISBN 0-935702-96-2. - Fowler, C. M. R. (1990).
*The solid earth*. Cambridge, UK: Cambridge University Press. ISBN 0-521-38590-3.S-wave.