S wave

For the lowest-energy electronic wavefunction in atomic physics, see atomic orbital. For the S wave on an electrocardiogram, see QRS complex.
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Plane shear wave
Propagation of a spherical S wave in a 2d grid (empirical model)

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 movement of an 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]

  1. ^ "Seismology | UPSeis | Michigan Tech". Michigan Technological University. Retrieved 2023-10-07.
  2. ^ "S wave". United States Geological Survey. Archived from the original on July 22, 2021.
  3. ^ "Why can't S-waves travel through liquids?". Earth Observatory of Singapore. Retrieved 2019-12-06.
  4. ^ 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. PMID 12206629.
  5. ^ "Do viscous fluids support shear waves propagation?". ResearchGate. Retrieved 2019-12-06.[unreliable source?]
  6. ^ "Lecture 16 Seismographs and the earth's interior". University of Illinois at Chicago. 17 July 1997. Archived from the original on 7 May 2002. Retrieved 8 June 2010.