Editing Seismic wave (section)

=== Body waves ===
Body waves travel through the interior of the Earth along paths controlled by the material properties in terms of [[density]] and [[Young's modulus|modulus]] (stiffness). The density and modulus, in turn, vary according to temperature, composition, and material phase.  This effect resembles the [[refraction]] of [[light wave]]s. Two types of particle motion result in two types of body waves: '''''Primary''''' and '''''Secondary''''' waves.  This distinction was recognized in 1830 by the French mathematician [[Siméon Denis Poisson]].<ref>{{cite journal |last1=Poisson |first1=S. D. |title=Mémoire sur la propagation du mouvement dans les milieux élastiques |journal=Mémoires de l'Académie des Sciences de l'Institut de France |date=1831 |volume=10 |pages=549–605 |url=https://www.biodiversitylibrary.org/item/55253#page/789/mode/1up |trans-title=Memoir on the propagation of motion in elastic media |language=fr}}</ref>
[[File:Seismic wave travel through Earth.png|thumb|Patterns of seismic wave travel through Earth's mantle and core. S waves can not travel through the liquid outer core, so they leave a shadow on Earth's far side. P waves do travel through the core, but P wave refraction bends seismic waves away from P wave shadow zones.]]

==== Primary waves ====
{{Main|P wave}}
Primary waves (P waves) are compressional waves that are [[longitudinal wave|longitudinal]] in nature.  [[P waves]] are pressure waves that travel faster than other waves through the earth to arrive at seismograph stations first, hence the name "Primary". These waves can travel through any type of material, including fluids, and can travel nearly 1.7 times faster than the [[S waves]]. In air, they take the form of sound waves, hence they travel at the [[speed of sound]]. Typical speeds are 330&nbsp;m/s in air, 1450&nbsp;m/s in water and about 5000&nbsp;m/s in [[granite]].

==== Secondary waves ====
{{Main|S wave}}
Secondary waves (S waves) are shear waves that are [[transverse wave|transverse]] in nature. Following an earthquake event, S waves arrive at seismograph stations after the faster-moving P waves and displace the ground perpendicular to the direction of propagation. Depending on the propagational direction, the wave can take on different surface characteristics; for example, in the case of horizontally polarized S waves, the ground moves alternately to one side and then the other. S waves can travel only through solids, as fluids (liquids and gases) do not support [[shear stress]]es. S waves are slower than P waves, and speeds are typically around 60% of that of P waves in any given material. Shear waves can not travel through any liquid medium,<ref>{{cite web|url=http://www.burkemuseum.org/static/earthquakes/cur-seismic.html|title=Seismic Waves|publisher=[[Burke Museum of Natural History and Culture]]|access-date=March 24, 2019}}</ref> so the absence of S waves in earth's outer core suggests a liquid state.