Editing Seismology (section)

===Body waves===

There are two types of body waves, pressure waves or primary waves (P waves) and [[Shearing (physics)|shear]] or secondary waves ([[S wave]]s). P waves are [[longitudinal wave]]s associated with [[Compression (geology)|compression]] and [[rarefaction|expansion]], and involve particle motion parallel to the direction of wave propagation. P waves are always the first waves to appear on a seismogram as they are the waves that travel fastest through solids. [[S waves]] are [[transverse wave]]s associated with shear, and involve particle motion perpendicular to the direction of wave propagation. S waves travel more slowly than P waves so they appear later than P waves on a seismogram. Because of their low shear strength, fluids cannot support transverse elastic waves, so S waves travel only in solids.<ref name=Gubbins1990>{{harvnb|Gubbins|1990}}</ref>