Wave that travels through the Earth or a planetary body
The propagation velocity of a seismic wave depends on density and elasticity of the medium as well as the type of wave.
Velocity tends to increase with depth through Earth’s crust and mantle, but drops sharply going from the mantle to Earth’s outer core
Body waves travel through the interior of the Earth.
Surface waves travel across the surface. Surface waves decay more slowly with distance than body waves which travel in three dimensions.
Amplitude of surface waves is larger than that of body waves, so surface waves tend to cause more damage.
Different travel times of seismic waves are used to locate their epicentre
Refraction or reflection of seismic waves is used for research into Earth’s internal structure
Scientists sometimes generate and measure vibrations to investigate shallow, subsurface structures.
Compressional waves: Seismic, longitudinal body waves
Shear waves: Seismic, transverse body waves
P-Waves
Primary waves (P-waves) are compressional waves that are 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
S-Waves
Secondary waves (S-waves) are shear waves that are 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 stresses.
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’t travel through any liquid medium, so the absence of S-waves in earth’s outer core suggests a liquid state.