P-wave

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Plane P-wave

Representation of the propagation of a P-wave on a 2d grid (empirical shape)

Of the two types of elastic body waves (named because they travel through the body of the Earth) that are produced by earthquakes and recorded by seismometers. The name primary comes from the fact that they have the highest velocity of all seismic waves and are thus the first to arrive at any seismic station, the other body wave type being S- or secondary waves. Sound, as a pressure wave and a longitudinal wave, is also a P-wave. This means that the particles in the body of the Earth have vibrations along or parallel to the direction of travel of the wave energy.

P-wave shadow zone (from USGS)

$v_p= \sqrt{ \frac {k+\frac{4}{3}\mu} {\rho}}$

where:

k is the modulus of incompressibility
$μ$ is the modulus of rigidity; and
$ρ$ the density of the material through which the wave is propagating

Of these density shows the least variaton so the velocity is mostly controlled by k and μ

Almost all the information we have on the structure of the Earth's deep interior is derived from observations of the travel times, reflections, refractions and phase transitions of seismic body waves, or normal modes. Body waves travel through the fluid layers of the Earth's interior, but P-waves are refracted slightly when they pass through the transition between the semisolid mantle and the liquid outer core. As a result, there is a P-wave "shadow zone" between 104° and 140°, where the initial P-waves are not registered on seismometers.