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P-wave

P-waves are one of the types of elastic waves, also called seismic waves, that can travel through gasses (such as sounds), elastic solids and liquids, including the Earth. The waves can be produced by earthquakes and recorded by seismometers. The name P-wave stands for primary wave, as the P-wave is the fastest wave among the elastic waves, compared to the S-waves.

The polarization of P-wave is always longitudinal (in isotropic and homogeneous solids), meaning the particles in the body of a solid have vibrations along or parallel to the direction of travel of the wave energy.

Speed of P-waves

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

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 variation so the velocity is mostly controlled by K and μ.

The elastic moduli P-wave modulus, M, is defined so that M = K + 4μ / 3 and thereby $v_p = \sqrt{M/\rho}$.

Polarization

The polarization of P-wave is always longitudinal (in isotropic and homogeneous solids), 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.

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. In contrast, "S" waves do not travel through liquids, rather, they are attenuated.