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P-wave
From Wikipedia, the free encyclopedia
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P-waves are one of the types of elastic waves, also called seismic waves, that can travel through 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 (secondary waves).
Sound, being a pressure and longitudinal wave in air, is also a P-wave.
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.
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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 
.
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.
- "Photo Glossary of Earthquakes". U.S. Geological Survey". Retrieved on October 10, 2005.
