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P-waves are type of elastic wave, also called seismic waves, that can travel through gases (such as sounds), elastic solids and liquids, including the Earth. P-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 among the elastic waves, compared to the S-waves and surface waves.
In isotropic and homogeneous solids, the polarization of a P-wave is always longitudinal; thus, the particles in the solid have vibrations along or parallel to the travel direction of the wave energy.
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Speed of P-waves
The speed of P-waves is given by
where K is the bulk modulus (the modulus of incompressibility) μ, the first Lamé parameter, is the shear modulus (modulus of rigidity), ρ is the density of the material through which the wave propagates, and λ is second Lamé parameter.
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 
.
Typical values for P-wave velocity in earthquakes are in the range 5 to 8 km/s .[1]
Polarization
In isotropic and homogeneous solids, the polarization of P-waves is always longitudinal. This means that the particles in the body have vibrations along or parallel to the direction of travel of the wave energy.
As an earthquake warning
Earthquake advance warning is possible by detecting the non-destructive primary waves that travel more quickly through the earth's crust than do the destructive secondary and Rayleigh waves. The amount of advance warning depends on the delay between the arrival of the P-wave and other destructive waves, generally on the order of seconds up to about a minute maximum for deep, distant, large quakes. The effectiveness of advance warning depends on accurate detection of the P-waves and compensation for ground vibrations caused by local activity (such as trucks or construction work).
P-wave shadow zone
Almost all the information available 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.
See also
References
- "Photo Glossary of Earthquakes". U.S. Geological Survey". http://earthquake.usgs.gov/learning/glossary.php. Retrieved March 8 2009.
External links
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