P-waves are a type of elastic wave, also called seismic waves, that can travel through
gasGas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...
es (as
soundSound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...
waves), solids and liquids, including the
EarthEarth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
. P-waves are produced by earthquakes and recorded by
seismographsSeismometers are instruments that measure motions of the ground, including those of seismic waves generated by earthquakes, volcanic eruptions, and other seismic sources...
. The name P-wave is often mistakenly said to stand either for
primary wave, as it has the highest velocity and is therefore the first to be recorded; or
pressure wave, as it is formed from alternating compressions and
rarefactionRarefaction is the reduction of a medium's density, or the opposite of compression.A natural example of this is as a phase in a sound wave or phonon. Half of a sound wave is made up of the compression of the medium, and the other half is the decompression or rarefaction of the medium.Another...
s. Plane waves were originally referred to as primus(P)-waves), and secundus(S)-waves, as when these waves were first being studied, it was still contentious what kinds of elastic waves these arrivals actually represented.
In isotropic and homogeneous solids, the mode of propagation of a P-wave is always
longitudinalLongitudinal waves, as known as "l-waves", are waves that have the same direction of vibration as their direction of travel, which means that the movement of the medium is in the same direction as or the opposite direction to the motion of the wave. Mechanical longitudinal waves have been also...
; thus, the particles in the solid have vibrations along or parallel to the travel direction of the wave energy.
Velocity
The velocity of P-waves in a
homogeneousIn general, homogeneity is defined as the quality or state of being homogeneous . For instance, a uniform electric field would be compatible with homogeneity...
isotropic medium is given by
where
K is the
bulk modulusThe bulk modulus of a substance measures the substance's resistance to uniform compression. It is defined as the pressure increase needed to decrease the volume by a factor of 1/e...
(the modulus of incompressibility),
is the
shear modulus (modulus of rigidity, sometimes denoted as
G and also called the second
Lamé parameter),
is the
densityThe mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
of the material through which the wave propagates, and
is the first
Lamé parameter.
Of these, density shows the least variation, so the velocity is mostly
controlled by
K and
μ.
The elastic moduli
P-wave modulusIn linear elasticity, the P-wave modulus M, also known as the longitudinal modulus, is one of the elastic moduli available to describe isotropic homogeneous materials....
,
, is defined so that
and thereby
Typical values for P-wave velocity in earthquakes are in the range 5 to 8 km/s. The precise speed varies according to the region of the Earth's interior, from less than 6 km/s in the Earth's crust to 13 km/s through the core.
Seismic waves in the Earth
The seismic waves of both
P-type and
S-type in the Earth are monitored to probe the interior structure of the Earth. Discontinuities in velocity as a function of depth are indicative of changes in phase or composition. Differences in arrival times of waves originating in a seismic event like an earthquake as a result of waves taking different paths allow mapping of the Earth's inner structure.
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,
reflectionReflection is the change in direction of a wavefront at an interface between two differentmedia so that the wavefront returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves...
s,
refractionRefraction is the change in direction of a wave due to a change in its speed. It is essentially a surface phenomenon . The phenomenon is mainly in governance to the law of conservation of energy. The proper explanation would be that due to change of medium, the phase velocity of the wave is changed...
s 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
mantleThe mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....
and the liquid outer core. As a result, there is a P-wave "
shadow zoneA seismic shadow zone is an area of the Earth's surface where seismographs cannot detect an earthquake after its seismic waves have passed through the Earth. When an earthquake occurs, seismic waves radiate out spherically from the earthquake's focus...
" between 105° and 143° from the earthquake's focus, where the initial P-waves are not registered on seismometers. In contrast, S-waves do not travel through liquids, rather, they are
attenuatedIn physics, attenuation is the gradual loss in intensity of any kind of flux through a medium. For instance, sunlight is attenuated by dark glasses, X-rays are attenuated by lead, and light and sound are attenuated by water.In electrical engineering and telecommunications, attenuation affects the...
.
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 waveRayleigh waves are a type of surface acoustic wave that travels on solids. They are produced on the Earth by earthquakes, in which case they are also known as "ground roll", or by other sources of seismic energy such as ocean waves an explosion or even a sledgehammer impact...
s, in the same way that lightning flashes reaches our eyes before we hear the thunder during a storm. 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 60–90 seconds for deep, distant, large quakes such as Tokyo would have received before the
2011 Tohoku earthquake and tsunamiThe 2011 earthquake off the Pacific coast of Tohoku, also known as the 2011 Tohoku earthquake, or the Great East Japan Earthquake, was a magnitude 9.0 undersea megathrust earthquake off the coast of Japan that occurred at 14:46 JST on Friday, 11 March 2011, with the epicenter approximately east...
. The effectiveness of advance warning depends on accurate detection of the P-waves and rejection of
ground vibrationsGround vibrations is a technical term that is being used to describe mostly man-made vibrations of the ground, in contrast to natural vibrations of the Earth studied by seismology. For example, vibrations caused by explosions, construction works, railway and road transport, etc - all belong to...
caused by local activity (such as trucks or construction) otherwise false-positive warnings will result. Technology currently in use known as the QuakeGuard system employs this technique to automate emergency response procedures that protect against loss of life and reduce property damage.
See also
- S-Wave
A type of seismic wave, the S-wave, secondary wave, or shear wave is one of the two main types of elastic body waves, so named because they move through the body of an object, unlike surface waves....
- Lamb waves
Lamb waves propagate in solid plates. They are elastic waves whose particle motion lies in the plane that contains the direction of wave propagation and the plate normal . In 1917, the English mathematician Horace Lamb published his classic analysis and description of acoustic waves of this type....
- Longitudinal wave
Longitudinal waves, as known as "l-waves", are waves that have the same direction of vibration as their direction of travel, which means that the movement of the medium is in the same direction as or the opposite direction to the motion of the wave. Mechanical longitudinal waves have been also...
- Love wave
In elastodynamics, Love waves are horizontally polarized shear waves guided by an elastic layer, which is "welded" to an elastic half space on one side while bordering a vacuum on the other side...
- Surface wave
In physics, a surface wave is a mechanical wave that propagates along the interface between differing media, usually two fluids with different densities. A surface wave can also be an electromagnetic wave guided by a refractive index gradient...
- Earthquake Early Warning (Japan)
The is a warning issued just after an earthquake in Japan is detected. The warnings are issued mainly by the Japan Meteorological Agency , and they issue tips on how to react to the warnings.-Introduction:...
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