Seismic wave
From Wikipedia, the free encyclopedia
A seismic wave is a wave that travels through the Earth, most often as the result of a tectonic earthquake, sometimes from an explosion. Seismic waves are also continually excited by the pounding of ocean waves and the wind. Seismic waves are studied by seismologists, and measured by a seismograph, which records the output of a seismometer, or geophone. For seismic studies of oil reservoirs, hydrophones may give additional information.
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[edit] Types of seismic wave
There are two types of seismic wave, namely, 'body wave' and 'surface wave'. Other modes of wave propagation exist than those described in this article, but they are of comparatively minor importance. An excellent audience demonstration for seismic waves is shown in slinky seismology.
[edit] Body waves
Body waves travel through the interior of the Earth. They follow raypaths bent by the varying density and composition of the Earth's interior. This effect is similar to the refraction of light waves. Body waves transmit the first-arriving tremors of an earthquake, as well as many later arrivals. There are two kinds of body waves: primary (P-waves) and secondary (S-waves).
[edit] P waves
P waves are longitudinal or compressional waves, which means that the ground is alternately compressed and dilated in the direction of propagation. These waves generally travel slightly less than twice as fast as S waves and can travel through any type of material. In air, these pressure waves take the form of sound waves, hence they travel at the speed of sound. Typical speeds are 330 m/s in air, 1450 m/s in water and about 5000 m/s in granite. P waves are sometimes called "primary waves", and are not as destructive as the S waves and surface waves that follow them.
[edit] S waves
S waves are transverse or shear waves, which means that the ground is displaced perpendicularly to the direction of propagation. In the case of horizontally polarized S waves, the ground moves alternately to one side and then the other. S waves can travel only through solids, as fluids (liquids and gases) do not support shear stresses. Their speed is about 60% of that of P waves in a given material. S waves are sometimes called "secondary waves", and are several times larger in amplitude than P waves.
[edit] Surface waves
Surface waves are analogous to water waves and travel just under the Earth's surface. They travel more slowly than body waves. Because of their low frequency, long duration, and large amplitude, they can be the most destructive type of seismic wave. There are two types of surface waves: Rayleigh waves and Love waves. Theoretically, surface waves can be understood as systems of interacting P and/or S waves.
[edit] Rayleigh waves
Rayleigh waves, also called ground roll, are surface waves that travel as ripples similar to those on the surface of water. The existence of these waves was predicted by John William Strutt, Lord Rayleigh, in 1885. They are slower than body waves, and supposedly can readily be seen during an earthquake in an open space like a parking lot where the cars move up and down with the waves.
[edit] Love waves
Love waves are surface waves that cause horizontal shearing of the ground. They are named after A.E.H. Love, a British mathematician who created a mathematical model of the waves in 1911. They usually travel slightly faster than Rayleigh waves.
[edit] P and S waves in Earth's mantle and core
When an earthquake occurs, seismographs near the epicenter, out to about 90° distance, are able to record both P and S waves, but those at a greater distance no longer detect the S wave. Since shear waves cannot pass through liquids, this phenomenon is the basis of the theory that the Earth has a liquid outer core, as demonstrated by Richard Dixon Oldham. This principle has also been used to show, by seismic testing, that the Moon has a solid core.
[edit] Some principles of locating an event
In the case of local or nearby earthquakes, the difference in the arrival times of the P and S waves can be used to determine the distance to the event. In the case of earthquakes that have occurred at global distances, four or more P-wave arrivals permits the computation of a unique location on the planet. Typically, dozens or even hundreds of P-wave arrivals are used to calculate hypocenters. The misfit generated by an hypocenter calculation is known as "the residual". Residuals of 0.5 second or less are typical, meaning most reported P arrivals fit the computed hypocenter that well. Typically a location program will start by assuming the event occurred at a depth of about 33 km; then it minimizes the residual by adjusting depth. Most events occur at depths shallower than about 40 km, but some occur as deep as 700 km.
A quick way to determine the distance from a location to the origin of a seismic wave less than 200 km away is to take the difference in arrival time of the P wave and the S wave in seconds and multiply by 8 kilometers per second. Modern seismic arrays use more complicated earthquake location techniques.
At teleseismic distances, the first arriving P waves have necessarily travelled deep into the mantle, and perhaps have even refracted into the outer core of the planet, before travelling back up to the Earth's surface where the seismographic stations are located. The waves travel more quickly than if they had traveled in a straight line from the earthquake. This is due to the appreciably increased velocities within the planet, and is termed Huygens' Principle. Density in the planet increases with depth, which would slow the waves, but the modulus of the rock increases much more, so deeper means faster. Therefore, a longer route can take a shorter time.
The travel time must be calculated very accurately in order to compute a precise hypocenter. Since P waves move at many kilometers per second, being off on travel-time calculation by even a half second can mean an error of many kilometers in terms of distance. In practice, P arrivals from many stations are used and the errors cancel out, so the computed epicenter likely to be quite accurate, on the order of 10-50 km or so around the world. Dense arrays of nearby sensors such as those that exist in California can provide accuracy of roughly a kilometer, and much greater accuracy is possible when timing is measured directly by cross-correlation of seismogram waveforms.
