Seismic waves are waves that travel through the Earth, most often as the result of a tectonic earthquake, sometimes from an explosion. A wave is a disturbance that propagates through Space and Time, usually with transference of Energy. EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 An earthquake is the result of a sudden release of energy in the Earth 's crust that creates Seismic waves Earthquakes are recorded with a Seismometer An explosion is a sudden increase in Volume and release of Energy in an extreme manner usually with the generation of high Temperatures and the release Seismic waves are also continually excited by the pounding of ocean waves and the wind. Ocean surface waves are Surface waves that occur on the Free surface of the Ocean. Wind is the flow of Air or other Gases that compose an Atmosphere (including but not limited to the Earth's) Seismic waves are studied by seismologists, and measured by a seismograph, which records the output of a seismometer, or geophone. Seismology (from Greek grc σεισμός seismos, "earthquake" and grc -λογία -logia) is the scientific study of Earthquakes Seismometers (from Greek Seism - "the shakes" - and Metro - "I measure" are instruments that measure and record motions of the ground including Seismometers (from Greek Seism - "the shakes" - and Metro - "I measure" are instruments that measure and record motions of the ground including The term geophone derives from the Greek word "geo" meaning " earth " and "phone" meaning "sound" For seismic studies of oil reservoirs, hydrophones may give additional information. A hydrophone (Greek "hydro" = "water" and "phone" = "sound" is a Microphone designed to be used underwater for recording or listening
Contents |
Types of seismic wave
There are two types of seismic waves, body waves and surface waves. 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.
Body waves
Body waves travel through the interior of the Earth. They follow raypaths bent by the varying density and modulus (stiffness) of the Earth's interior. The density of a material is defined as its Mass per unit Volume: \rho = \frac{m}{V} Different materials usually have different The density and modulus, in turn, vary according to temperature, composition, and phase. This effect is similar to the refraction of light waves. Refraction is the change in direction of a Wave due to a change in its Speed. Light, or visible light, is Electromagnetic radiation of a Wavelength that is visible to the Human eye (about 400–700 Body waves transmit the first-arriving tremors of an earthquake, as well as many later arrivals. There are two kinds of body waves: primary and secondary.
P waves
P waves (primary waves) are longitudinal or compressional waves, which means that the ground is alternately compressed and dilated in the direction of propagation. P-wave can also refer to a type of electronic wavefunction in atomic physics see Atomic orbital. Longitudinal waves are waves that have vibrations along or parallel to their direction of travel that is waves in which the motion of the medium is in the same direction as the motion In solids, these waves generally travel almost 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. Sound is a vibration that travels through an elastic medium as a Wave. Typical speeds are 330 m/s in air, 1450 m/s in water and about 5000 m/s in granite. When generated by an earthquake they are less destructive than the S waves and surface waves that follow them, due to their smaller amplitudes.
S waves
S waves (secondary waves) are transverse or shear waves, which means that the ground is displaced perpendicularly to the direction of propagation. S-wave can also refer to the lowest energy electronic wavefunction in atomic physics see Atomic orbital. A transverse wave is a moving Wave that consists of oscillations occurring perpendicular to the direction of energy transfer 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 several times larger in amplitude than P waves for earthquake sources.
Surface waves
Surface waves are analogous to water waves and travel just under the Earth's surface. In Physics, surface wave can refer to a Mechanical wave that propagates along the interface between differing media usually two fluids with different densities 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. Rayleigh waves, also known as the Rayleigh-Lamb Wave or "ground roll" are a type of Surface wave. In Elastodynamics, Love waves are essentially horizontally polarized shear waves (SH waves guided by an elastic layer which is "welded" to an elastic half space on Theoretically, surface waves can be understood as systems of interacting P and S waves which you can read in the few paragraphs below.
Rayleigh waves
Rayleigh waves, also called ground roll, are surface waves that travel as ripples similar to those on the surface of water. Rayleigh waves, also known as the Rayleigh-Lamb Wave or "ground roll" are a type of Surface wave. The existence of these waves was predicted by John William Strutt, Lord Rayleigh, in 1885. John William Strutt 3rd Baron Rayleigh OM (12 November 1842 &ndash 30 June 1919 was an English Physicist who with William Ramsay, discovered Year 1885 ( MDCCCLXXXV) was a Common year starting on Thursday (link will display the full calendar of the Gregorian calendar (or a Common They are slower than body waves, roughly 70% of the velocity of S waves.
Rayleigh waves have been asserted to be visible during an earthquake in an open space like a parking lot where the cars move up and down with the waves, however, reports among seismologists suggest that the apparent motion may be due to distortion of the human eye during shaking. Anecdotally, placing people on "shake tables" causes the room to appear to ripple. In any case, waves of the reported amplitude, wavelength, and velocity of the "visible waves" have never been recorded instrumentally.
Love waves
Love waves are surface waves that cause horizontal shearing of the ground. In Elastodynamics, Love waves are essentially horizontally polarized shear waves (SH waves guided by an elastic layer which is "welded" to an elastic half space on They are named after A.E.H. Love, a British mathematician who created a mathematical model of the waves in 1911. Augustus Edward Hough Love ( 17 April 1863, Weston-super-Mare - 5 June 1940, Oxford) often known as A Year 1911 ( MCMXI) was a Common year starting on Sunday (link will display the full calendar of the Gregorian calendar (or a Common year They usually travel slightly faster than Rayleigh waves, about 90% of the S wave velocity.
P and S waves in Earth's mantle and core
When an earthquake occurs, seismographs near the epicenter, out to about 90 Km distance, are able to record both P and S waves, but those at a greater distance no longer detect the high frequencies of the first S wave. The epicenter or epicentre is the point on the Earth 's surface that is directly above the Hypocenter or focus the point where an Earthquake Since shear waves cannot pass through liquids, this phenomenon was original evidence for the now well-established observation that the Earth has a liquid outer core, as demonstrated by Richard Dixon Oldham. Richard Dixon Oldham ( July 31, 1858 &ndash July 15, 1936) was an Irish geologist who in 1906 argued that the Earth This kind of observation has also been used to argue, by seismic testing, that the Moon has a solid core, although recent geodetic studies suggest the core is still molten.
Usefulness of P and S waves in 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 geographically diverse observing stations (using a common clock) recording P-wave arrivals permits the computation of a unique time and location on the planet for the event. Circadian Locomotor Output Cycles Kaput, or Clock is a gene which encodes proteins regulating Circadian rhythm. Typically, dozens or even hundreds of P-wave arrivals are used to calculate hypocenters. The hypocenter or hypocentre (literally 'below the center' from the Greek υπόκεντρον) refers to the site of an earthquake or to that of a nuclear The misfit generated by a hypocenter calculation is known as "the residual". Residuals of 0. 5 second or less are typical for distant events, residuals of 0. 1-0. 2 s typical for local events, 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. The second ( SI symbol s) sometimes abbreviated sec, is the name of a unit of Time, and is the International System of Units The kilometre ( American spelling: kilometer) symbol km is a unit of Length in the Metric system, equal to one thousand Modern seismic arrays use more complicated earthquake location techniques. The primary purpose of a Seismometer is to locate the initiating points of Earthquake Hypocenters The secondary purpose of determining the 'size' or Moment
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. The Huygens–Fresnel principle (named for Dutch Physicist Christiaan Huygens, and French physicist Augustin-Jean Fresnel 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. The density of a material is defined as its Mass per unit Volume: \rho = \frac{m}{V} Different materials usually have different 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. In Signal processing, cross-correlation is a measure of similarity of two waveforms as a function of a time-lag applied to one of them A seismogram is a graph output by a Seismograph. It is a record of the ground motion at a measuring station
See also
External links
- Illustrated discussion of seismic waves - Purdue.edu
- Earthquake Basics - Seismic Waves
- Seismic Waves by EduMedia
Dictionary
seismic wave
-noun
- Any of several forms of vibrational waves that travel through the Earth as the result of an earthquake or underground explosion.
Dapyx Software network: MP3 Explorer | Ebook Manager | Zenithic