seismic waves
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Seismic waves are
waves The United States Naval Reserve (Women's Reserve), better known as the WAVES (for Women Accepted for Volunteer Emergency Service), was the women's branch of the United States Naval Reserve The United States Navy Reserve (USNR), known as the Un ...

waves
of
energy In , energy is the that must be to a or to perform on the body, or to it. Energy is a ; the law of states that energy can be in form, but not created or destroyed. The unit of measurement in the (SI) of energy is the , which is the ...

energy
that travel through
Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wit ...

Earth
's layers, and are a result of
earthquake An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth Earth is the third planet from the Sun and the only astronomical object known t ...

earthquake
s,
volcanic eruptions Several types of volcanic eruptions—during which lava of pāhoehoe lava, Hawaii, United States , Iceland in 1984 Lava is molten Rock (geology), rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) ...
, magma movement, large
landslide Landslides, also known as landslips, are several forms of mass wasting Mass wasting, also known as mass movement, is a general term for the movement of rock (geology), rock or soil down slopes under the force of gravity. It differs from othe ...

landslide
s and large man-made
explosion An explosion is a rapid expansion in volume associated with an extremely vigorous outward release of energy, usually with the generation of high temperatures and release of high-pressure gases. Supersonic explosions created by high explosives ...

explosion
s that give out low-frequency acoustic energy. Many other natural and anthropogenic sources create low-amplitude waves commonly referred to as
ambient vibrations
ambient vibrations
. Seismic waves are studied by
geophysicists Geophysics () is a subject of natural science Natural science is a branch of science Science (from the Latin word ''scientia'', meaning "knowledge") is a systematic enterprise that Scientific method, builds and Taxonomy (general), or ...

geophysicists
called
seismologists Seismology (; from Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following period ...
. Seismic wave fields are recorded by a
seismometer A seismometer is an instrument that responds to ground noises and shaking such as caused by earthquakes An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of ...

seismometer
,
hydrophone A hydrophone ( grc, ὕδωρ + φωνή, , water + sound) is a microphone A microphone, colloquially called a mic or mike (), is a device – a transducer – that converts sound In physics Physics (from grc, φυσική ( ...

hydrophone
(in water), or
accelerometer An accelerometer is a tool that measures proper acceleration. Proper acceleration is the acceleration In mechanics Mechanics (Ancient Greek, Greek: ) is the area of physics concerned with the motions of physical objects, more specificall ...

accelerometer
. The propagation
velocity The velocity of an object is the rate of change of its position with respect to a frame of reference In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (p ...
of seismic waves depends on
density The density (more precisely, the volumetric mass density; also known as specific mass), of a substance is its per unit . The symbol most often used for density is ''ρ'' (the lower case Greek letter ), although the Latin letter ''D'' can also ...

density
and
elasticity Elasticity often refers to: *Elasticity (physics), continuum mechanics of bodies that deform reversibly under stress Elasticity may also refer to: Information technology * Elasticity (data store), the flexibility of the data model and the clu ...
of the medium as well as the type of wave. Velocity tends to increase with depth through Earth's crust and mantle, but drops sharply going from the mantle to
outer core Earth's outer core is a fluid layer about thick and composed of mostly iron Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transi ...
. Earthquakes create distinct types of waves with different velocities; when reaching seismic observatories, their different
travel times
travel times
help scientists to locate the source of the
hypocenter A hypocenter (or hypocentre; ) is the point of origin of an earthquake or a subsurface nuclear explosion. In seismology, it is a synonym of the focus. Outside seismology, the term ''hypocenter'' is also used as a synonym for ''ground zero'', t ...
. In geophysics, the refraction or reflection of seismic waves is used for research into the structure of
Earth's interior The internal structure of Earth, structure of the solid Earth, or simply structure of Earth refers to concentric spherical layers subdividing the Solid earth, i.e., excluding Earth's atmosphere File:Atmosphere gas proportions.svg, Compositi ...
, and man-made vibrations are often generated to investigate shallow, subsurface structures.


Types

Among the many types of seismic waves, one can make a broad distinction between ''body waves'', which travel through the Earth, and ''surface waves'', which travel at the Earth's surface. (Also se
errata
Other modes of wave propagation exist than those described in this article; though of comparatively minor importance for earth-borne waves, they are important in the case of
asteroseismology Asteroseismology is the study of oscillations in stars. Because a star's different oscillation modes are sensitive to different parts of the star, they inform astronomers about the internal structure of the star, which is otherwise not directly po ...
. * Body waves travel through the interior of the Earth. * Surface waves travel across the surface. Surface waves decay more slowly with distance than body waves which travel in three dimensions. * Particle motion of surface waves is larger than that of body waves, so surface waves tend to cause more damage.


Body waves

Body waves travel through the interior of the Earth along paths controlled by the material properties in terms of
density The density (more precisely, the volumetric mass density; also known as specific mass), of a substance is its per unit . The symbol most often used for density is ''ρ'' (the lower case Greek letter ), although the Latin letter ''D'' can also ...

density
and modulus (stiffness). The density and modulus, in turn, vary according to temperature, composition, and material phase. This effect resembles the
refraction In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force ...

refraction
of
light wave Light or visible light is electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies ...
s. Two types of particle motion result in two types of body waves: ''Primary'' and ''Secondary'' waves.


Primary waves

Primary waves (P-waves) are compressional waves that are
longitudinal Longitudinal is a geometric term of location which may refer to: * Longitude ** Line of longitude, also called a meridian (geography), meridian * Longitudinal engine, an internal combustion engine in which the crankshaft is oriented along the long ...

longitudinal
in nature.
P-waves A P wave (primary wave or pressure wave) is one of the two main types of elastic body waves, called seismic waves in seismology. P waves travel faster than other seismic waves and hence are the first signal from an earthquake to arrive at any ...
are pressure waves that travel faster than other waves through the earth to arrive at seismograph stations first, hence the name "Primary". These waves can travel through any type of material, including fluids, and can travel nearly 1.7 times faster than the
S-waves __NOTOC__ In seismology Seismology (; from Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided ...
. In air, they take the form of sound waves, hence they travel at the
speed of sound The speed of sound is the distance travelled per unit of time by a sound wave as it propagates through an elasticity (solid mechanics), elastic medium. At , the speed of sound in air is about , or one kilometre in or one mile in . It depends s ...
. Typical speeds are 330 m/s in air, 1450 m/s in water and about 5000 m/s in
granite Granite () is a coarse-grained (phaneritic A phanerite is an igneous rock Igneous rock (derived from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Lat ...

granite
.


Secondary waves

Secondary waves (S-waves) are shear waves that are
transverse Transverse may refer to: *Transverse engine, an engine in which the crankshaft is oriented side-to-side relative to the wheels of the vehicle *Transverse flute, a flute that is held horizontally *Euler force, Transverse force (or ''Euler force''), ...

transverse
in nature. Following an earthquake event, S-waves arrive at seismograph stations after the faster-moving P-waves and displace the ground perpendicular to the direction of propagation. Depending on the propagational direction, the wave can take on different surface characteristics; for example, 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. S-waves are slower than P-waves, and speeds are typically around 60% of that of P-waves in any given material. Shear waves can't travel through any liquid medium, so the absence of S-waves in earth's outer core suggests a liquid state.


Surface waves

Seismic surface waves travel along the Earth's surface. They can be classified as a form of mechanical surface waves. They are called surface waves, as they diminish as they get further from the surface. They travel more slowly than seismic body waves (P and S). In large earthquakes, surface waves can have an amplitude of several centimeters.


Rayleigh waves

Rayleigh waves, also called ground roll, are surface waves that travel as ripples with motions that are similar to those of waves on the surface of water (note, however, that the associated particle motion at shallow depths is retrograde, and that the restoring force in Rayleigh and in other seismic waves is elastic, not gravitational as for water waves). The existence of these waves was predicted by John William Strutt,
Lord Rayleigh John William Strutt, 3rd Baron Rayleigh, (; 12 November 1842 – 30 June 1919) was a British scientist who made extensive contributions to both theoretical A theory is a rational type of abstract thinking about a phenomenon A phenome ...

Lord Rayleigh
, in 1885. They are slower than body waves, roughly 90% of the velocity of S waves for typical homogeneous elastic media. In a layered medium (like the crust and
upper mantle The upper mantle of Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. About 29% of Earth's surface is land consisting of continent A continent is one of several large landmasses. ...
) the velocity of the Rayleigh waves depends on their frequency and wavelength. See also
Lamb wavesLamb waves propagate in solid plates or spheres. They are elastic waves whose particle motion lies in the plane that contains the direction of wave propagation and the plane normal (the direction perpendicular to the plate). In 1917, the English mat ...
.


Love waves

Love waves are horizontally polarized
shear wave __NOTOC__ In seismology Seismology (; from Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided ...
s (SH waves), existing only in the presence of a semi-infinite medium overlain by an upper layer of finite thickness. 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, about 90% of the S wave velocity, and have the largest amplitude.


Stoneley waves

A Stoneley wave is a type of boundary wave (or interface wave) that propagates along a solid-fluid boundary or, under specific conditions, also along a solid-solid boundary. Amplitudes of Stoneley waves have their maximum values at the boundary between the two contacting media and decay exponentially towards the depth of each of them. These waves can be generated along the walls of a fluid-filled
borehole A borehole is a narrow shaft Shaft may refer to: Rotating machine elements * Shaft (mechanical engineering), a rotating machine element used to transmit power * Line shaft, a power transmission system * Drive shaft, a shaft for transferring tor ...

borehole
, being an important source of coherent noise in
vertical seismic profileIn geophysics, vertical seismic profile (VSP) is a technique of seismic measurements used for correlation with surface seismic data. The defining characteristic of a VSP (of which there are many types) is that either the energy source, or the detecto ...
s (VSP) and making up the low frequency component of the source in sonic logging. The equation for Stoneley waves was first given by Dr. Robert Stoneley (1894–1976), Emeritus Professor of Seismology, Cambridge.


Normal modes

Free oscillations of the Earth are standing waves, the result of interference between two surface waves traveling in opposite directions. Interference of Rayleigh waves results in ''spheroidal oscillation S'' while interference of Love waves gives ''toroidal oscillation T''. The modes of oscillations are specified by three numbers, e.g., nSlm, where ''l'' is the angular order number (or ''spherical harmonic degree'', see Spherical harmonics for more details). The number ''m'' is the azimuthal order number. It may take on 2''l''+1 values from −''l'' to +''l''. The number ''n'' is the ''radial order number''. It means the wave with ''n'' zero crossings in radius. For spherically symmetric Earth the period for given ''n'' and ''l'' does not depend on ''m''. Some examples of spheroidal oscillations are the "breathing" mode 0S0, which involves an expansion and contraction of the whole Earth, and has a period of about 20 minutes; and the "rugby" mode 0S2, which involves expansions along two alternating directions, and has a period of about 54 minutes. The mode 0S1 does not exist because it would require a change in the center of gravity, which would require an external force. Of the fundamental toroidal modes, 0T1 represents changes in Earth's rotation rate; although this occurs, it is much too slow to be useful in seismology. The mode 0T2 describes a twisting of the northern and southern hemispheres relative to each other; it has a period of about 44 minutes. The first observations of free oscillations of the Earth were done during the great Chile earthquake 1960, 1960 earthquake in Chile. Presently periods of thousands of modes are known. These data are used for determining some large scale structures of the Earth interior.


P and S waves in Earth's mantle and core

When an earthquake occurs, seismographs near the epicenter 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. 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. This kind of observation has also been used to argue, by Exploration geophysics, seismic testing, that the Moon has a solid core, although recent geodetic studies suggest the core is still molten.


Notation

The path that a wave takes between the focus and the observation point is often drawn as a ray diagram. An example of this is shown in a figure above. When reflections are taken into account there are an infinite number of paths that a wave can take. Each path is denoted by a set of letters that describe the trajectory and phase through the Earth. In general an upper case denotes a transmitted wave and a lower case denotes a reflected wave. The two exceptions to this seem to be "g" and "n". For example: * ScP is a wave that begins traveling towards the center of the Earth as an S wave. Upon reaching the outer core the wave reflects as a P wave. * sPKIKP is a wave path that begins traveling towards the surface as an S-wave. At the surface it reflects as a P-wave. The P-wave then travels through the outer core, the inner core, the outer core, and the mantle.


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, three 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. Typically, dozens or even hundreds of P-wave arrivals are used to calculate
hypocenter A hypocenter (or hypocentre; ) is the point of origin of an earthquake or a subsurface nuclear explosion. In seismology, it is a synonym of the focus. Outside seismology, the term ''hypocenter'' is also used as a synonym for ''ground zero'', t ...
s. 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. 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 Speed of sound#Three-dimensional solids, 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 is 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.


See also

*Adams–Williamson equation *Helioseismology *Reflection seismology


References


Sources

*


External links


EDT: A MATLAB Website for seismic wave propagation
{{DEFAULTSORT:Seismic Wave Seismology Surface waves