query: a seismic wave that travels along the earth surface

pos: Seismic wave the Earth's crust, up to 13 km/s in the deep mantle. Earthquakes create distinct types of waves with different velocities; when reaching seismic observatories, their different travel times help scientists to locate the source of the hypocenter. In geophysics the refraction or reflection of seismic waves is used for research into the structure of the Earth's interior, and man-made vibrations are often generated to investigate shallow, subsurface structures. 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. Other modes

pos: Surface wave "ground wave" is a guided wave that propagates close to the surface of the Earth. In seismology, several types of surface waves are encountered. Surface waves, in this mechanical sense, are commonly known as either "Love waves" (L waves) or "Rayleigh waves". A seismic wave is a wave that "travels through the Earth, often as the result of an earthquake or explosion." Love waves have transverse motion (movement is perpendicular to the direction of travel, like light waves), whereas Rayleigh waves have both longitudinal (movement parallel to the direction of travel, like sound waves) and transverse motion. Seismic waves are

pos: Surface wave inversion Surface wave inversion Seismic inversion involves the set of methods which seismologists use to infer properties through physical measurements. Surface-wave inversion is the method by which elastic properties, density, and thickness of layers in the subsurface are obtained through analysis of surface-wave dispersion. The entire inversion process requires the gathering of seismic data, the creation of dispersion curves, and finally the inference of subsurface properties. Surface waves are seismic waves that travel at the surface of the earth, along the air/earth boundary. Surface waves are slower than P-waves(compressional waves) and S-waves(transverse waves). Surface waves are classified into two basic types,

pos: Seismic tomography Seismic tomography Seismic tomography is a technique for imaging the subsurface of the Earth with seismic waves produced by earthquakes or explosions. P-, S-, and surface waves can be used for tomographic models of different resolutions based on seismic wavelength, wave source distance, and the seismograph array coverage. The data received at seismometers are used to solve an inverse problem, wherein the locations of reflection and refraction of the wave paths are determined. This solution can be used to create 3D images of velocity anomalies which may be interpreted as structural, thermal, or compositional variations. Geoscientists use these images to

pos: Seismic wave 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. Seismic surface waves travel along the Earth's surface. They can be classified as a form of mechanical surface waves.

pos: Seismic tomography based on visual perceptions of color, such as the change from orange to red being more subtle than blue to yellow. The degree of color saturation can also visually skew interpretations. These factors should be considered when analyzing images. Seismic tomography Seismic tomography is a technique for imaging the subsurface of the Earth with seismic waves produced by earthquakes or explosions. P-, S-, and surface waves can be used for tomographic models of different resolutions based on seismic wavelength, wave source distance, and the seismograph array coverage. The data received at seismometers are used to solve an inverse problem, wherein

pos: Love wave Love wave In elastodynamics, Love waves, named after Augustus Edward Hough Love, are horizontally polarized surface waves. The Love wave is a result of the interference of many shear waves (S–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. In seismology, Love waves (also known as Q waves ("Q"uer: German for lateral)) are surface seismic waves that cause horizontal shifting of the Earth during an earthquake. Augustus Edward Hough Love predicted the existence of Love waves mathematically in 1911. They form a distinct class,

pos: Surface wave inversion inversion. Elastic properties of the earth are those properties which affect the propagation of elastic waves. These properties are Lamé parameters and are used to relate stress to strain in isotropic media through Hooke’s law. Density is also related to elastic parameters through velocity equations for compressional and shear waves. Two main data gathering techniques are employed in gathering surface wave information. The two methods are spectral analysis of surface waves (SASW) and multi-channel analysis of surface waves (MASW). These techniques use either passive or active sources. Passive sources are simply ambient noise, while active sources include traditional seismic sources

pos: Surface wave inversion modify seismic equipment and experimental procedures to remove surface wave information from the data. Earthquake seismologists however require the information seismic surface waves provide and thus design their equipment to amplify and gather as much information on these waves as possible. The work by early earthquake seismologists to extract substantial information from surface wave data was the basis for surface wave inversion theory. The usefulness of surface waves in determining subsurface elastic properties arises from the way in which they disperse. Dispersion (geology) is the way in which surface waves spread out as they travel across the surface of the

pos: Surface wave Surface waves have been studied as part of transmission lines and some may be considered as single-wire transmission lines. Characteristics and utilizations of the electrical surface wave phenomenon include: The surface plasmon polariton (SPP) is an electromagnetic surface wave that travels along an interface between two media with different dielectric constants. It exists under the condition that the permittivity of one of the materials forming the interface is negative, while the other one is positive, as is the case for the interface between air and a lossy conducting medium below the plasma frequency. The wave propagates parallel to the interface

pos: Seismic magnitude scales the amplitude of short-period (~1 sec.) Lg waves, a complex form of the Love wave which, although a surface wave, he found provided a result more closely related the scale than the scale. Lg waves attenuate quickly along any oceanic path, but propagate well through the granitic continental crust, and Mb is often used in areas of stable continental crust; it is especially useful for detecting underground nuclear explosions. Surface waves propagate along the Earth's surface, and are principally either Rayleigh waves or Love waves. For shallow earthquakes the surface waves carry most of the energy of the earthquake, and

pos: Surface wave inversion Rayleigh waves and Love waves. Rayleigh waves travel in a longitudinal manner (the wave motion is parallel to the direction of wave propagation) with particle motion in a retrograde elliptical motion (Figure 1). The Rayleigh waves result from the interaction between P-waves and vertically polarized S-waves. Conversely, Love waves travel in a traverse manner (Figure 1) (the wave motion is perpendicular to the direction of wave propagation), consisting of horizontally polarized S-waves. In seismology, surface waves are collected along with other seismic data, but are traditionally considered noise and an impedance in interpreting deeper reflection and refraction information. Seismologists usually

pos: Seismic wave 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, 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

pos: Surface wave inversion more time for the waves to reach each geophone, giving the longer wavelengths more time to disperse. The shot gather is modified to minimize the influence of body waves. As the data is gathered, the spectral analyzer is able to generate the dispersion curves for the survey area in real time. The multi-channel analysis of surface waves (MASW) technique can be performed similar to a traditional seismic acquisition whereby there is a geophone spread that is acquiring seismic data. The resulting data is processed by picking out the surface wave arrivals from the acquired distance vs. time plot. Based on

pos: Surface wave magnitude Surface wave magnitude The surface wave magnitude (formula_1) scale is one of the magnitude scales used in seismology to describe the size of an earthquake. It is based on measurements in Rayleigh surface waves that travel primarily along the uppermost layers of the Earth. It is currently used in People's Republic of China as a national standard(GB 17740-1999) for categorising earthquakes. Surface wave magnitude was initially developed in the 1950s by the same researchers who developed the local magnitude scale M in order to improve resolution on larger earthquakes: Recorded magnitudes of earthquakes during that time, commonly attributed to Richter,

pos: Surface wave magnitude corresponding to A in s, and T is the period corresponding to A in s. Vladimír Tobyáš and Reinhard Mittag proposed to relate surface wave magnitude to local magnitude scale M, using Other formulas include three revised formulae proposed by CHEN Junjie et al.: and Surface wave magnitude The surface wave magnitude (formula_1) scale is one of the magnitude scales used in seismology to describe the size of an earthquake. It is based on measurements in Rayleigh surface waves that travel primarily along the uppermost layers of the Earth. It is currently used in People's Republic of China as a

pos: Earthquake duration magnitude Earthquake duration magnitude The concept of Earthquake Duration Magnitude - originally proposed by Bisztricsany in 1958 using surface waves only - is based on the realization that on a recorded earthquake seismogram, the total length of the seismic wavetrain - sometimes referred to as the CODA - reflects its size. Thus larger earthquakes give longer seismograms [as well as stronger seismic waves] than small ones. The seismic wave interval measured on the time axis of an earthquake record - starting with the first seismic wave onset until the wavetrain amplitude diminishes to at least 10% of its maximum recorded value

pos: Tsunami earthquake is that the release of seismic energy occurs at long periods (low frequencies) relative to typical tsunamigenic earthquakes. Earthquakes of this type do not generally show the peaks of seismic wave activity associated with ordinary events. A tsunami earthquake can be defined as an undersea earthquake for which the surface wave magnitude M differs markedly from the moment magnitude M, because the former is calculated from surface waves with a period of about 20 seconds, whereas the latter is a measure of the total energy release at all frequencies. The displacements associated with tsunami earthquakes are consistently greater than those

pos: Deep-focus earthquake Deep-focus earthquakes give rise to minimal surface waves. Their focal depth causes the earthquakes to be less likely to produce seismic wave motion with energy concentrated at the surface. The path of deep-focus earthquake seismic waves from focus to recording station goes through the heterogeneous upper mantle and highly variable crust only once. Therefore, the body waves undergo less attenuation and reverberation than seismic waves from shallow earthquakes, resulting in sharp body wave peaks. The pattern of energy radiation of an earthquake is represented by the moment tensor solution, which is graphically represented by beachball diagrams. An explosive or implosive

pos: Seismic metamaterial are body waves and surface waves. Both of these have different modes of wave propagation. Computations showed that seismic waves traveling toward a building, could be directed around the building, leaving the building unscathed, by using "seismic metamaterials". The very long wavelengths of earthquake waves would be shortened as they interact with the metamaterials; the waves would pass around the building so as to arrive in phase as the earthquake wave proceeded, as if the building was not there. The mathematical models produce the regular pattern provided by Metamaterial cloaking. This method was first understood with electromagnetic cloaking metamaterials -

pos: Surface wave inversion log. The discrepancy in overall detail should have no effect in evaluating the subsurface. There are many advantages to using surface waves to image the subsurface. For one, surface wave inversion readily images low-velocity zones. Refraction methods cannot see low-velocity zones because such a zone would bend the traversing wave deeper instead of towards the surface. Surface wave inversion is also non-invasive as well as cost effective. There are a few disadvantages to this method as well. The resolution of the surface wave inversion method is not nearly as resolved as a seismic collection done in a wellbore. There is

pos: Seismic tomography these areas will improve when more data becomes available. The uneven distribution of earthquakes naturally biases models to better resolution in seismically active regions. The type of wave used in a model limits the resolution it can achieve. Longer wavelengths are able to penetrate deeper into the earth, but can only be used to resolve large features. Finer resolution can be achieved with surface waves, with the trade off that they cannot be used in models of the deep mantle. The disparity between wavelength and feature scale causes anomalies to appear of reduced magnitude and size in images. P- and

pos: November 11 2018 seismic event November 11 2018 seismic event On November 11, 2018, a seismic event occurred about off the coast of Mayotte, an overseas French territory. It was recorded by seismograms almost away. Despite this, no one felt it. The seismic waves lasted for over 20 minutes. Most earthquakes have P-waves and S-waves. At the end there are long-period surface waves. The Mayotte event lacked P-waves and S-waves, but did experience a long-period surface wave. Additionally, the signal released by the earthquake was a clean zigzag, while most earthquake waves have multiple frequencies. The cause of the event is not yet known, but

pos: November 11 2018 seismic event scientists from the French Geological Survey believe it may have been caused by an underwater volcano, and also related to an earthquake swarm nearby. November 11 2018 seismic event On November 11, 2018, a seismic event occurred about off the coast of Mayotte, an overseas French territory. It was recorded by seismograms almost away. Despite this, no one felt it. The seismic waves lasted for over 20 minutes. Most earthquakes have P-waves and S-waves. At the end there are long-period surface waves. The Mayotte event lacked P-waves and S-waves, but did experience a long-period surface wave. Additionally, the signal released

pos: Surface wave detection by animals in surface wave detection is not entirely clear; Xenopus with all of their lateral line organs destroyed are still able to respond to surface waves in an oriented fashion. Compared to animals with an intact system they respond somewhat less accurately to anterior stimuli and much less accurately to stimuli from behind. Areas of the Xenopus brain responding to surface waves have been identified. It has been speculated that both Xenopus and the European grassfrog use their seismic sensory capabilities to locate conspecifics in breeding ponds. Dolomedes spiders hunt by waiting at the edge of a pool or stream. They

neg: Ground wave propagation out from the transmitter along the surface of the Earth. Instead of just travelling in a straight line the radio signals tend to follow the curvature of the Earth. This is because currents are induced in the surface of the earth and this action slows down the wave-front in this region, causing the wave-front of the radio communications signal to tilt downwards towards the Earth. With the wave-front tilted in this direction it is able to curve around the Earth and be received well beyond the horizon. As the wavefront of the ground wave travels along the Earth's surface it

neg: Rayleigh wave Rayleigh wave Rayleigh waves are a type of surface acoustic wave that travel along the surface of solids. They can be produced in materials in many ways, such as by a localized impact or by piezo-electric transduction, and are frequently used in non-destructive testing for detecting defects. Rayleigh waves are part of the seismic waves that are produced on the Earth by earthquakes. When guided in layers they are referred to as Lamb waves, Rayleigh–Lamb waves, or generalized Rayleigh waves. Rayleigh waves are a type of surface wave that travel near the surface of solids. Rayleigh waves include both longitudinal

neg: SKS wave hits the core-mantle boundary as it leaves the core, some energy is converted back to shear energy before the wave travels through the mantle. SKS waves can be observed by seismometers roughly 60-141° from an earthquake according to the Preliminary reference Earth model. SKS wave An SKS wave is a type of seismic wave, with the three letters denoting the type of wave-motion through the mantle and outer core. The first S indicates that the wave travels as a shear or s-wave through the mantle, the K shows that the wave travels as a compressional or p-wave through the outer

neg: Reflection seismology the geologic structure that generated the reflections. Interpretation of large surveys is usually performed with programs using high-end three-dimensional computer graphics. In addition to reflections off interfaces within the subsuface, there are a number of other seismic responses detected by receivers and are either unwanted or unneeded: The airwave travels directly from the source to the receiver and is an example of coherent noise. It is easily recognizable because it travels at a speed of 330 m/s, the speed of sound in air. A Rayleigh wave typically propagates along a free surface of a solid, but the elastic constants and

neg: Mantle plume measured in numerous different ways, including surface heat flow, petrology, and seismology. Thermal anomalies produce anomalies in the speeds of seismic waves, but unfortunately so do composition and partial melt. As a result, wave speeds cannot be used simply and directly to measure temperature, but more sophisticated approaches must be taken. Seismic anomalies are identified by mapping variations in wave speed as seismic waves travel through Earth. A hot mantle plume is predicted to have lower seismic wave speeds compared with similar material at a lower temperature. Mantle material containing a trace of partial melt (e.g., as a result of

neg: Reflection seismology "V" is the seismic wave velocity and "ρ" (Greek "rho") is the density of the rock. When a seismic wave travelling through the Earth encounters an interface between two materials with different acoustic impedances, some of the wave energy will reflect off the interface and some will refract through the interface. At its most basic, the seismic reflection technique consists of generating seismic waves and measuring the time taken for the waves to travel from the source, reflect off an interface and be detected by an array of receivers (or geophones) at the surface. Knowing the travel times from the

neg: Supershear earthquake Supershear earthquake A supershear earthquake is an earthquake in which the propagation of the rupture along the fault surface occurs at speeds in excess of the seismic shear wave (S-wave) velocity. This causes an effect analogous to a sonic boom. During seismic events along a fault surface the displacement initiates at the focus and then propagates outwards. Typically for large earthquakes the focus lies towards one end of the slip surface and much of the propagation is unidirectional (e.g. the 2008 Sichuan and 2004 Indian Ocean earthquakes). Theoretical studies have in the past suggested that the upper bound for propagation

neg: Core–mantle boundary commonly used in reference to a decrease in seismic velocity with depth that is sometimes observed at about 100 km below the Earth's oceans. Core–mantle boundary The core–mantle boundary (CMB in the parlance of solid earth geophysicists) of the Earth lies between the planet's silicate mantle and its liquid iron-nickel outer core. This boundary is located at approximately 2891 km (1796 mi) depth beneath the Earth's surface. The boundary is observed via the discontinuity in seismic wave velocities at that depth due to the differences between the acoustic impedances of the solid mantle and the molten outer core. P-wave velocities

neg: Reflection seismology density of air are very low compared to those of rocks so the surface of the Earth is approximately a free surface. Low velocity, low frequency and high amplitude Rayleigh waves are frequently present on a seismic record and can obscure signal, degrading overall data quality. They are known within the industry as ‘Ground Roll’ and are an example of coherent noise that can be attenuated with a carefully designed seismic survey. The Scholte wave is similar to ground roll but occurs at the sea-floor (fluid/solid interface) and it can possibly obscure and mask deep reflections in marine seismic records.

neg: Spherical surface acoustic wave (SAW) sensor above mentioned parameters is very small due to limited path offered to Surface acoustic wave by planar sensor. In case of Spherical sensors surface acoustic wave make several round trips along the equator of a ball as shown in fig, which offer longer paths to Surface acoustic wave hence even smaller change in parameters is amplified with multiple turns, which increases the sensitivity of the sensor considerably. Spherical surface acoustic wave (SAW) sensor Spherical surface acoustic wave sensors use a type of surface acoustic wave (SAW) that travels along the surface of a medium exhibiting elasticity with exponentially decaying amplitude

neg: Spherical surface acoustic wave (SAW) sensor Spherical surface acoustic wave (SAW) sensor Spherical surface acoustic wave sensors use a type of surface acoustic wave (SAW) that travels along the surface of a medium exhibiting elasticity with exponentially decaying amplitude along depth. MEMS-IDT technology allows the use of SAW waves to sense various gases. Sensitivity up to 10 ppm of hydrogen using a spherical Ball SAW device is obtained. Conventional planar SAW sensors are based on principle that the parameters such as amplitude, speed and phase of Surface acoustic wave changes on adsorption of gas molecules. Limitation of planar SAW based sensors is that the change in

neg: Seismic inverse Q filtering velocity dispersion, seismic data with enhanced vertical resolution can yield correct timings for lithological identification. However, Wang's outline of the subject is excellent and to follow his path, inverse Q filtering can be introduced based on the 1-D one-way propagation wave equation. He introduce this equation:. where U(r,w) is the plane wave of radial frequency w at travel distance r, k(w) is the wavenumber and i is the imaginary unit. Reflection seismograms record the reflection wave along the propagation path r from the source to reflector and back to the surface. With this approach Wang assumes that the plane wave

neg: Surface acoustic wave the propagation of a wave front, which appears similar to seismic activities. The waves are generated at the excitation centre and spread out along the surface of a solid material. An electric pulse induces them to generate SAWs that propagate like the waves of an earthquake. Interdigital transducer acts as sender and as receiver. When one is in sender mode, the two most distant ones act as receivers. The SAWs travel along the surface of the measuring tube, but a portion will couple out to the liquid. The decoupling angle depends on the liquid respectively the propagation velocity of the

neg: Geothermal exploration exploration. Seismic waves propagate and interact with subterranean components and respond accordingly. Two sub categories exist that are relevant to the source of the seismic signal. Active seismology relies on using induced/man-made vibrations at or near the surface. Passive seismology uses earthquakes, volcanic eruptions or other tectonic activity as sources. Passive seismic studies use natural wave propagation through the earth. Geothermal fields are often characterized by increased levels of seismicity. Earthquakes of lesser magnitude are much more frequent than ones of larger magnitude. Therefore, these micro earthquakes (MEQ), registering below 2.0 magnitude on the Richter scale, are used to reveal

neg: P-wave propagation of a P-wave is always longitudinal; thus, the particles in the solid vibrate along the axis of propagation (the direction of motion) of the wave energy. Primary and secondary waves are body waves that travel within the Earth. The motion and behavior 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

neg: Shear wave splitting matrix with eigenvalues corresponding to the two shear wave velocities. The resulting split waveform is Where formula_22 is the time delay between the slow and fast shear waves and formula_23 is the angle between the polarizations of the slow and fast shear waves. These two parameters can be individually estimated from multiple component seismic recordings (Fig. 5). Figure 6 is a schematic animation showing the process of shear wave splitting and the seismic signature generated by the arrivals of two polarized shear waves at the surface recording station. There is one incident shear wave (blue) traveling vertically along the center

neg: SKS wave SKS wave An SKS wave is a type of seismic wave, with the three letters denoting the type of wave-motion through the mantle and outer core. The first S indicates that the wave travels as a shear or s-wave through the mantle, the K shows that the wave travels as a compressional or p-wave through the outer core, and the final S indicates an s-wave, returning through the mantle. As shear waves cannot be transmitted through the liquid Outer core, some of the s-wave energy is converted to p-wave energy as the wave hits the Core–mantle boundary. When the wave

neg: Core–mantle boundary Core–mantle boundary The core–mantle boundary (CMB in the parlance of solid earth geophysicists) of the Earth lies between the planet's silicate mantle and its liquid iron-nickel outer core. This boundary is located at approximately 2891 km (1796 mi) depth beneath the Earth's surface. The boundary is observed via the discontinuity in seismic wave velocities at that depth due to the differences between the acoustic impedances of the solid mantle and the molten outer core. P-wave velocities are much slower in the outer core than in the deep mantle while S-waves do not exist at all in the liquid portion of

neg: Near-surface geophysics identifies these layer boundaries by the reflections. Seismic reflection is used for imaging of nearly horizontal layers in the Earth. The method is much like echo sounding. It can be used to identify folding and faulting, and to search for oil and gas fields. On a regional scale, profiles can be combined to get sequence stratigraphy, making it possible to date sedimentary layers and identify eustatic sea level rise. Seismic refraction can be used not only to identify layers in rocks by the trajectories of the seismic waves, but also to infer the wave speeds in each layer, thereby providing

neg: Surface wave inversion subsurface profiles. Foti, S., Comina, C., Boiero, D., Socco, L. V., 2009, Non-uniqueness in surface-wave inversion and consequences on seismic site response analyses: Soil Dynamics and Earthquake Engineering, v. 29, p. 982-993. Kennett, B.L.N., 1976, The inversion of surface wave data: Pure and Applied Geophysics, v. 114, p 747-751. Luke, B., Calderon-Macias, C., 2007, Inversion of seismic surface wave data to resolve complex profiles: Journal of Geotechnical and Geoenvironmental Engineering, v. 133, p. 155-165. Lai, C. G., Foti, S., and Rix, G. J., 2005, Propagation of data uncertainty in surface wave inversion: Journal of Environmental & Engineering Geophysics, v. 10,

neg: Seismic refraction usually more-readily identifiable within the seismic recording as compared to the other seismic waves. S-wave refraction evaluates the shear wave generated by the seismic source located at a known distance from the array. The wave is generated by horizontally striking an object on the ground surface to induce the shear wave. Since the shear wave is the second fastest wave, it is sometimes referred to as the secondary wave. When compared to the compression wave, the shear wave is approximately one-half (but may vary significantly from this estimate) the velocity depending on the medium. i - critical angle V -

neg: Seismic site effects with an incidence formula_4. It thus generates: The refracted wave originates a reflected wave when reaching the free surface libre; its amplitude and incidence are denoted formula_9 and formula_8 respectively. This latter wave will be reflected and refracted several times at the base and the top of the surficial layer. If the layer is softer than the half-space, the surface motion amplitude can be larger than formula_3 thus leading to the "amplification of seismic waves" or "seismic site effetcs". When the geological interfaces are not horizontal, it also possible to study seismic site effetcs by considering the "basin effects" due

neg: 2001 Kunlun earthquake is unusually large, with significant faulting being observed up to 60 km from the main rupture trace. This deformation occurs in two swathes, ca. 20 and 60 km from the main fault trace. Pre-existing lineaments and geomorphological features suggest that this earthquake-triggered displacement occurred on existing faults. The co-seismic surface rupture extended for more than 400 km, making it the longest zone of co-seismic surface rupture so far observed. An analysis of the propagation speed indicates that the rupture propagated at a normal velocity along the original segment, but increased in velocity to above the S-wave velocity after the jump

neg: Surface wave permittivity of one of the materials forming the interface is negative, while the other one is positive, as for example the interface between air and a lossy conducting medium such as the terrestrial transmission line, below the plasma frequency. Arising from original analysis by Arnold Sommerfeld and Jonathan Zenneck of the problem of wave propagation over a lossy earth, it exists as an exact solution to Maxwell's equations. Surface wave In physics, a surface wave is a mechanical wave that propagates along the interface between differing media. A common example is gravity waves along the surface of liquids, such as

neg: Seismic metamaterial Seismic metamaterial A seismic metamaterial, is a metamaterial that is designed to counteract the adverse effects of seismic waves on artificial structures, which exist on or near the surface of the earth. seismic metamaterials were still in the development stage. More than a million earthquakes are recorded each year, by a worldwide system of earthquake detection stations. The propagation velocity of the seismic waves depends on density and elasticity of the earth materials. In other words, the speeds of the seismic waves vary as they travel through different materials in the earth. The two main components of a seismic event

neg: Mechanical wave vibrate parallel to the direction of the wave. It consists of multiple compressions and rarefactions. The rarefaction is the farthest distance apart in the longitudinal wave and the compression is the closest distance together. The speed of the longitudinal wave is increased in higher index of refraction, due to the closer proximity of the atoms in the medium that is being compressed. Sound is considered a longitudinal wave. This type of wave travels along a surface that is between two media. An example of a surface wave would be waves in a pool, or in an ocean, lake, or any

neg: Multidimensional seismic data processing seismic profile. This involves gathering seismic traces from points at the same depth and summing them. This is referred to as "Common depth-point stacking" or "Common midpoint stacking". Simply speaking, when these traces are merged, the background noise cancels itself out and the seismic signal add up, thus improving the SNR. Assuming a seismic wave formula_27 travelling upwards towards the surface, where formula_28 is the position on the surface and formula_29 is the depth. The wave's propagation is described by: formula_30 Migration refers to this wave's backward propagation. The two-dimensional Fourier transform of the wave at depth formula_31 is given

neg: Microseism water waves. For wave trains with a very small difference in frequency (and thus wavenumbers), this pattern of wave groups may have the same velocity as seismic waves, between 1500 and 3000 m/s, and will excite acoustic-seismic modes that radiate away. As far as seismic and acoustic waves are concerned, the motion of ocean waves in deep water is, to the leading order, equivalent to a pressure applied at the sea surface. This pressure is nearly equal to the water density times the wave orbital velocity squared. Because of this square, it is not the amplitude of the individual wave

neg: Mathematical Q models reflection wave along the propagation path r from the source to reflector and back to the surface. Equation (1.1) has an analytical solution given by: where k is the wave number. When the wave propagates in inhomogeneous seismic media the propagation constant k must be a complex value that includes not only an imaginary part, the frequency-dependent attenuation coefficient, but also a real part, the dispersive wave number. We can call this K(w) a propagation constant in line with Futterman. k(w) can be linked to the phase velocity of the wave with the formula: To obtain a solution that can

neg: Adams–Williamson equation Adams–Williamson equation The Adams–Williamson equation, named after L. H. Adams and E. D. Williamson, is an equation used to determine density as a function of radius, more commonly used to determine the relation between the velocities of seismic waves and the density of the Earth's interior. Given the average density of rocks at the Earth's surface and profiles of the P-wave and S-wave speeds as function of depth, it can predict how density increases with depth. It assumes that the compression is adiabatic and that the Earth is spherically symmetric, homogeneous, and in hydrostatic equilibrium. It can also be applied

neg: Clapotis infrasonic microbaroms into the atmosphere, and seismic signals called microseisms coupled through the ocean floor to the solid Earth. Clapotis has been called the bane and the pleasure of sea kayaking. Clapotis In hydrodynamics, the clapotis (from ) is a non-breaking standing wave pattern, caused for example, by the reflection of a traveling surface wave train from a near vertical shoreline like a breakwater, seawall or steep cliff. The resulting clapotic wave does not travel horizontally, but has a fixed pattern of nodes and antinodes. These waves promote erosion at the toe of the wall, and can cause severe damage

neg: Surface wave the earth’s surface. The wavefronts initially are vertical, but the ground, acting as a lossy dielectric, causes the wave to tilt forward as it travels. This directs some of the energy into the earth where it is dissipated, so that the signal decreases exponentially. Most long-distance LF "longwave" radio communication (between 30 kHz and 300 kHz) is a result of groundwave propagation. Mediumwave radio transmissions (frequencies between 300 kHz and 3000 kHz), including AM broadcast band, travel both as groundwaves and, for longer distances at night, as skywaves. Ground losses become lower at lower frequencies, greatly increasing the coverage of

neg: Surface acoustic wave local oscillator has been mixed with the received signal, and so a filter with a fixed frequency and high Q provides excellent removal of unwanted or interference signals. In these applications, SAW filters are almost always used with a phase locked loop synthesized local oscillator, or a varicap driven oscillator. In seismology surface acoustic waves travelling along the Earth's surface play an important role, since they can be the most destructive type of seismic wave produced by earthquakes. In recent years, attention has been drawn to using SAWs to drive microfluidic actuation and a variety of other processes. Owing to

neg: Seismic wave structures of the Earth interior. 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 seismic testing, that the Moon has a solid core, although recent geodetic studies suggest the core is still molten.

neg: Seismic wave 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. Body waves travel through the interior of the Earth along paths controlled by the material properties in terms of density and modulus (stiffness). The density and modulus, in turn, vary according to temperature, composition, and material phase. This effect resembles the refraction of light waves. Two types of particle motion result in two types of body waves: Primary and Secondary waves. Primary waves (P-waves) are compressional waves that are longitudinal in nature. P waves

neg: Seismic migration subsurface from active source seismic data collected at the surface, seabed, boreholes, etc., and therefore is used on industrial scales by oil and gas companies and their service providers on digital computers. Explained in another way, this process attempts to account for wave dispersion from dipping reflectors and also for the spatial and directional seismic wave speed (heterogeneity) variations, which cause wavefields (modelled by ray paths) to bend, wave fronts to cross (caustics), and waves to be recorded at positions different from those that would be expected under straight ray or other simplifying assumptions. Finally, this process often attempts to

neg: Seismic tomography and core based on the availability of data and types of seismic waves that penetrate the region at a suitable wavelength for feature resolution. The accuracy of the model is limited by availability and accuracy of seismic data, wave type utilized, and assumptions made in the model. P-wave data are used in most local models and global models in areas with sufficient earthquake and seismograph density. S- and surface wave data are used in global models when this coverage is not sufficient, such as in ocean basins and away from subduction zones. First-arrival times are the most widely used, but

neg: Surface acoustic wave wave which is specific to the liquid. On the other side of the measuring tube, portions of the wave will couple into the tube and continue their way along its surface to the next interdigital transducer. Another portion will be coupled out again and travels back to the other side of the measuring tube where the effect repeats itself and the transducer on this side detects the wave. That means excitation of any one transducer here will lead to a sequence of input signals on two other transducers in the distance. Two of the transducers send their signals in the

neg: Hyperspace aimed a photon of light at the Moon, it will follow a curved path, since gravity bends all things. The space along which the photon travels is, in fact, curved because gravity curves space itself. Just like traveling along the surface of water; if the surface of the water is swelled in a wave, then it would still be possible to travel in a straight line "through" the water (traveling underneath the wave,) but it would require more effort than just traveling along the curved surface of the water. It is still possible to travel in a straight line to

neg: Inge Lehmann Inge Lehmann Inge Lehmann (13 May 1888 – 21 February 1993) was a Danish seismologist and geophysicist. In 1936, she discovered that the Earth has a solid inner core inside a molten outer core. Before that, seismologists believed Earth's core to be a single molten sphere, being unable, however, to explain careful measurements of seismic waves from earthquakes, which were inconsistent with this idea. Lehmann analysed the seismic wave measurements and concluded that Earth must have a solid inner core and a molten outer core to produce seismic waves that matched the measurements. Other seismologists tested and then accepted Lehmann's

neg: Earthquake casualty estimation the hypocenter, or, more accurately expressed, from the entire area of rupture. To calculate the intensity of shaking at a given settlement, the computer looks up the attenuation (decrease in amplitude) for seismic waves that travel the distance to the settlement in question. Such calculations are similar to those made to assess the seismic hazard, part of the field of engineering seismology. Errors are again introduced through the heterogeneity of the Earth. The loss of energy along the wave path is not exactly the same in all parts of the world. Examples are shown in Figure 2. For poorly studied

neg: Reflection seismology The velocity of these waves varies with wavelength, so they are said to be dispersive and the shape of the wavetrain varies with distance. A head wave refracts at an interface, travelling along it, within the lower medium and produces oscillatory motion parallel to the interface. This motion causes a disturbance in the upper medium that is detected on the surface. The same phenomenon is utilised in seismic refraction. An event on the seismic record that has incurred more than one reflection is called a "multiple". Multiples can be either short-path (peg-leg) or long-path, depending upon whether they interfere with

neg: Dispersive body waves w at travel distance r, k is the wavenumber and i is the imaginary unit. Reflection seismograms record the reflection wave along the propagation path r from the source to reflector and back to the surface. Equation (1.1) has an analytical solution given by Where k is the wave number. When the wave propagates in inhomogeneous seismic media the propagation constant k must be a complex value that includes not only an imaginary part, the frequency-dependent attenuation coefficient, but also a real part, the dispersive wavenumber. We can call this K(w) a propagation constant in line with Futterman. Here a(w)

neg: Seismic communication Seismic communication Seismic or vibrational communication is a process of conveying information through mechanical (seismic) vibrations of the substrate. The substrate may be the earth, a plant stem or leaf, the surface of a body of water, a spider’s web, a honeycomb, or any of the myriad types of soil substrates. Seismic cues are generally conveyed by surface Rayleigh or bending waves generated through vibrations on the substrate, or acoustical waves that couple with the substrate. Vibrational communication is an ancient sensory modality and it is widespread in the animal kingdom where it has evolved several times independently. It has

neg: Shadow zone Shadow zone A seismic shadow zone is an area of the Earth's surface where seismographs can only barely 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. The primary seismic waves are refracted by the liquid outer core of the Earth and are not detected between 104° and 140° (between approximately ) from the epicenter. The secondary seismic waves cannot pass through the liquid outer core and are not detected more than 104° (approximately ) from the epicenter. P waves that have been converted

neg: Adare Basin has a velocity of 4900 m/s, followed by a layer at 3.1 km with 5700 m/s wave velocity. The final layer lies 3.3 km beneath the surface and displays velocities of 6000 m/s. Seismic transects, using sonobuoys, reveal additional crustal structures beneath layers of sediment, which the Moho is defined in the basin by a seismic velocity of 8000 m/s. The total lithospheric thickness of the basin is thicker along the central Trough and along the southwest end closest to the continental shelf.These reading measure to a basement depth of 5–6 km. Fast velocities detected in shallower depths are consistent

neg: Michael Schoenberg vertical seismic profiling, and surface seismic. In 1990, he transferred to the seismic research department of Schlumberger Cambridge Research in Cambridge, U.K. where he worked on the application of surface seismic to characterization of the subsurface. His 1980 paper on "Elastic wave behavior across linear slip interfaces" in the Journal of the Acoustical Society of America has found wide application in geophysics, the seismic characterization of fractures and fractured reservoirs, materials science and ultrasonic non-destructive testing. He received the 1997 Conrad Schlumberger Award of the EAGE (European Association of Geoscientists and Engineers) "In recognition of his important contribution to seismic

neg: Surface wave (TW) of Von Bekesy, resulted from an acoustic surface wave of the basilar membrane into the cochlear duct. His theory purported to explain every feature of the auditory sensation owing to these passive mechanical phenomena. Jozef Zwislocki, and later David Kemp, showed that that is unrealistic and that active feedback is necessary. Ground wave refers to the propagation of radio waves parallel to and adjacent to the surface of the Earth, following the curvature of the Earth. This radiative ground wave is known as the Norton surface wave. Other types of surface wave are the non-radiative Zenneck surface wave or

neg: Seismic wave 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 S, which involves an expansion and contraction of the whole Earth, and has a period of about 20 minutes; and the "rugby" mode S, which involves expansions along two alternating directions, and has a period of about 54 minutes. The mode S does not exist because

neg: Gardner's relation If formula_3 is measured in m/s, formula_11 and the equation is: This equation is very popular in petroleum exploration because it can provide information about the lithology from interval velocities obtained from seismic data. The constants formula_4 and formula_5 are usually calibrated from sonic and density well log information but in the absence of these, Gardner's constants are a good approximation. Gardner's relation Gardner's relation, or Gardner's equation, named after G. H. F. Gardner and L. W. Gardner, is an empirically derived equation that relates seismic P-wave velocity to the bulk density of the lithology in which the wave travels.

neg: Seismic tomography archives were established. These developments occurred concurrently with advancements in computing power that were required to solve inverse problems and generate theoretical seismograms for model testing. In 1977, P-wave delay times were used to create the first seismic array-scale 2D map of seismic velocity. In the same year, P-wave data were used to determine 150 spherical harmonic coefficients for velocity anomalies in the mantle. The first model using iterative techniques, required when there are a large numbers of unknowns, was done in 1984. This built upon the first radially anisotropic model of the Earth, which provided the required initial reference

neg: Seismic site effects may notice that the acceleration amplitude strongly decreases first and then increases when the seismic waves reach the alluvial deposit on which Mexico City has been founded. In case of horizontal soil layering (constant thickness, cf Fig.1), we may analyze seismic site effects theoretically. One considers a shear wave (formula_2) (i.e. polarized perpendicularly to the figure) reflected and refracted wave at the interface between both media and reflected at the free surface. Considering Fig.1, we may analyze the propagation of the various waves in the sedimentary layer (formula_17) and in the half-space (formula_18). Assuming both media as linear elastic and

neg: South-pointing chariot and solar time. If the south-pointing chariot were built perfectly accurately, using a differential gear, and if it travelled on an Earth that was perfectly smooth, it would have interesting properties. It would be a mechanical compass that transports a direction, given by the pointer, along the path it travels. Mathematically the device performs parallel transport along the path it travels. The chariot can be used to detect straight lines or geodesics. A path on a surface the chariot travels along is a geodesic if and only if the pointer does not rotate with respect to the base of the

neg: Mantle plume a seismological subdivision of the Earth. It appears to be compositionally distinct from the overlying mantle, and may contain partial melt. Two very large, broad, large low-shear-velocity provinces, exist in the lower mantle under Africa and under the central Pacific. It is postulated that small plumes rise from their surface or their edges. Their low seismic velocities were thought to suggest that they are relatively hot, although it has recently been shown that their low wave velocities are due to high density caused by chemical heterogeneity. Various lines of evidence have been cited in support of mantle plumes. There is

neg: Ground wave propagation objects including the earth's surface and any hills, or large buildings that may be present. In addition to this there is a surface wave. This tends to follow the curvature of the Earth and enables coverage beyond the horizon. It is the sum of all these components that is known as the ground wave. Beyond the horizon the direct and reflected waves are blocked by the curvature of the Earth, and the signal is purely made up of the diffracted surface wave. It is for this reason that surface wave is commonly called ground wave propagation. The radio signal spreads

neg: Surface wave inversion group because each wavelet travels at a different speed. A group velocity is basically the speed at which a wave group travels. A phase velocity is the speed at which an individual wave travels, having its own characteristic wavelength and frequency. Fourier theory tells us that a sharp impulse is made up of infinite frequency content in phase at one point. If each frequency travels at the same speed, that peak will remain intact. If each frequency travels at a different speed, that peak will spread out (Figure 3). This spreading out is dispersion. Phase and group velocity are both

neg: Earthquake Earthquake An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth, resulting from the sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in size from those that are so weak that they cannot be felt to those violent enough to toss people around and destroy whole cities. The seismicity, or seismic activity, of an area is the frequency, type and size of earthquakes experienced over a period of time. The word "tremor" is also used for non-earthquake seismic rumbling. At the Earth's surface, earthquakes

neg: Interplate earthquake Interplate earthquake An interplate earthquake is an earthquake that occurs at the boundary between two tectonic plates. Earthquakes of this type account for more than 90 percent of the total seismic energy released around the world. If one plate is trying to move past the other, they will be locked until sufficient stress builds up to cause the plates to slip relative to each other. The slipping process creates an earthquake with relative displacement on either side of the fault, resulting in seismic waves which travel through the Earth and along the Earth's surface. Relative plate motion can be lateral

neg: Surface wave inversion dispersion data: Bulletin of the Seismological Society of America, v. 86, p. 436-444. Kallivokas, L.F., Fathi, A., Kucukcoban, S., Stokoe II, K.H., Bielak, J., Ghattas, O., (2013). Site characterization using full waveform inversion: Soil Dynamics and Earthquake Engineering, v. 47, p. 62-82. Foti, S., Lai, C.G., Rix, G.J., and Strobbia, C., (2014). Surface Wave Methods for Near-Surface Site Characterization, CRC Press, Boca Raton, Florida (USA), 487 pp., <https://www.crcpress.com/product/isbn/9780415678766> Surface wave inversion Seismic inversion involves the set of methods which seismologists use to infer properties through physical measurements. Surface-wave inversion is the method by which elastic properties, density, and thickness of

neg: Kola Superdeep Borehole found at the depth of about , where the velocity of seismic waves has a discontinuity. Instead the change in the seismic wave velocity is caused by a metamorphic transition in the granite rock. In addition, the rock at that depth had been thoroughly fractured and was saturated with water, which was surprising. This water, unlike surface water, must have come from deep-crust minerals and had been unable to reach the surface because of a layer of impermeable rock. Microscopic plankton fossils were found 6 kilometers (4 mi) below the surface. Another unexpected discovery was a large quantity of hydrogen

neg: Seismic migration industry as well as academic circles. Seismic waves are elastic waves that propagate through the Earth with a finite velocity, governed by the elastic properties of the rock in which they are travelling. At an interface between two rock types, with different acoustic impedances, the seismic energy is either refracted, reflected back towards the surface or attenuated by the medium. The reflected energy arrives at the surface and is recorded by geophones that are placed at a known distance away from the source of the waves. When a geophysicist views the recorded energy from the geophone, they know both the

neg: Reflection seismology Reflection seismology Reflection seismology (or seismic reflection) is a method of exploration geophysics that uses the principles of seismology to estimate the properties of the Earth's subsurface from reflected seismic waves. The method requires a controlled seismic source of energy, such as dynamite or Tovex blast, a specialized air gun or a seismic vibrator, commonly known by the trademark name Vibroseis. Reflection seismology is similar to sonar and echolocation. This article is about surface seismic surveys; for vertical seismic profiles, see VSP. Reflections and refractions of seismic waves at geologic interfaces within the Earth were first observed on recordings of

neg: Slowness (seismology) Slowness (seismology) Slowness (s) is a quantity introduced in Seismology which is the reciprocal of velocity. Thus travel time of a wave is the distance that the wave travels times the slowness of the medium ( in seismology, it refers to different layers of Earth exhibiting different densities) Thus, Slowness = 1/Velocity If we divide the path followed by the ray into N segments numbered i = 1, 2 .. N the total travel time of the wave is formula_1 Therefore, the residual travel time is due to departures in slowness experienced by the wave along the raypath. Measurements of

neg: Earthscope method of producing a three-dimensional image of the internal structures of a solid object (such as the human body or the earth) by the observation and recording of differences in the effects on the passage of energy waves impinging on those structures. The waves of energy are P-waves generated by earthquakes and are recording the wave velocities. The high quality data that is being collected by the permanent seismic stations of USArray and the Advanced National Seismic System (ANSS) will allow the creation of high resolution seismic imaging of the Earth’s interior below the United States. Seismic tomography helps constrain

neg: Seismic metamaterial to have no limitation regarding the radiated frequency. The cloak itself demonstrates no forward or back scattering, hence, the seismic cloak becomes an effective medium. In 2012, researchers held an experimental field-test in France, with the aim to highlight analogy with phononic crystal. Seismic metamaterial A seismic metamaterial, is a metamaterial that is designed to counteract the adverse effects of seismic waves on artificial structures, which exist on or near the surface of the earth. seismic metamaterials were still in the development stage. More than a million earthquakes are recorded each year, by a worldwide system of earthquake detection stations.

neg: Multidimensional seismic data processing further classified into various categories, depending on the configuration and type of sources and receivers used. For example, zero-offset vertical seismic profiling (ZVSP), walk-away VSP etc. The source (which is typically on the surface) produces a wave travelling downwards. The receivers are positioned in an appropriate configuration at known depths. For example, in case of vertical seismic profiling, the receivers are aligned vertically, spaced approximately 15 meters apart. The vertical travel time of the wave to each of the receivers is measured and each such measurement is referred to as a “check-shot” record. Multiple sources may be added or a

neg: Seismoelectrical method Seismoelectrical method The seismoelectrical method (which is different from the electroseismic physical principle) is based on the generation of electromagnetic fields in soils and rocks by seismic waves. This technique is still under development and in the future it may have applications like detecting and characterizing fluids in the underground by their electrical properties, among others, usually related to fluids (porosity, transmissivity, physical properties). When a seismic wave encounters an interface, it creates a charge separation at the interface forming an electric dipole. This dipole radiates an electromagnetic wave that can be detected by antennae on the ground surface. As

neg: Seismic interferometry impulse response of the subsurface can be reconstructed using only an extended record of background noise, initially only for the surface and direct wave arrivals. Crosscorrelations of seismic signals from both active and passive sources at the surface or in the subsurface can be used to reconstruct a valid model of the subsurface. Seismic interferometry can produce a result similar to traditional methods without limitations on the diffusivity of the wavefield or ambient sources. In a drilling application, it is possible to utilize a virtual source to image the subsurface adjacent to a downhole location. This application is increasingly utilized

neg: Gardner's relation Gardner's relation Gardner's relation, or Gardner's equation, named after G. H. F. Gardner and L. W. Gardner, is an empirically derived equation that relates seismic P-wave velocity to the bulk density of the lithology in which the wave travels. The equation reads: where formula_2 is bulk density given in g/cm, formula_3 is P-wave velocity given in ft/s, and formula_4 and formula_5 are empirically derived constants that depend on the geology. Gardner et al. proposed that one can obtain a good fit by taking formula_6 and formula_7. Assuming this, the equation is reduced to: where the unit of formula_9 is feet/s.

neg: Microseism Microseism In seismology, a microseism is defined as a faint earth tremor caused by natural phenomena. Sometimes referred to as a "hum", it should not be confused with the anomalous acoustic phenomenon of the same name. The term is most commonly used to refer to the dominant background seismic and electromagnetic noise signals on Earth, which are caused by water waves in the oceans and lakes. Characteristics of microseism are discussed by Bhatt. Because the ocean wave oscillations are statistically homogenous over several hours, the microseism signal is a long-continuing oscillation of the ground. The most energetic seismic waves that

neg: Total internal reflection many types of waves, such as electromagnetic waves in general or sound waves. When a wave reaches a boundary between different materials at a angle of <25° with different refractive indices, the wave will in general be partially refracted at the boundary surface, and partially reflected. However, if the angle of incidence is greater (i.e. the direction of propagation is closer to being parallel to the boundary) than the critical angle – the angle of incidence at which light is refracted such that it travels along the boundary – then the wave will not cross the boundary, but will instead

neg: Seismic wave 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: In the case of

neg: Seismic magnitude scales the first second. A modification – the "Moscow-Prague formula" – was proposed in 1962, and recommended by the IASPEI in 1967; this is the basis of the standardized M scale (Ms_20, M(20)). A "broad-band" variant (Ms_BB, M(BB)) measures the largest velocity amplitude in the Rayleigh-wave train for periods up to 60 seconds. The M scale used in China is a variant of M calibrated for use with the Chinese-made "type 763" long-period seismograph. The MLH scale used in some parts of Russia is actually a surface wave magnitude. Other magnitude scales are based on aspects of seismic waves that only

neg: Geophysical MASINT very low-frequency sound waves in the atmosphere produced by natural and man-made events. Closely related to the microbarographs, but detecting pressure waves in water, are hydro-acoustic sensors, both underwater microphones and specialized seismic sensors that detect the motion of islands. US Army Field Manual 2-0 defines seismic intelligence as "The passive collection and measurement of seismic waves or vibrations in the earth surface." One strategic application of seismic intelligence makes use of the science of seismology to locate and characterize nuclear testing, especially underground testing. Seismic sensors also can characterize large conventional explosions that are used in testing the high-explosive

neg: Structure of the Earth the crust, upper mantle, lower mantle, outer core, and inner core. The geologic component layers of Earth are at the following depths below the surface: The layering of Earth has been inferred indirectly using the time of travel of refracted and reflected seismic waves created by earthquakes. The core does not allow shear waves to pass through it, while the speed of travel (seismic velocity) is different in other layers. The changes in seismic velocity between different layers causes refraction owing to Snell's law, like light bending as it passes through a prism. Likewise, reflections are caused by a large