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43-517: ULVZs are discovered by the delay and scattering of body waves that reflect and diffract on or are refracted by the core-mantle boundary . Different body waves types give different constraints on the dimensions or velocity contrasts of the ULVZ. Even though ULVZs are discovered in places, it remains difficult to map out their extent and constrain their density and velocity. Usually trade-offs between various parameters exist. In general though, ULVZs appear to be

86-877: A planet , dwarf planet , or natural satellite . It is usually distinguished from the underlying mantle by its chemical makeup; however, in the case of icy satellites, it may be distinguished based on its phase (solid crust vs. liquid mantle). The crusts of Earth , Mercury , Venus , Mars , Io , the Moon and other planetary bodies formed via igneous processes and were later modified by erosion , impact cratering , volcanism, and sedimentation. Most terrestrial planets have fairly uniform crusts. Earth, however, has two distinct types: continental crust and oceanic crust . These two types have different chemical compositions and physical properties and were formed by different geological processes. Planetary geologists divide crust into three categories based on how and when it formed. This

129-444: 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 hypocenters . 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

172-727: A fluid-filled borehole , being an important source of coherent noise in vertical seismic profiles (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. 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., n S l , where l

215-419: A hundred to a thousand kilometers across and tens of kilometers thick (although existing thinner or smaller ULVZs might fall below the resolution of seismology). Their shear wave velocity reduction is on the order of −10 to −30% and the compressional wave velocity reduction tends to be weaker. ULVZs are hypothesized to be enriched in iron , be partially molten or a combination of both, or result from

258-526: A layered medium (e.g., the crust and upper mantle ) the velocity of the Rayleigh waves depends on their frequency and wavelength. See also Lamb waves . Love waves are horizontally polarized shear waves (SH waves), existing only in the presence of a layered medium. They are named after Augustus Edward Hough 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

301-408: 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

344-435: A seismic observatory, their different travel times help scientists locate the quake's hypocenter . In geophysics, the refraction or reflection of seismic waves is used for research into Earth's internal structure . Scientists sometimes generate and measure vibrations to investigate shallow, subsurface structure. Among the many types of seismic waves, one can make a broad distinction between body waves , which travel through

387-568: Is a mechanical wave of acoustic energy that travels through the Earth or another planetary body . It can result from an earthquake (or generally, a quake ), volcanic eruption , magma movement, a large landslide and a large man-made explosion that produces low-frequency acoustic energy. Seismic waves are studied by seismologists , who record the waves using seismometers , hydrophones (in water), or accelerometers . Seismic waves are distinguished from seismic noise (ambient vibration), which

430-524: Is a planet's "original" crust. It forms from solidification of a magma ocean. Toward the end of planetary accretion , the terrestrial planets likely had surfaces that were magma oceans. As these cooled, they solidified into crust. This crust was likely destroyed by large impacts and re-formed many times as the Era of Heavy Bombardment drew to a close. The nature of primary crust is still debated: its chemical, mineralogic, and physical properties are unknown, as are

473-491: Is debated. The anorthosite highlands of the Moon are primary crust, formed as plagioclase crystallized out of the Moon's initial magma ocean and floated to the top; however, it is unlikely that Earth followed a similar pattern, as the Moon was a water-less system and Earth had water. The Martian meteorite ALH84001 might represent primary crust of Mars; however, again, this is debated. Like Earth, Venus lacks primary crust, as

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516-463: 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. Crust (geology) In geology , the crust is the outermost solid shell of

559-490: Is needed to create tertiary crust, and Earth is the only planet in the Solar System with plate tectonics. Earth's crust is a thin shell on the outside of Earth, accounting for less than 1% of Earth's volume. It is the top component of the lithosphere , a division of Earth's layers that includes the crust and the upper part of the mantle . The lithosphere is broken into tectonic plates that move, allowing heat to escape from

602-457: Is persistent low-amplitude vibration arising from a variety of natural and anthropogenic sources. The propagation velocity of a seismic wave depends on density and elasticity 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 Earth's outer core . Earthquakes create distinct types of waves with different velocities. When recorded by

645-522: Is roughly 800 by 250 km (roughly the size of Florida) and is 10–15 km high. Its material appears 45% slower in shear wave velocity, 15% slower in compressional wave velocity and 10% denser. Additionally, the ULVZ appears to lie in a gap of the Pacific LLSVP (not represented in the illustration here), leading to the hypothesis that this slow material is pushed to the center by surrounding large piles. Body wave (seismology) A seismic wave

688-433: 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

731-407: 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

774-457: Is very dense. Its shear wave velocity reduction is roughly 20% compared to surrounding material. It remains speculative if there is a correlation between this large ULVZ and the presence of the strongest hotspot flux at the surface; potentially the ULVZ could be an anchor to a whole-mantle plume. The Samoan is another mega-ultra-low velocity zone which lies directly beneath the Samoa hotspot . This zone

817-409: The "breathing" mode 0 S 0 , which involves an expansion and contraction of the whole Earth, and has a period of about 20 minutes; and the "rugby" mode 0 S 2 , which involves expansions along two alternating directions, and has a period of about 54 minutes. The mode 0 S 1 does not exist because it would require a change in the center of gravity, which would require an external force. Of

860-421: The adiabatic rise of mantle causes partial melting. Tertiary crust is more chemically-modified than either primary or secondary. It can form in several ways: The only known example of tertiary crust is the continental crust of the Earth. It is unknown whether other terrestrial planets can be said to have tertiary crust, though the evidence so far suggests that they do not. This is likely because plate tectonics

903-494: The refraction of light waves . Two types of particle motion result in two types of body waves: Primary and Secondary waves. This distinction was recognized in 1830 by the French mathematician Siméon Denis Poisson . Primary waves (P waves) are compressional waves that are longitudinal in nature. P waves are pressure waves that travel faster than other waves through the earth to arrive at seismograph stations first, hence

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946-431: The Earth's surface where the seismographic stations are located. The waves travel more quickly than if they had traveled in a straight line from the earthquake. This is due to the appreciably increased velocities within the planet, and is termed Huygens' Principle . Density in the planet increases with depth, which would slow the waves, but the modulus of the rock increases much more, so deeper means faster. Therefore,

989-542: The Earth, and surface waves , which travel at the Earth's surface. 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 . 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

1032-461: 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 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

1075-408: The S wave velocity. 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 away from the contact. These waves can also be generated along the walls of

1118-645: The ULVZs can also be controlled by the presence of thermo-chemical piles (or LLSVPs ). The denser ULVZ material heaps up at the edges of these piles. The Hawaiian ULVZ appears to be the largest ULVZ mapped to date. It sits on the core-mantle boundary slightly to the west of the Hawaiian hotspot at the northern boundary of the Pacific large low-shear-velocity province . It is mapped out to be roughly 1000 km across and 20 km high. Its large aspect ratio dynamically suggests it

1161-557: The absence of S waves in earth's outer core suggests a liquid state. Seismic surface waves travel along the Earth's surface. They can be classified as a form of mechanical surface wave . Surface waves diminish in amplitude as they get farther from the surface and propagate more slowly than seismic body waves (P and S). Surface waves from very large earthquakes can have globally observable amplitude of several centimeters. Rayleigh waves, also called ground roll, are surface waves that propagate with motions that are similar to those of waves on

1204-403: 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

1247-423: The crust ranges between about 20 and 120 km. Crust on the far side of the Moon averages about 12 km thicker than that on the near side . Estimates of average thickness fall in the range from about 50 to 60 km. Most of this plagioclase-rich crust formed shortly after formation of the Moon, between about 4.5 and 4.3 billion years ago. Perhaps 10% or less of the crust consists of igneous rock added after

1290-481: The different areas of application, a wide variety of nomenclatures have emerged historically, the standardization of which – for example in the IASPEI Standard Seismic Phase List – is still an ongoing process. The path that a wave takes between the focus and the observation point is often drawn as a ray diagram. Each path is denoted by a set of letters that describe the trajectory and phase through

1333-453: The entire planet has been repeatedly resurfaced and modified. Secondary crust is formed by partial melting of mostly silicate materials in the mantle, and so is usually basaltic in composition. This is the most common type of crust in the Solar System. Most of the surfaces of Mercury, Venus, Earth, and Mars comprise secondary crust, as do the lunar maria . On Earth secondary crust forms primarily at mid-ocean spreading centers , where

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1376-587: 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 not travel through any liquid medium, so

1419-463: The formation of the initial plagioclase-rich material. The best-characterized and most voluminous of these later additions are the mare basalts formed between about 3.9 and 3.2 billion years ago. Minor volcanism continued after 3.2 billion years, perhaps as recently as 1 billion years ago. There is no evidence of plate tectonics . Study of the Moon has established that a crust can form on a rocky planetary body significantly smaller than Earth. Although

1462-475: The fundamental toroidal modes, 0 T 1 represents changes in Earth's rotation rate; although this occurs, it is much too slow to be useful in seismology. The mode 0 T 2 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 1960 earthquake in Chile . Presently

1505-491: The igneous mechanisms that formed them. This is because it is difficult to study: none of Earth's primary crust has survived to today. Earth's high rates of erosion and crustal recycling from plate tectonics has destroyed all rocks older than about 4 billion years , including whatever primary crust Earth once had. However, geologists can glean information about primary crust by studying it on other terrestrial planets. Mercury's highlands might represent primary crust, though this

1548-416: The interior of Earth into space. A theoretical protoplanet named " Theia " is thought to have collided with the forming Earth, and part of the material ejected into space by the collision accreted to form the Moon. As the Moon formed, the outer part of it is thought to have been molten, a " lunar magma ocean ". Plagioclase feldspar crystallized in large amounts from this magma ocean and floated toward

1591-489: 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 . In air, they take the form of sound waves, hence they travel at the speed of sound . Typical speeds are 330 m/s in air, 1450 m/s in water and about 5000 m/s in granite . Secondary waves (S waves) are shear waves that are transverse in nature. Following an earthquake event, S waves arrive at seismograph stations after

1634-446: 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 . The naming of seismic waves is usually based on the wave type and its path; due to the theoretically infinite possibilities of travel paths and

1677-421: The periods of thousands of modes have been observed. These data are used for constraining large scale structures of the Earth's 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

1720-431: The presence of carbon. Different scenarios have been proposed for the iron enrichment: iron could be leaking from the core, have accumulated over past subduction , or be remnants of a basal magma ocean . Both silicate perovskite and periclase (which are thought to be present in the lowermost mantle) show reduced velocities with increasing iron at these pressures and temperatures. Experiments with iron and water under

1763-411: The same conditions form an iron peroxide FeO 2 H x that will contribute to ULVZ. ULVZs have higher density than their surroundings to remain stable on the core-mantle boundary. In a general mantle convection setting, the density contrast as well as the amount of material available would control the morphology/shape of the ULVZ. So far a range of sizes for ULVZs has been found. The location and shape of

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1806-459: The surface of water (note, however, that the associated seismic particle motion at shallow depths is typically 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 , in 1885. They are slower than body waves, e.g., at roughly 90% of the velocity of S waves for typical homogeneous elastic media. In

1849-444: The surface. The cumulate rocks form much of the crust. The upper part of the crust probably averages about 88% plagioclase (near the lower limit of 90% defined for anorthosite ): the lower part of the crust may contain a higher percentage of ferromagnesian minerals such as the pyroxenes and olivine , but even that lower part probably averages about 78% plagioclase. The underlying mantle is denser and olivine-rich. The thickness of

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