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Topic: Seismic wave

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	Seismic wave - Wikipedia, the free encyclopedia
		Seismic waves are also continually excited by the pounding of ocean waves and the wind.
		Seismic waves are studied by seismologists, and measured by a seismograph, seismometer, or geophone.
		P waves are longitudinal or compressional waves, which means that the ground is alternately compressed and dilated in the direction of propagation.
	en.wikipedia.org /wiki/Seismic_wave (1071 words)

	Seismic Evidence for Internal Earth Structure
		Seismic velocities depend on the material properties such as composition, mineral phase and packing structure, temperature, and pressure of the media through which seismic waves pass.
		Mohorovicic (1909) interpreted this to mean that the seismic waves recorded beyond 200 km from the earthquake source had passed through a lower layer with significantly higher seismic velocity.
		Seismic velocities tend to gradually increase with depth in the mantle due to the increasing pressure, and therefore density, with depth.
	www.columbia.edu /~vjd1/earth_int.htm (1141 words)

	Seismic Refraction
		This wave is refracted in the subsurface and detected by a series of sensitive geophones deployed along the ground.
		When the seismic wave returns to the surface its arrival is detected by a series of geophones (seismic spread) and recorded on a seismograph.
		Each seismic refraction “spread” consists of a series of 12 or 24 geophones placed along the line at a set distance or “geophone interval.” The geophone interval is generally 10 to 50 feet depending on the desired resolution and the desired depth of exploration.
	www.nga.com /Geo_ser_Seismic_refraction.htm (487 words)

	Seismograms Offer Insight Into Oklahoma City Bombing
		The duration of the seismic signal increases as seismic waves propagate outward from the Federal Building, as is common to all seismograms as the numbers and lengths of ray paths increase.
		Even without identifying the phases of the two low-frequency wave trains recorded about 10 s apart at FNO after the bombing and demolition, both their similarity (Figure 4a) and development as seismic energy propagates outward from the Federal Building (Figure 3b) indicate that they are associated with the propagation of seismic waves.
		The particle motion of the second low-frequency wave at FNO (wave train 3 in Figure 3b) is vertically polarized and retrograde elliptical, which is characteristic of seismic waves known as Rayleigh waves.
	www.agu.org /sci_soc/eosholzer.html (2546 words)

	Earthquakes & Earth's Interior
		Seismic waves emanating from the focus can travel in several ways, and thus there are several different kinds of seismic waves.
		If the seismic wave velocity in the rock above an interface is less than the seismic wave velocity in the rock below the interface, the waves will be refracted or bent upward relative to their original path.
		The Mantle - Seismic wave velocities increase abruptly at the Moho.
	www.tulane.edu /~sanelson/geol111/earthint.htm (3470 words)

	Seismic Waves
		Seismic Waves: Because of the elastic properties of Earth materials (rocks) and the presence of the Earth's surface, four main types of seismic waves propagate within the Earth.
		Demonstrating the S or Shear wave is performed in a similar fashion except that the person who creates the shear disturbance does so by moving his or her hand quickly up and then down.
		Similarly, using this "elastic rebound" method for the S waves, one person holding the end of the stretched slinky should use their other hand to grab one of the coils about 10-12 coils away from the end of the slinky.
	web.ics.purdue.edu /~braile/edumod/slinky/slinky4.htm (2460 words)

	Downhole Seismic
		In downhole seismic surveys a triaxial geophone transducer is incrementally lowered down a borehole while seismic waves are generated from either the surface or another borehole.
		Downhole seismic data provides detailed information on stratigraphy and the engineering properties of subsurface soils and rock that is not available from surface seismic surveys and thus are often used for the enhancement and quality control of surface seismic interpretations.
		A downhole seismic survey (also called a borehole velocity survey) is conducted by measuring the time for seismic waves generated by an impulsive source at the surface to travel to a sensor located at a sequence of depths in the borehole.
	www.nga.com /Geo_ser_Downhole_Seismic.htm (521 words)

	Earthquakes: Seismic Waves
		Seismic waves are recorded on a seismogram at a seismograph station.
		Variations in seismic wave velocity are used to infer the properties of Earth’s interior.
		The difference in arrival time between P and S waves on a seismogram can be used to determine the distance of the station from the earthquake source and the amplitude (height) of the S wave recorded at the station can be used to determine earthquake magnitude (see Measurement of Earthquakes).
	www.mhhe.com /earthsci/geology/mcconnell/eqk/swaves.htm (959 words)

	Seismic Methods
		There are two major classes of seismic waves: body waves, which pass through the volume of a material; and, surface waves, that exist only near a boundary.
		Compressional waves in fluids, e.g., water and air, are commonly referred to as acoustic waves.
		Sources: The seismic source may be a hammer striking the ground or an aluminum plate or weighted plank, drop weights of varying sizes, rifle shot, a harmonic oscillator, waterborne mechanisms, or explosives.
	www.freewebtown.com /geophysics2/seis-method.html (2049 words)

	Seismic wave speed tester
		The speed with which seismic waves can pass through samples of earth material is estimated indirectly using ultrasonic interferometery.
		This attenuation apparatus measures the seismic wave speed through samples of earth materials by exerting a torsional force on the sample.
		By integrating a knowledge of the macroscopic physical properties of earth materials (such as strength, seismic wave speeds and attenuation) with an understanding of their microstructure the group is providing deep insights on how the Earth's mantle behaves.
	www.anu.edu.au /CSEM/machines/SeismicWaveTester.htm (713 words)

	Hasbrouck Geophysics, Inc.: Buried Contamint Pit (Shear-Wave Seismic)
		Seismic wave transmission differences expected between disturbed and undisturbed material can be detected by shear-wave seismic surveys.
		Seismic energy was generated by striking a 20-pound (lb) sledgehammer horizontally against a rectangular steel fixture coupled to the ground.
		The seismic energy was received with uniquely designed two-spike, 40-Hz horizontal geophones (the spikes were located on both ends of the geophone and offered excellent coupling to the ground and reduction of possible spurious vibrations).
	www.hasgeo.com /shear.htm (533 words)

	Earthquakes & Man-Made Shocks \| Glossary (Site not responding. Last check: 2007-10-11)
		P-wave: Primary wave: a seismic wave that pushes and pulls rocks, contracting and expanding them as it moves through them.
		S-wave: Secondary wave: a seismic wave that moves rocks from side to side as it moves through them.
		wave front: The edge of an advancing wave, which includes adjacent points that have the same rotation or variation considered in relation to a reference or assumed instant of starting.
	www.seed.slb.com /en/scictr/watch/seismology/glossary.htm (606 words)

	What Is Seismology and What Are Seismic Waves?
		Seismology is the study of earthquakes and seismic waves that move through and around the earth.
		Seismic waves are the waves of energy caused by the sudden breaking of rock within the earth or an explosion.
		The second type of body wave is the S wave or secondary wave, which is the second wave you feel in an earthquake.
	www.geo.mtu.edu /UPSeis/waves.html (492 words)

	[No title]
		Love waves are dispersive, that is, the wave velocity is dependent on frequency, generally with low frequencies propagating at higher velocity.
		The surface waves extend over a long time interval because surface wave propagation is dispersive (the velocity of propagation is dependent on the frequency of the wave).
		The first shear wave motion (illustrated in the particle motion diagram in the lower left of Figure 9) is arriving from down and to the south (diagonally from the lower left in the particle motion diagram).
	web.ics.purdue.edu /~braile/edumod/waves/WaveDemo.htm (4491 words)

	CVO Menu - Glossary of Earthquake and Related Terminology
		The amplitude of a seismic wave is the amount the ground moves as the wave passes by.
		Seismic waves are the vibrations from earthquakes that travel through the Earth; they are recorded on instruments called seismographs.
		tsunamis are seismic sea waves caused by earthquakes, submarine landslides, and, infrequently, by eruptions of island volcanoes.
	vulcan.wr.usgs.gov /Glossary/Seismicity/earthquake_terminology.html (2808 words)

	Seismic Surveys (Site not responding. Last check: 2007-10-11)
		Seismic methods have little use in metalliferous exploration because of the relatively small size and complicated geology of the typical ore deposit, and because of the high cost of seismic work.
		A seismic wave is usually generated by a small explosive charge, a shotgun shell, or a sledge hammer.
		Method: Seismic reflection is a geophysical technique in which acoustic waves, reflected directly from underground surfaces with density contrasts, are used to map soil and bedrock stratigraphy.
	www.digistar.mb.ca /minsci/finding/seismic1.htm (604 words)

	Seismic Simulations Home
		These range from a simple convolutional model, which produces the equivalent of ideal post-stack, time-migrated seismic data, to numerically solving the two-way wave equation, which we use to produce a synthetic version of pre-stack, unmigrated seismic data.
		Solution of the two-way wave equation simulates the full propagation of seismic waves, from their source, through multi-layered strata, to their arrival at surface receivers.
		The following simulations illustrate the relationship between the experimental strata and the synthetic seismic data produced by the convolution model, the one-way wave equation model, and the two-way wave equation model.
	www.nicholas.duke.edu /people/faculty/pratson/seismic/temp.htm (1142 words)

	Wave Interaction with Boundaries
		In this particular example the speed* at which seismic waves propagate in the layer is faster than the speed at which they propagate in the halfspace.
		Remember that the period of a wave is related to its wavelength through the speed at which the wave propagates through the medium.
		The change in curvature of the wavefront as the wave passes through the interface implies that the raypaths describing the direction of propagation of the wave change direction through the boundary.
	gretchen.geo.rpi.edu /roecker/AppGeo96/lectures/seismic/bound.html (723 words)

	Report On Seismic Activity Due To The Aircraft Impact At The Pentagon.
		If the plane impact to the Pentagon generated strong enough ground motion that could be propagated through the Earth's crust as elastic waves (seismic waves) and recorded at sensitive seismographic stations around the source, we could determine absolute time of the impact by using the arrival times of P, S or surface seismic waves.
		Hence,the seismic phase Pg (P waves propagating through Earth's crust) would propagate with nearly constant speed of about 6 km/s over the record section, where as the phase Pn (critically refracted P waves propagating through top of the uppermost mantle) propagates with an apparent speed of about 8 km/s.
		S wave arrivals with 3.5 km/s are indicated by solid line, which is usually the beginning of Lg waves.
	911research.wtc7.net /mirrors/guardian/WTC/Seismic/WTC_PENT_KIM.htm (1925 words)

	Seismic Wave Speeds and Rock Properties
		It's clear from the previous examples that variations in the speed at which seismic waves propagate through the Earth* can cause variations in seismic waves recorded at the Earth's surface.
		For example, we've shown that reflected waves can be generated from a planar boundary in seismic wave speed that can be recorded at the Earth's surface.
		Where Vp and Vs are the P and S wave velocities of the medium, &rho is the density of the medium, and &mu and k are referred to as the shear and bulk modulii of the media.
	gretchen.geo.rpi.edu /roecker/AppGeo96/lectures/seismic/evel.html (730 words)

	CVO Website - Tsunamis and Seismic Sea Waves
		A series of waves may reach the coast at intervals of 5 to 40 minutes; the first wave is frequently not the largest.
		Rather, tsunamis are seismic sea waves caused by earthquakes, submarine landslides, and, infrequently, by eruptions of island volcanoes.
		As a wave approaches land, however, its bottom is slowed down by contact with the shallowing sea floor, whereas the top is slowed much less and catches up with the bottom.
	vulcan.wr.usgs.gov /Glossary/Tsunami/description_tsunami.html (1764 words)

	Faults and Earthquakes
		Near-surface earthquakes are measured in terms of their surface waves, but deep earthquakes don't produce much surface waves.
		Seismic waves travel faster through deeper parts of the Earth, and..
		Seismic tomography is a method of using seismic signals to map the earth's interior in three dimensions.
	www.uwgb.edu /dutchs/EarthSC202Notes/quakes.htm (1902 words)

	Geophysics : Radiation of Seismic Shock Waves (Site not responding. Last check: 2007-10-11)
		A seismic shock wave is a wave that travels outward from the site of an earthquake through the earth.
		In a homogeneous (unvarying) media, a seismic wave would radiate in all directions as a growing sphere.
		Seismic waves can also be used to find archaelogocal remains underground.
	www.seed.slb.com /qa2/FAQView.cfm?ID=917 (230 words)

	index
		On the pictures above and left are shown the traces of volcanic activity, with water, land and ice as the epicenters of the earthquakes.
		Buildings and structures are deformed and ruined as a result of a horizontal shock wave.
		"Seismic Star" is capable of reducing the level of impulse surplus pressure on structures by 40%, which would result in insignificant damage or none of it.
	www.seismicstar.com (277 words)

	Seismic Waves (Site not responding. Last check: 2007-10-11)
		P waves shake the ground in the direction they are propagating, while S waves shake perpendicularly or transverse to the direction of propagation.
		Although wave speeds vary by a factor of ten or more in the Earth, the ratio between the average speeds of a P wave and of its following S wave is quite constant.
		The average velocity of the wave is just the slope of the line connecting arrivals, or the change in distance divided by the change in time.
	www.seismo.unr.edu /ftp/pub/louie/class/100/seismic-waves.html (1064 words)

	Wave Theory Research Applications
		The societal and commercial benefits of wave propagation research can be measured most directly by its contribution to seismology's principal extra-scientific applications: 1) earthquake hazard mitigation, 2) the discrimination of underground nuclear explosions from earthquakes and quarry blasts, and 3) shallow crustal imaging for resource exploitation and environmental monitoring.
		Because the scattered wave field can obscure the aspects of the seismic source mechanism that are useful for discrimination, methods for incorporating three-dimensional structure into synthetic seismogram algorithms, such as those described in the previous section, contribute to the goal of verifying treaties that limit underground nuclear tests.
		wave was originally mistaken for a Love wave in the granitic crust by early researchers, and its detailed waveform is difficult to model.
	www.agu.org /revgeophys/park01/node3.html (1203 words)

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