Seismic wave is composed ofSourceTo spread aroundVibration, referring toSourceRadiativeelastic wave。It can be divided intolongitudinal wave(P wave)transverse wave(S-wave) (both P-wave and S-wave belong to body wave) andsurface wave(L wave) three types.When an earthquake occurs, the medium in the source area breaks and moves rapidly, and this disturbance forms aWave source。Because of the continuity of the earth's medium, this wave propagates to the interior and surface of the earth, formingcontinuous medium Inelastic wave。
seismologyOne of the main contents of is to study the information brought by seismic waves.Seismic wave is a kind ofmechanical movementThe spread of the earth is caused by the elasticity of the earth's medium.Its nature andacoustic waveVery close, so it is also called ground acoustic wave.But ordinary sound waves propagate in the fluid, while seismic waves propagate in the earth medium, so it is much more complicated. Seismic waves and light waves have some similarities in calculation.Wave opticsIn the case of short wave, it can transition togeometrical optics , which simplifies the calculation;Similarly, under certain conditions, the concept of seismic wave can be usedSeismic rayInstead, geometric seismology was formed.However, light wave is only shear wave, while seismic wave has both longitudinal and transverse parts, so in specific calculation, seismic wave is much more complex.[1]
Chinese name
seismic wave
Foreign name
seismic waves
Principle
The medium in the source area vibrates to form a wave source
Media
Earth medium
Communication mode
Longitudinal wave, transverse wave and surface wave
Seismic waves can be divided into three types according to their propagation modes:longitudinal wave, shear wave andsurface wave。The longitudinal wave is the advancing wave, and the propagation speed in the crust is 5.5~7 km/second, first to reach the epicenter, also known asP-wave, it makes the ground vibrate up and down, which is less destructive.The shear wave is a shear wave: its propagation speed in the crust is 3.2~4.0 km/s. The second one reaches the epicenter, also known as S wave, which makes the ground shake back and forth, left and right, and is destructive.Surface waveL-waveIs a mixed wave generated after the P-wave and S-wave meet on the surface.Its long wave and strong amplitude can only propagate along the ground surface, which is the main factor causing strong damage to buildings.[2]
Scope of influence
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overview
When water waves encounter an interface, such as a steep bank, they will be reflected back from the boundary, forming a column of water waves sent out of the bank, overlapping with the water waves sent in the bank.When the ocean wave slants into the shoal, the wave moves slowly when the sea depth becomes shallow, and falls behind the wave in the deeper sea.The result is that the wave bends towards shallow water.thereforewavefrontTurn more and more parallel to the beach before they hit the shore.The term refraction describes the phenomenon that the direction of wave front changes due to the change of conditions on the propagation path.Reflection andrefractionIs also the light passing throughlensAnd prisms.
In an elastic solid with density ρ, two kinds ofelastic wave。
P-wave, velocity vP=√ (k+4/3 μ)/ρ.
Granite: vP=5.5 km/s;
Water: vP=1.5 km/s.
S-wave, velocity vS=√μ/ρ.
Granite:VS=3.0 km/s;
Water: vS=0 km/s.
Phenomenon introduction
Like sound, light or water waves, seismic waves can also be reflected or refracted on one side of the boundary, but different from other waves, when seismic waves incident on a reflecting surface in the earth, for example, when a P wave shoots at the boundary surface at an angle, it is not only divided into a reflected P wave and a refracted P wave, but also produces a reflected S wave and a refracted S wave. The reason is that,The rock on the boundary of the incident point is not only squeezed, but also sheared.
In other words, one incident P wave generates four typesConverted wave。Wave pattern multiplication from one wave pattern to another also occurs when the SV wave is obliquely incident on the internal boundary, resulting in reflected and refracted P and SV waves.In this case, the reflected and refracted S-wave is always SV type, because when the incident SV wave arrives, the rock particle moves laterally in an incident plane perpendicular to the ground.Conversely, if the incident S-wave is horizontalpolarizationSH type, the particle moves back and forth in the direction perpendicular to the incident plane and parallel to the boundary interface, and there is no extrusion or vertical deformation on the discontinuous interface, so that no corresponding new P wave and SV wave will be generated, only one of the SH typereflected waveAnd oneRefracted wave。Visually analyzing from the physical image, there is no shear on the reflecting interface of the vertically incident P waveweightThere is only reflected P wave and no reflected SV wave or SH wave at all.The limitations of wave mode conversion discussed above are helpful to fully understand the complexity and interpretation of ground motionSeismogramThe various images of seismic waves in are crucial.
Buildings built on thick soil, such as those on sediments in alluvial valleys along rivers, are prone to serious damage during earthquakes, which is also due to wave amplification and enhancement.When we vibrate two springs connected together, the weak spring will have a larger vibration amplitude.Similarly, when the S-wave is transmitted from the deep underground, it passes through the deep rock with greater rigidity to the alluvium with less rigidityWeak rockAnd soil will increase the amplitude by 4 times or more, depending on the frequency of waves and the thickness of alluvium.In 1989CaliforniaSan Francisco built on sand and alluvial fill during the Loma Pratt earthquakeCoastal areaThe damage of the houses in this area is greater than that of similar houses built on solid foundation nearby.
application area
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seismic wave
Used by seismologists, geophysicists and engineersseismograph、detector(Geophone) to record seismic waves. Early instruments used the pendulum principle andAnalog signalRecording seismic waves, modern instruments usePiezotransistoranddigital signals Process seismic waves.When the medium is changed, the seismic wave will have different transmission speed, and will be generated on the interfacerefractionThese characteristics are used to understand the internal structure of the earth.
In March 2015, American scientists revealed the underground structure by using the simulation diagram drawn by the velocity of seismic waves.This simulation map shows the mantle beneath the Pacific Ocean. The slower seismic waves are red and orange, and the faster seismic waves are green and blue.The 3D simulation map showing the interior of the earth was drawn by a research team led by Professor Jerome Trump of Princeton University.The goal of their research is to map the whole mantle by the end of the year.The depth of the mantle reaches 1865 miles.
Seismic resonance
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Conceptual explanation
seismic wave
The reflection and refraction of seismic waves can sometimes make seismic energy concentrate in a geological structure, such as an alluvial valley, where there are soft rocks or soil near the surface.1985, which will be discussed latermexico cityAnd the special distribution areas severely damaged during the 1989 Loma Pratt earthquake can be explained by this reason (Fig. 2.7).Its effect is that in a room, sound waves can beMultiple reflectionForm echo and gather energy.During an earthquake, P and S waves are transmitted from afar and refracted into the valley. Their velocities arerigiditySmall rocks decrease, and they propagate under the valley bottom until they are close to the edge of the valley, and part of the energy is refracted back into the basin.In this way, waves begin to propagate back and forth, similar to water waves in a pond.Different P waves and S waves are interleaved, and the rotating wave crest is superimposed on the incoming wave crest, causing amplitude changes.At this time, the phase of each superimposed wave is the key, because when the phase of the intersecting wave is the same, the energy will be strengthened.Through this“Positive interference”Seismic energy is gathered in some frequency bands.If there is no geometric diffusion and friction dissipation of waves, that is, the vibrating rock and soil convert some wave energy into heat,Interference of wavesThe amplitude increase caused by this may lead to disastrous consequences.
The effect of seismic waves in limited geological structures can be understood from another perspective.Like the cross water wave seen in the pond, the interference seismic wave can producestanding waveOn the surface, the interference wave seems to stand still, and the ground seems to vibrate purely up and down.Similarly, when the strings of a stringed instrument such as a harp are plucked, standing waves also occur.Generally speaking, during an earthquake, many P waves and S waves with different frequencies and amplitudes are often excited in a river valley or similar structure. Soft soil can enhance the movement in many frequency bands, as in music, producing significant overtones or high-order vibration modes.If enough seismic wave recording instruments are deployed, this overtone can sometimes be recognized.
Specific cases
Since the 18th century, mathematicians have analyzed the vibration of an elastic ball.In 1911, the British mathematician Love predicted that a steel ball as big as the earth would have a basic vibration with a period of about one hour and a overtone with a smaller period.However, more than half a century after Loew's prediction,seismologyHome is still uncertain whether even the largest earthquake has enough energy to shake the earth and produce deep seismic music.It is not hard to imagine that seismologists have observed for the first timeFree oscillation of the earthHow time is ecstatic.May 1960Chile EarthquakeAt that time, there were only a few extremely long cycleseismographIt was clearly recorded that the extremely long period of seismic fluctuations lasted for many days, and the longest period of vibration measured was 53 minutes, which was similar to the 60 minutes predicted by Leff.The analysis of these ground motion records gives clear evidence for the first time that theFree oscillationIt is indeed observed.
summary
Vibration state of elastic rope
When an earthquake source releases energy, the resonance vibration of the earth will continue in the way of no longer being stressed, and its vibration frequency only depends on the nature of the elastic earth itself.The exact basic principle of mathematical simulation is still similar to the analysis of plucking string instruments.The Greeks realized more than 2000 years ago that the harmonic of music only depends on the length, density and tension of the strings (Figure 2.8).This free vibration is called intrinsic vibration.Similarly, the intrinsic vibration in the earth that has been stirred depends on the size and density of its geological structure and the internalModulus of elasticity。
There are only two different types of intrinsic vibrations in an elastic sphere.One is called T-shaped or annular oscillation, which only includes the horizontal movement of the earth's rocks;The particles of rocksphere——Reciprocating motion on the earth's surface or some internal interfaces.The second type is called S-shaped or spherical oscillation. The motion components of spherical oscillation are both in the radial direction and horizontal direction.
Seismic surface wave
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The movement of rocks near the surface during the propagation of Love wave and Rayleigh wave
When P and S waves reach the free surface of the earth or are located at the interface of layered geological structures, other types of seismic waves will be generated under certain conditions.The most important of these waves isRayleigh waveandLefbo。These two types of waves propagate along the earth's surface;The amplitude of rock vibration gradually decreases to zero with the increase of depth.Because of thesesurface waveOnly when the energy is captured on the surface can it spread along or near the surface, otherwise these waves will be reflected downward into the earth, where there is only a short life.These waves are similar to the "whisper gallery" at St. Paul's Cathedral in LondonEcho Wall of Temple of Heaven）Only when your ears are close to the wall can you hear the whispers coming from the opposite wall.Love wave is the simplest type of seismic surface wave.They were named after Loew, who first described them in 1912.As shown in Figure 2.9, this type of wave makes the movement of rock particle similar to SH wave, and the movement is not verticaldisplacement。The rock movement is from side to side in the horizontal plane in a direction perpendicular to the propagation direction.Although Leff waves do not include waves of vertical ground motion, they can be the most destructive in earthquakes because they often have large amplitude and can cause horizontal shear under the building foundation.
On the contrary, Rayleigh surface waves have quite different ground motions.It was first described by Lord Rayleigh in 1885. They are the nearest water like waves in seismic waves.The rock particle moves forward, upward, backward and downward, making a vertical plane along the wave propagation direction, and the particle moves in the plane to draw an ellipse.The velocities of Love wave and Rayleigh wave are always smaller than those of P wave, and are equal to or smaller than those of S wave.From the ground motion similarity, the spherical (S-shaped) free oscillation is the propagation of Rayleigh wavestanding wave, annular (T-shaped) free oscillationLefbocorresponding.
Seismic wave sequence
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Because the velocities of different seismic wave types are different, their arrival times are also different, forming a group of sequences, which explains the feeling we experience after the ground starts shaking during the earthquake.
fromSourceThe first wave to reach a certain place is the "push and pull" P wave.They usually shoot out of the ground at a steep angle of inclination, thus causing ground movement in the vertical direction. Vertical shaking is generally easier to withstand than horizontal shaking, so they are not the most destructive waves.Because the propagation speed of S wave is about half of P wave, the relatively strong S wave arrives later.It includes SH and SV waves: the former is in the horizontal plane, and the latter vibrates in the vertical plane.The duration of S wave is longer than that of P wave.The earthquake makes the buildings shake up and down mainly by the action of P wave, and the buildings shake laterally by the action of S wave.
Just after or at the same time as the S wave,LefboStart arriving.The ground began to sway laterally perpendicular to the wave propagation direction.Although witnesses often claim that the source direction can be determined according to the shaking direction, the Love wave makes it difficult to determine the source direction according to the feeling of ground shaking.The next is the Rayleigh wave propagating across the earth's surface, which makes the ground shake in both longitudinal and vertical directions.These waves may last for many cycles, and are well known as "rock movement" when they cause large earthquakes.Because they followDistance attenuationThe rate of P wave or S wave is slower than that of P wave or S wave, and what is perceived or recorded for a long time when the distance from the source is large is mainlysurface wave。
Similar to the last section of the music, the surface wave train forms an important part of the seismic record, called seismic coda.The tail of the seismic wave actually contains the path of scattering through the complexRock structureP wave, S waveLefboAnd Rayleigh waves.The continuous wave cycle in the wake may play a role in the destruction of buildings, and promote the collapse of buildings that have been weakened by strong S waves that arrived earlier.
The propagation of surface waves into long coda is an example of wave dispersion.This effect occurs when various types of waves pass through media with varying physical properties or scales.A closer look at the water waves in the pond shows that the waves with short wavelengths propagate in front of the waves with longer wavelengths.The speed of a wave crest is not constant but depends on the wavelength of the wave.When a stone hit the water, the original waves began to be distinguished according to different wavelengths with the development of time, and then the shorter wave ridges and wave troughs gradually spread to the front of the long wavessurface waveThere are similar phenomena in communication.
The wavelength of different seismic waves varies greatly, from thousands of meters long to tens of meters short, so the seismic waves are likely to have frequency dispersion.Figure 2.11 shows the variation of particle motion of rock with depth from the ground to the deeper part of a typical surface wave.Since it is a surface wave, most of the wave energy is captured near the surface of the earth. After reaching a certain depth, the rock is actually not affected by the transmission of surface waves. This depth depends on the wavelength. The longer the wavelength, the deeper the wave penetrates the earth.Generally speaking, the deeper the rock in the earth, theseismic velocity The faster, so long period (long wavelength) surface waves generally propagate faster than short period (short wavelength) surface waves.This difference in wave velocity causes the surface wave to disperse and stretch the long wave train.However, in contrast to water waves, the longer surface waves arrive first.
As the depth increases, the ellipse becomes smaller and finally disappears. The elliptical movement may be clockwise or counterclockwise
We also need to understand another nature of waves to complete the full understanding of the wonderful world of seismic wave movement, which isWave diffraction(diffraction）Phenomenon.When a row of water waves encounters an obstacle, such as a vertical pipe protruding from the water surface, most of the energy of the wave energy is reflected away, but some waves will enter the shadow around the pipe, so the water behind the pipe is not completely calm.In fact, the diffraction of all types of waves, whether water, acoustic or seismic, causes them to shift from a straight path, dimly illuminating the area behind the obstacle.
The theory and observation agree that the long wave deflects more towards the quiet zone than the short wave.In other words, diffraction, like dispersion, is a function of wavelength.The most important point for geological interpretation is P wave and S wavesurface waveNot completely prevented by abnormal rock inclusions, some seismic energy is diffracted around geological structures, and others are refracted through them.
Types of seismic waves
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Seismic waves are mainly divided into two types:surface wave, one isSolid wave。Surface waves are transmitted only on the surface, and solid waves can pass through the interior of the earth.
Solid wave(Body Wave): It is transmitted inside the earth and divided intoP-waveandS-waveTwo.
P-wave: P represents primary or compression, which is one oflongitudinal waveThe vibration direction of particles is parallel to the advancing direction of wave front. Among all seismic waves, the advancing speed is the fastest and the earliest.P wave can be transmitted in solid, liquid or gas.
S-wave: S refers to the secondary or shear force, the forward speed is only second to P wave, and the particle vibration direction is perpendicular to the forward direction of the wavetransverse wave。The S-wave can only be transmitted in the solid and cannot pass through the liquidearth 's core。
Using the different transmission speeds of P wave and S wave and the travel time difference between them, simple earthquake location can be made.
Loewwave(Love Wave): The particle vibration direction is perpendicular to the wave forward direction, but the vibration only occurs in the horizontal direction, without vertical component, similar to the S wave, the difference is that the lateral vibration amplitude will decrease with the increase of depth.
rayleighwave(Rayleigh wave): also known as ground roll wave, particles move in a way similar to sea waves. On the vertical plane, particles vibrate in an anticlockwise ellipse, and the vibration amplitude will decrease with the increase of depth.
