A tidal wave with a wave height of half an inch was detected on the ocean near the Pacific coast of the Americas.This is the first time such waves have been detected on the high seas. People may think that the tidal waves mentioned here are not the same as the tsunamis that destroyed houses and factories on the coast.But in fact, the huge tidal waves in the Pacific Ocean are only a little higher, perhaps just a few inches high.These unusual waves are very long and flow very fast. When they overlap along the coast, trouble will come.On the ocean surface of the continental shelf, the accumulated tidal waves suddenly become bigger than the houses and form a huge wave with rapid speed. Everything is swept away everywhere, causing very serious damage.Therefore, it is meaningful to take some measures to detect them on the high seas[1]。
This half inch tidal wave was observed by Dr. J. Filloux, a French oceanographer who is now working at the Scripps Institution of Oceanography in California.Veru and his research team have installed some pressure detectors at different locations on the seafloor in the northeastern Pacific Ocean.These detectors record the weight of water on them twice every minute, and can sensitively detect the micro change of water level height[1]。
Veru often collects some recorded results and brings them back to California for analysis.He has found a clear record of the passage of a series of tidal waves in one of the records.Tsunami is not caused by tides, but by the sudden change of ocean water level caused by undersea earthquakes.At the beginning of the Flor record, there were violent fluctuations, along which an earthquake occurred in the ocean near the coast of Mexico can be traced.Flou believes that the wave recorded this time is not from the change of ocean water level, but caused by the first wave of the earthquake passing through the seabed rocks and reaching the observation point.After 90 minutes or so, the pressure value shows a slow and regular oscillation, which seems to have nothing to do with the rock mass movement, but it must be related to the wave of the ocean surface.In about two hours, the sea water seemed to rise and fall four times above and below half an inch.The arrival of the wave and the time required for its propagation are exactly the time people expected for the arrival of a tsunami caused by the Mexican earthquake and the time required for its propagation[1]。
In this way, Veru did detect the tsunami propagation, and he should be able to detect the same situation again with his other detectors.It is reasonable to say that as long as the pressure wave signal can be automatically transmitted to the land through the surface buoy or satellite within the time when the alarm takes effect, it can be used to issue an alarm.The only warning is the signs of earthquakes in the areas that we already know can cause tsunamis. However, given that only about one fifth of the destructive waves caused by these earthquakes, people are sometimes reluctant to take these warnings seriously.Combined with the analysis of pressure wave detection results and seismic observation results, it is possible to issue more reliable warnings about the impending tsunami and its size, but the cost of the satellite recording system is prohibitive[1]。
Causes
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EarthlyrotationIt determines the replacement of day and night on the earth, and makes some natural geographical processes on the surface have the rhythm of day and night, and also makes moving objects such asAir mass、ocean currentAnd flowing water.The rotation of the earth causes the difference of the earth at the same timeMeridianIt has different local time, and also makes the tide change into tide wave opposite to the rotation direction.
Some studies indicate that some tidal waves are caused bymethaneA large bubble rises to the surface of the water.This huge bubble will be generated because the methane is blocked when it is discharged from the sea bottom, and then the pressure increases until the gas suddenly bursts and quickly rises to the sea surface, thus generating tides.Research shows that such bubbles can easily and quickly sink the model ship.
Tidal wave and seismic wave
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Crustal deformation is the effect of the crust on the tectonic stress andExtraterrestrial objectsDeformation under tidal force and other loads.With the different frequency of the force, the crust will have different periods of vibration.Earth tide and earthquake are two extreme cases of ground vibration period, and their vibration waveforms are called solid tide wave and seismic wave respectively.Solid tide waves include gravity, tilt and strain solid tide waves, hereinafter referred to as tide waves."Wave is also called wave. The process of vibration transmission is a form of energy transmission", and vibration is "the process of objects changing back and forth through the balance position".Since both tidal waves and seismic waves are called waves, they have some common characteristics in the process of vibration propagation. However, due to the different force sources that generate these two waves, they also have many differences[2]。
Similarities between tidal wave and seismic wave
(1) The vibration response of the solid earth to different excitation forces Tidal wave and seismic wave are both the ground response of the solid earth to the tidal force and source excitation, and its response form is a kind of vibration[2]。
(2) Elastic behavior of the earth
Since the earth medium is between fluid and rigid body, generally speaking, the magnitude or form of crustal deformation is related to the acting time of the force. If the acting time of the deformation force is very short, or the deformation force changes rapidly, the earth is elastic.The analysis of the earth tide shows that the response of the tectonic circle to the weak stress change is basically elastic under the action of a longer stress cycle.For seismic waves with high propagation speed, transient stress applied to rocks and small energy loss, the assumption that rocks are completely elastic is more acceptable[2]。
(3) Different vibration forms of simple harmonic motion
According to document [7], "simple harmonic motion is a vibration of a particle or object along a straight line in the form of periodic motion relative to the central point", and its universal expression can generally be expressed as a cosine function varying with time[2]。
Differences between tidal wave and seismic wave
(1) Different power sources
The force source of tidal wave is tidal force.Tidal force refers to the difference between the universal gravitation of the celestial body to a certain point of the earth and the universal gravitation to the center of the earth.The reason why we should subtract the universal gravitation of the celestial body from the universal gravitation of the celestial body to a certain point of the earth is that when the earth moves horizontally around the common center of mass of the earth and the moon, all the particles of the earth will produce the same force as the universal gravitation of the celestial body to the center of the earth.The same force everywhere will not deform the earth, but will accelerate the whole earth like a particle, and at the same time, the tidal force at the earth center will be 0[2]。
In 1922, Dudson expressed the above tidal generating potential as the sum of 386 waves of amplitude coefficient multiplied by the earth coefficient of the corresponding tidal generating potential according to the argument number.
Seismic waves are generated by source disturbances.The source models mainly include fault source and volume source.The volume source is used for artificial earthquake, and the fault source is used for tectonic earthquake.The fault source is related to the relative sliding of the fault plane in the crust.The fracture of fault plane caused by sliding of fault plane can be considered as the superposition of many point fractures.Therefore, seismic waves can be regarded as the superposition of pulses from many different seismic phases[2]。
(2) Different energy transmission modes
Tidal action is actually a mechanism to transfer energy from one celestial body to another [Chuan. The energy transmission of tidal waves of a particle on the earth is carried out in the form of potential energy and kinetic energy conversion associated with the weak vibration of each particle (except the earth center) relative to its equilibrium position.Since the tide generating force of celestial bodies is continuous, the energy transmission of tidal waves is also persistent. However, the energy transmission is limited to a very small range of simple harmonic vibration of particles. It cannot spread around like seismic waves. Except at this point, tidal waves at this point cannot be observed in other places.
Seismic waves are vibrations that travel from the earthquake source to all directions.The seismic energy is provided by the source and propagates around at the wave speed of various seismic waves. A strong earthquake can be observed by seismic stations around the world.We can personally feel the seismic waves that sense earthquakes, while tidal waves can only be observed by extremely sensitive instruments because of their small amplitude[2]。
(3) Different recording waveforms
Tidal wave is composed of long period wave, daily wave, half day wave and 1/3 day wave.On the background of the diurnal wave, we can clearly distinguish the double peaks and double valleys of the semidiurnal wave.Because of the existence of celestial bodies, the tidal wave curve is a continuous curve.
Seismic waves are composed of many pulse wave groups called seismic phases, which are caused by energy propagating along different rays through the earth.In the record of an earthquake, any change in amplitude and frequency is called seismic phase. The main seismic phases are P, S and L, which represent longitudinal wave, shear wave and surface wave respectively.From P wave to L wave, the period is longer than one another, and the period ranges from a fraction of a second to 50 minutes. The longest periods correspond to the free oscillation of the earth excited by large earthquakes.Because the pulse wave groups that make up the seismic phase have a certain duration, the waveforms of different seismic phases overlap each other, resulting in interference, which makes the seismogram present complex graphics, so that
Generally, only the start of seismic phase can be identified ["]. With the attenuation of seismic energy, the seismic record curve will decay quickly[2]。
(4) Different spectrum structures
The spectrum of tidal wave is line spectrum, because it is composed of many different frequencies of partial wave superposition.
The frequency spectrum of seismic waves is continuous spectrum, because seismic waves are composed of many pulses called seismic phases.Earthquakes produce complex vibrations with short duration and quite wide frequency spectrum.Therefore, the spectrum of seismic wave is continuous spectrum, but not flat spectrum[2]。
(5) Different observation means
Tidal waves include gravity, tilt and strain earth tides.Gravity and tilt earth tide shall be observed by gravimeter and tiltmeter respectively.The strain gauge is used to observe the strain earth tide, and the strain observation is limited to the expansion and contraction of the surface in a certain direction.The shear strain component cannot be observed yet and can only be calculated indirectly.
The same seismograph can record P wave (expansion and contraction), S wave (shear) and surface wave on the ground successively.Although the equipotential planes of P wave and S wave near the earthquake source are very close, the wave front is separated at a far place, so it can be recorded successively by the same seismograph.After S wave, surface wave propagating near the surface can also be recorded.
However, with theMicroelectronicsWith the rapid progress of computer technology and communication technology, digital observation technology has been widely used in crustal deformation and seismic observation.Due to the large dynamic range, wide frequency band and high resolution of digital observation, today's strain gauge can record seismic strain waves in addition to earth tide.Broadband seismographEarth tide can also be recorded[2]。
(6) Waveform generation is different
Tidal waves are superimposed by many wave groups, and each wave group is composed of many sub waves with very close angular frequencies.Therefore, the superposition of these waves will produce a modulation phenomenon, which is called beat.
Seismic waves include body wave and surface wave. Body wave will reflect, refract and diffract on the interface, and surface wave will produce polarization on the interface.In layered media or media with velocity varying with depth, frequency dispersion also occurs. At this time, seismic energy propagates at group velocity instead of phase velocity, resulting in more and more scattered surface wave groups and attenuation of amplitude[2]。
Main characteristics of tidal waves
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The force source of tidal waves is the tidal force of celestial bodies.Tidal force is essentially gravitational force.According to Newton's classical theory, the universal gravitation is a force beyond the distance and an instantaneous force, but in fact there is no such force.According to Einstein's general theory of relativity, an accelerating mass will radiate gravitational waves, and a celestial body will also radiate gravitational waves when it moves in orbit.The hypothesis that gravitational waves propagate at the speed of light is the core of Einstein's general theory of relativity, which constructs the concept of space-time.This theory has never been tested since it came out in 1915. Up to now, it has been confirmed by American scientists ①.Therefore, the time when gravitational waves emitted by celestial bodies in the same epoch act on the earth is different, but considering the space scale of the earth, the time difference of gravitational waves acting on the earth's points may not be considered.Of course, this is just a statement, because some scientists claim that the speed of light is not fixedEinstein's Theory of RelativityChallenges are presented.Therefore, the nature of gravity or gravity is still inconclusive.However, at any moment, the elastic earth becomes a slightly elongated ellipsoid along the direction of the celestial body under the tidal force.The general feature of the lunar gravitational field, which cannot be "eliminated" by free fall, is the tidal effect. The magnitude of its tidal deformation is inversely cubic with the distance away from the lunar attraction center, rather than the inverse square law of gravity[2]。
As a response to the tidal force, the earth can be regarded as an elasticLinear time invariant system。Therefore, in essence, the tidal wave at any point on the earth can be considered as the superposition of the system's response to each simple harmonic component in the tide generating potential.Each component has only three elements: amplitude, angular frequency and initial phase.Unlike seismic waves, the wave form of tidal waves is the propagation of source disturbance. It is only a time-lapse vibration of a physical quantity (such as gravity, tilt, strain) of a particle of the elastic earth relative to the equilibrium position, and does not have the propagation speed of particle vibration in the earth's interior and surface.In this sense, tidal wave is not a wave in essence, but a superposition of a simple harmonic vibration of a particle[2]。
Another characteristic of tidal wave is that its theoretical value can be calculated in advance.The rigid body seismic model is used for gravity and tilt earth tide, while the Gutenberg Brent earth model is used for strain earth tide.The ratio of the transfer function of the real earth's response to the tidal force to the transfer function of the earth model's response to the tidal force is called tidal admittance.Tidal admittance is a complex number. Its amplitude is called tidal factor, and its phase is called phase lag. Tidal admittance can be calculated by using the Vinidikov harmonic analysis method.The tidal factors of gravity, tilt and body strain earth tide observations are linear combinations of Love numbers.The closer the Earth model is to the real Earth, the closer the tidal factor is to 1, and the closer the phase lag is to zero[2]。
disaster
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typhoon“Davy”After landing in northern Vietnam in September 2003, we continued to move from north to south and arrived atNanding Province, causing tidal waves up to 2.6 meters high, which is the highest tidal wave record in the past 40 years.In addition, the typhoon also damaged many coastal embankments, thousands of trees and telegraph poles were blown down, and telephone communication was also interrupted.Research shows that tidal waves, also known as freak waves, can roll up to 30 meters (100 feet) high, enough to sink a large ship in an instant.Tidal waves are very rare, butocean currentTidal waves occasionally occur.Now, tide waves are assumed to be the cause of many ships missing without reason in recent years[3]。
