Seismic wave
Seismic waves, also called seismic waves, triggered by an earthquake through the hearth process and propagate radially inside the earth from there. On their way through the bowels of the earth, these waves can be refracted, reflected, diffracted, scattered, absorbed and converted. The propagation velocity of seismic waves depends on the wave type and the material that passed through the waves. In particular, the material properties based research into the Earth's interior based on triggered by earthquakes ( seismology ) or by blasting or vibration ( artificial) caused ( seismology) seismic waves. Earthquake waves are long wavelength. The maximum is in the range of less than one cycle per second, sometimes above 20 Hertz. In severe earthquake, the fundamental of the earth is stimulated, which lasts 54 minutes.
- 5.1 Love waves
- 5.2 Rayleigh waves
- 5.3 Scholte waves
Seismic "rays" and seismic waves
In this article propagations of seismic waves are described in terms of approximate solutions, which could be referred to as " seismic rays". The relationship between " seismic rays" and " seismic waves " is corresponding to that between ray optics and wave optics. A more detailed description of the propagation of seismic waves is possible on partial differential equations, the so-called wave equations. These mathematical techniques similar to those of Erdspektroskopie.
Magneto hydrodynamic effects
Also inside the Earth exists, the Earth's magnetic field. The seismic waves through acoustic waves as a conducting fluid in a magnetic field. A circumferential approach is therefore to MGD. The occurring MHD waves are not described in ( used in the article) approximation of seismic rays.
Division into body and surface waves
First basic distinction is made between body waves (P- and S- waves) and surface waves ( Love and Rayleigh waves).
Body waves
The names of the following described primary waves (P- waves) and secondary waves (S- waves ) refer to the fact that the former spread faster. At a remote location from the epicenter, the first P-wave and later the S waves are recorded. From the time difference between the onset of the P and S- waves, the distance can be calculated on the hearth. Can at least three different places the distance to the epicenter and the direction of the waves are determined in this way, the epicenter can be determined within the measurement accuracy.
P-waves
The P- waves or primary waves vibrate in the direction of propagation ( longitudinal wave ) and can propagate in solids, liquids and gases. It involves compression waves (also: pressure or compression waves). An everyday example of compression waves of sound in the air or in the water. The P- waves can propagate in solid rocks, but also in liquids such as water or the quasi-fluid parts of the Earth's interior. As with sound waves in the air, the particles are herein pushed and pulled in the ground, said movement being in the direction of propagation of the wave.
The propagation velocity of P- waves can be calculated with the following formula:
Where K is the bulk modulus, shear modulus and the density of the material through which the wave propagates.
Although the density in the ground increases with depth, the effect is more than compensated for in the above formula by a stronger increase of K and μ, so that in general an increase in the observed rate of P waves with depth.
In the earth's crust the velocity of P- waves is between 5000 to 7000 m / s (compared to the speed of sound in air 340 m / s in granite about 5000 m / s, in water about 1500 m / s) and about 8000 m / s in the Earth's mantle and core.
S- waves
The S- waves or secondary waves oscillate transversely to the direction of propagation ( transverse wave ). Because they lead to Verscherung of the propagation medium, they are also called shear waves. S- waves can propagate in solids, but not liquids or gases, as the latter two no ( significant ) shear resistance have. One can therefore recognize and liquid regions inside the earth because there is no running S- waves.
The propagation velocity of S- waves is calculated with the following formula:
With typical values of the elastic constants within the Earth results in a first approximation for the ratio of the approximate value of. This result for the S-wave velocities of 3000-4000 m / s in the Earth's crust and about 4500 m / s in the Earth's mantle.
Surface waves
In addition to the P and S - waves, there are the surface wave. They arise that P- or S- waves are refracted into the surface of the earth into it. As with the S- waves, the particle motion or vibration is perpendicular to the propagation direction. However, they are distinguished in that they run out at the surface and that the amplitudes of the waves decrease with depth. The energy of the surface waves increases also with the distance r only by a factor of 1 / r and not as that of the body waves by a factor of 1/r2 (in each case, neglecting damping). The surface waves propagate in the vertical and horizontal vibrations.
Seismic waves are also known from the solar surface. The measured propagation velocity is 50 km / s
Love waves
The Love waves are named after the British mathematician AEH Love, who was the first aufstellte 1911, a mathematical model for the propagation of these waves. They are the fastest surface waves propagate at approximately 2000-4400 m / s (depending mainly on the frequency and thus the penetration depth in the earth's crust ), but slower than the S- waves. The ground movement takes place in a horizontal direction perpendicular to the propagation direction.
Rayleigh waves
The Rayleigh waves were named after Lord Rayleigh, who had in 1885 the existence of these waves mathematically proven before they were ever observed. For Rayleigh waves the ground up in a retrograde elliptical motion similar to sea waves, ie the rolling movement takes place counter to the direction of propagation of the Rayleigh wave. This rolling moves the ground both up and down and back and forth in the direction of propagation of the wave. The propagation velocity is dependent mainly on the wavelength, approximately 2000-4000 m / s Most vibrations are sensed during an earthquake, are typically Rayleigh waves, the amplitudes can be much greater than that of the other types of waves. The destructive effect of earthquakes therefore is largely due to these wave type.
Scholte waves
Scholte waves are interfacial waves along the interface of " liquid-solid ", so for example the sea floor spreading. They are, as well as Rayleigh waves from the P- type SV. This means that they are elliptically polarized in the radial - vertical plane. If the ground is layered so the Scholte is dispersive. The Scholtewellen then has frequency-dependent propagation velocities. In addition, form in addition to fundamental mode ( fundamental frequency ) and higher order modes from ( harmonics).
Propagation in the Earth's interior
The density of the rocks takes to the interior of the earth to go. For each transition from a less dense to a more dense rock is increased, the propagation velocity of a seismic wave in the reverse case, the propagation velocity is decreased. According to Snell's law of refraction is a seismic wave that is, with increasing depth " from away Lot " broken, with decreasing depth they will " go to the Lot " broken. This is the cause of the typical course of seismic waves, which can be seen in profile as an arc ( see figure).