Earth Spectroscopy

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The Erdspektroskopie studied the spectra of oscillations of the earth's body . Both natural vibrations of the earth's body after being excited by an earthquake and the vibrations forced by tides and the earth's rotation are examined.

Natural vibrations of the earth were postulated at the beginning of the 20th century. The physical model at that time was that of a steel ball (earth) that is struck by an earthquake and then roars for a while. It's like a bell struck with a clapper . The effect was first demonstrated with the help of gravimeters after the strong earthquake in Valdivia in 1960 . One possible form of oscillation is the periodic change in the earth's radius, which can be around 1 mm in the case of very strong earthquakes. In this oscillation mode, a complete oscillation lasts 20.5 minutes.

With the tides it has long been postulated that the earth's body with the continental plates had to somehow adapt to the tidal force , but that this adaptation was not perfect because otherwise there would be no ebb and flow in the oceans . These assumptions have now been replaced by measurements of the tidal movement of the earth's surface ( earth tides ), combined with the knowledge that ebb and flow are complex oscillation processes with strong material flows. The repercussions of the tides on the earth and moon as a supplier of energy and angular momentum are important for the forced vibrations of the earth's body : the moon moves away from the earth by about 4 cm per year and slows the earth's rotation, with the days per year increasing by 20 Lengthen microseconds and the earth's axis changes in its orientation.

Earth spectroscopy works with similar tools and methods as seismology and can be understood as one area of ​​it. In contrast to seismology, earth spectroscopy understands the earth as a system that is generally capable of vibrations and is not “only” traversed by seismic waves .

practice

Since the earth has a very large volume, its natural acoustic vibrations are comparatively slow, which also applies to the forced vibrations (the tide , for example, has a period of about 12 hours). Earth spectroscopy has only become possible since the use of broadband seismometers .

theory

Vibrations of the earth's body , which are largely composed of electrically conductive fluids , i. H. Liquids or plasmas and which is in its own magnetic field , the earth's magnetic field , are theoretically classified under magnetohydrodynamics . According to the current state of mathematics , the occurring non-linear partial differential equations can not be solved analytically, so that acoustic simplifications or numerical methods can be used to compare theoretical statements with the measured spectra .

Such approximate solutions (acoustic solutions, without taking into account the electromagnetic field and independent of the radial component) contain spherical surface functions whose degeneracy, similar to the degeneracy in quantum mechanics , among other things due to the rotation of the earth and deviations from the symmetry in the boundary conditions (such as Position and shape of the tectonic plates ), is partially canceled.

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See also

literature

Contains some theoretical basics about swinging acoustic oscillations in spheres