Natural vibrations of the earth
This article was due to content flaws on the quality assurance side of Geosciences portal entered. This is done to increase the quality of the articles in the geosciences topic. Please help to correct the deficiencies or take part in the discussion . ( + )
The natural vibrations of the earth (also natural frequencies of the earth or earth hum ) consist of a superposition of dozens of low-frequency vibrations, which occur continuously in the earth's body, i.e. independent of events such as earthquakes or volcanic eruptions . However, there are seasonal influences.
A similar effect is also assumed with other planets (e.g. Mars ), one speaks then generally of planet hum . Such natural vibrations have also been proven for the sun .
term
The English term hum (English for "hum") is used internationally . However, this is ambiguous. On the one hand, this refers to low-frequency humming noises that are locally dependent and are not heard by everyone ( humming sound phenomenon ). The well-known microseisms , which are caused by the atmosphere and oceans, are also called hum or Earth hum .
properties
General
Around 60 different frequencies overlap in the range from 3 to 7 millihertz and form a mixture in the infrasound range that is no longer perceptible to the human ear . The vibrations are also so weak that they can only be registered with highly sensitive measuring devices.
An important property of this frequency mix is that it is not a series of overtones (similar to a bell that is struck), but rather a mix of different frequencies (similar to an orchestra).
species
There are two types of vibrations, spheroidal and toroidal. Both types of vibration each contribute about half to the overall effect.
Spheroid vibrations
The spheroidal vibrations are vertical. At the same time, there is a slight forward and backward movement, so that an overall elliptical movement is created. It takes place every minute . The deflections are a few tenths of a micrometer .
Toroidal vibrations
These are done horizontally, but twisted in a complicated way. Toroidal vibrations also occur every minute.
causes
General
While it is believed that spheroidal vibrations are well understood today, the toroidal vibrations are still a mystery.
So far, the cause of the spheroidal vibrations has been pressure fluctuations such as those caused by the movements of the sea and atmosphere .
The toroidal vibrations are caused by shear forces, the origin of which, however, is still unclear. There are numerous conjectures, all of which have been speculative so far and explain only small parts of the toroidal oscillations:
- Topographical effects from deep sea mountains were assumed, so mountains on the sea floor should divert pressure into shear forces.
- An invisible coupling should also transfer kinetic energy between the two forms of oscillation. This coupling is based on the shape of the earth, which is not an exact sphere, but slightly flattened at the poles.
- Earthquakes are supposed to stimulate the toroidal vibrations.
- Even the large-scale rotation of the air around low pressure areas has already been blamed.
The difficulty in explaining the toroidal vibrations also lies in the difficulty in measuring them. Local air pressure fluctuations superimpose the earth's toroidal natural vibrations. In order to measure them, underground observatories are therefore necessary, which isolate the measuring devices from such fluctuations.
Seasonal effects
The spheroidal vibrations are subject to seasonal effects. These are strongest when it is winter in the northern or southern hemisphere.
- From December to February the strongest vibrations come from the northwest,
- from June to August from the south.
One possible explanation for this is a theory proposed in 2004 by Junkee Rhie and Barbara Romanowicz of the University of California at Berkeley . According to this, storms could be the cause. Strong storms rage over the seas in winter and create waves that reach down to the sea floor and “knead” it. When winter is in the northern hemisphere, these storms particularly rage in the North Atlantic and North Pacific. In the winter of the southern hemisphere, they mainly romp in the seas around Antarctica.
However, the toroidal vibrations cannot be explained by this effect.
exploration
aims
In addition to explaining the toroidal vibrations, research into the hum of the earth promises a better understanding of the interaction between the atmosphere, solid earth and oceans, and this in turn leads to improved climate models . In addition, it is hoped to gain new knowledge about the internal structure of the earth, but also other planets, such as Mars.
prehistory
The idea of a " harmony of spheres " already existed in ancient Greece . The movements of the planets should produce planetary tones that are inaudible to humans . In 1619, the astronomer Johannes Kepler postulated such a harmony of spheres in his Harmonice Mundi . Although now scientifically outdated, the idea of the harmony of the spheres still finds interest in art and esotericism .
The Chinese astronomer Zhang Heng invented the first known seismoscope in AD 132 . A pendulum was already used in the device at that time.
In 1794 Ernst Florens Friedrich Chladni founded modern acoustics . He showed that acoustic vibrations are to be considered material- related independently of the air . In doing so, he paved the way for modern seismology , since seismic waves are ultimately nothing other than the vibrations of the materials that make up the earth.
Luigi Palmieri invented the electric seismograph around 1856.
In 1897 Emil Wiechert developed the first seismograph that was suitable for investigating the interior of the earth. In 1898 he became a professor at the University of Göttingen. In the same year his institute was transformed into the world's first institute for geophysics. From 1899 he carried out measurements with pendulums and light pointers in the observatory and from 1902 in the newly founded earthquake observatory in Göttingen , the oldest earthquake observatory in the world still in operation. He wrote books and made important discoveries. Wiechert thus became the founder of modern seismology.
In 1960 the American physicist Robert B. Leighton discovered the natural vibrations of the sun and thus founded helioseismology .
Traditionally, the focus of seismology has always been on the registration of earthquakes and seaquakes, volcanic eruptions, as well as artificially triggered vibrations (see seismics ), not least because such events cause far stronger vibrations than the hum of the earth. In the course of time, however, more and more sensitive measuring devices were developed, pendulums, seismometers and gravimeters, but also powerful computers, which ultimately created the technical prerequisites for the discovery and research of earth hum.
History and methods
GRSN observatories, in the extreme south-west the BFO
|
|
In 1998, the spheroidal oscillations were first discovered by Kazunari Nawa and Naoki Suda of Nagoya University through mathematical analysis of seismic data collected over several years.
Research into toroidal vibrations, on the other hand, caused problems for a long time. The reason is the fact that these are stronger than the spheroidal vibrations. However, this is due to local influences, such as fluctuations in air pressure. In order to exclude such local influences, underground observatories are used, e.g. B. the Black Forest Observatory (BFO) near Schiltach in Baden-Württemberg . The discovery of the toroidal vibrations finally succeeded Dieter Kurrle and Rudolf Widmer-Schnidrig at the BFO after they had compared the measurement series there with those from China and Japan.
Investigating toroidal vibrations is also more difficult because only a few observatories can provide suitable data. While there are over a hundred observatories worldwide that provide data for vertical oscillations, there are only four of the toroidal ones.
Below is a table of the stations and data channels whose data were used to study toroidal vibrations:
| station | Abbreviation | country | width | length | network | sensor | channel | Period |
|---|---|---|---|---|---|---|---|---|
| Black Forest Observatory | BFO | Germany | 48.33 ° N | 8.33 ° E | IRIS IDA | STS-1 | VHE | 1997-2006 |
| GRSN | STS-2 | LHE | 1996-2006 | |||||
| Baijiantuan | BJT | China | 40.02 ° N | 116.17 ° E | CDSN | STS-1 | VHN | 1994-2006 |
| Matsushiro | MAJO | Japan | 36.54 ° N | 138.21 ° E | IRIS USGS | STS-1 | VHE | 1996-2006 |
| Takato | TTO | Japan | 35.84 ° N | 138.12 ° E | F-net | STS-1 | LHN | 1999-2006 |
It is hoped that progress will also be made in the future through simulations using supercomputers.
Space missions are also planned for the future. The ESA ExoMars program planned to land on Mars in 2016 to a. also take seismic measurements. Something similar had already been planned in the late 1990s. In the CNES-led Netlander project , landing capsules for seismic measurements were to be deployed at four different locations on Mars. However, in 2003 the project was stopped.
music
- Kookoon : INNER EARTH. Seismosonic Symphony. 1999. Traumton Records . CD 4492-2.
- The Berlin sound engineer and composer Wolfgang Loos created a composition together with the geophysicist Frank Scherbaum from the Institute of Geosciences at the University of Potsdam from the vibrations of earth hums, earthquakes and volcanoes. With the help of mathematical processes, the frequencies, which are inaudible in themselves, had to be transmitted into the human hearing range.
literature
- Walter Zürn, Rudolf Widmer-Schnidrig: Global natural oscillations of the earth, Physik Journal, Volume 1, 2002, No. 10, pp. 49-55, online
Web links
- Ralf Butscher: The Symphony of the Earth. Image of Science, 1/2009. Konradin Medien GmbH, Leinfelden-Echterdingen. ISSN 0006-2375 .
- Homepage of the BFO
- The hum of the earth: natural oscillations, world of physics, DPG
swell
- ↑ Rhie, Junkee and Romanowicz, Barbara: Excitation of Earth's continuous free oscillations by atmosphere-ocean-seafloor coupling . Nature, 431, 552-556 (September 30, 2004). ISSN 0028-0836 . (in English)
- ↑ Naoki Suda, Kazunari Nawa and Yoshio Fukao: Earth's Background Free Oscillations . Science, Vol. 279, no. 5359, pp. 2089-2091 (March 27, 1998). ISSN 0036-8075 . (in English)
- ↑ Kurrle, Dieter and Widmer-Schnidrig, Rudolf: The horizontal hum of the Earth: A global background of spheroidal and toroidal modes. Geophysical Research Letters. Vol. 35 (2008), L06304. ISSN 0094-8276 . (in English)