Which seismic waves have the shortest wave length?
Seismology - a window to the interior of the earth
Earthquakes and volcanism are directly tangible effects of the ongoing dynamics of the earth. Time and again, these natural hazards have brought severe catastrophes over humanity. For science, however, they are a unique source of information about the structures and processes in the earth's interior.
Seismology, the science of earthquakes, has developed from a small, observational and measuring science to a large international company since the end of the last century. Today seismology finds numerous applications, for example in exploration in the petroleum industry and in assessing the earthquake hazard of possible locations for large, critical construction projects such as nuclear power plants, dams or bridges, or to detect atomic bomb tests.
Earthquakes are caused by fracture processes at the “active” edges of continental plates, that is, during subduction processes when one plate slides under another, or when continental plates move laterally against each other (see Fig. 1). Such a "sliding process" is by no means a constant process. Rather, it happens again and again that two opposing units “get caught”. If the plates move further, tension forces are built up, which are released in a sudden rupture process - the earthquake - with rapid displacements that can amount to a few meters. The tremendous impact of earthquakes can be seen in the fact that the energy released in seconds in a severe earthquake is around 1 percent of the annual energy consumption of the USA. The energy released during the break propagates in the form of pressure and shear waves (that is, “sound” waves oscillating along and across the direction of propagation) in the earth and on its surface. Depending on the strength of the quake, these waves are still registered at distances of a few thousand kilometers. This is precisely what makes them useful, because they contain information about the layers of the earth that they have traversed.
The location of the epicentres of earthquakes
Today, earthquake waves are registered with the help of a dense network of seismometers installed around the world, which are sensitive to a wide range of oscillation frequencies. The seismometers are combined to form an international information system. The data obtained in this way will in future enable tomographic, i.e. three-dimensional recordings of the earth's interior, on which details several tens of kilometers in size can still be recognized.
It makes sense to examine the earth's crust not only with the waves of the previously unpredictable natural earthquakes, but also with waves that have been created by deliberately triggered explosive detonations in shallow boreholes or with the help of large vibrating machines. With this reflection seismic, the seismic echoes returning from depths of up to 100 km are registered by several hundred seismometers on the earth's surface. The seismograms of the individual measuring stations are then superimposed with the help of special processes and then result in a physical image, a tomography of the examined subsurface. The resolution of reflection and surface seismics is determined by the wavelength of the sound waves traveling through the rock.
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