Seismic scale

The magnitude is a measure of the strength of the earthquake. Magnitudes are rare mainly determined from the amplitudes for other parameters of seismograms. These in turn are recorded on seismometers worldwide with seismographs. In contrast, the intensity of earthquakes - so its impact on people, buildings and landscape - without observing instruments.

• 3.1 Basis
• 3.2 comparability
• 3.3 Error in Press Releases
• 3.4 magnitude scales

Historical developments

The oldest magnitude scale is the well known from the media Richter scale, which was developed in the 1930s by Charles Francis Richter for the quantification of Californian earthquakes. Judge had recognized that a relationship between the maximum deflection in the seismogram and the distance is from the epicenter. The logarithmic relationship was found as suitable to zurückzuschließen from the decay of the amplitude on the magnitude of the earthquake. However, this magnitude scale refers to seismic waves, run their beam paths mostly through the earth's crust. Thus, the Richter scale is applicable only for use up to 600 to 1000 km Distance from the epicenter. It is therefore also known as Lokalbebenmagnitude (ML ).

In order to further compare distant earthquakes, led Beno Gutenberg, 1945, the so-called Oberflächenwellenmagnitude (MS ) a. In the same year he also presented the Raumwellenmagnitude ( mB ). Various conditions that were specified in part by the stove processes of earthquakes and partly by the limits of technical feasibility, led to the development of other magnitude scales.

That was due to the limited dynamics and the overdrive with strong local events to correctly determine the maximum deflections in the earlier analog recording devices is not always possible. Makeshift the Codamagnitude (Md ) has been developed for those cases for which the decay time of the Wellencoda, especially the Sg phase was used. In modern science is especially the moment magnitude scale use, which was developed in 1977 by Hiroo Kanamori and Tom Hanks.

Methodological bases

Historical definition

Judges looked at maximum amplitudes in seismograms (measured in micrometers [ microns ], ie 1/1000 mm ), which were recorded by standard Wood -Anderson seismometers of type. He put the logarithm of the amplitude values ​​as a function of the epicentral distance ( distance of the measuring instrument from the epicenter ) dar. He found that the maximum amplitudes of earthquakes of different intensities along more or less parallel curves decay with distance. He therefore defined the magnitude of an earthquake as the logarithm of the Maximalauschlag Standardseismometers. He used for scaling a reference distance of 100 km.

Local restrictions

Strictly speaking, the Richtermagnitude applies only to the California area, since the decrease in the amplitude depends on the nature of the rock material.

Subsequent development

Later, other magnitude scales developed. My basic principle is largely the same, but different phases of the wave field and their special physical properties, it can be exploited. Thus, in the surface acoustic wave, the true ground motion is derived from the seismogram and used for calculation of the magnitude, while the Raumwellenmagnitude mB based on the theoretically calculated correction of the amplitudes due to the decrease of the energy density of 1 / r ² with spherical waves and the attenuation occurring along the beam path.

With the introduction of WWSSN standard seismometer with a natural frequency of one hertz (equivalent to a natural period of a second), a calibration of the short-period wave components (English: short period, abbreviated as SP ) are common. The change was primarily due to the increasing interest in using seismological records for detection of underground nuclear explosions, which can be identified, among others, based on their frequency spectrum. To distinguish these short-period Raumwellenmagnitude is called mb.

A frequently used empirically developed relationship establishes a connection between the MS and the Oberflächenwellenmagnitude released during the earthquake seismic energy ES ( in joules ).

Consequently, magnitude scale 1 is an about 32 times greater energy release. A difference of 2 magnitude units already corresponds to the 1000-fold release of energy. An earthquake with the Oberflächenwellenmagnitude MS = 5.5 then has the seismic energy ES ≈ 3 GWh, which is released within a few seconds. The same seismic magnitude would produce an underground nuclear explosion with an equivalent of one megaton (Mt) chemical explosive. However, in the blast only about one percent seismic wave energy would be generated, while the remaining energy would flow in heat generation and in the crushing of rock material.

Errors due to saturation problems

Almost all magnitude scales behave problematic in the detection of particularly strong earthquakes ( saturation phenomenon). The reason for this is that the maximum amplitude not significantly increased in the upper part by the growth of the energy radiated by the earthquake.

In the saturation region, the scale gives the further growth of the energy radiated by the earthquake not properly again. This can not be inferred correctly on the energy released by the earthquake, and the magnitude of earthquakes in this area is virtually indistinguishable.

Saturation scale -free

The moment magnitude scale is derived solely from the seismic moment, and thus from the direct physical parameters of the epicenter. It does not reach saturation even for the most serious earthquake. and is therefore often used for very large events.

Diversity of the magnitude scales

Basis

The different methods for Magnitudenbestimmung based on the amplitudes of various phases of the seismic wave field. These differ in terms of the physical basis of their propagation. One major difference, for example in the energy spectrum, since the wave phases have different dominant frequencies and oscillation periods ( see table).

Comparability

Because of this nature- differences of the wave phases, the results differ the magnitude regulations different methods partially considerably from each other and are only partially comparable. This is especially true for very large earthquakes, when the saturation already described above comes into play.

This can be easily shown using the Chile earthquake of 1960: This event reached after the ( saturated ) Oberflächenwellenmagnitudenskala the value of MS = 8.5, while the moment magnitude MW = 9.5 gives the value and thus around 30 times higher energy release. For proper classification of the strength of an earthquake, the specification of a simple numerical value is not sufficient, it must always also the magnitude scale is based are called correctly.

Errors in press releases

In press releases on Erbebenereignissen incorrectly almost always spoken of the Richter scale. In particular, however high magnitude values ​​above about 6.5 are generally based on other magnitude scales, since the former is not designed for higher magnitudes. It is therefore advisable to check press releases based on publications by seismological services (see links).

Magnitude scales

Which method to determine the magnitude was used, the name can be seen. An index added: For this is the big " M" for " Magnitude" ( The Raumwellenmagnitude mb exception):

This magnitude scales represent a selection, for certain purposes even more or derived from the above-mentioned scale Magnitudenbeziehungen be used.

Others

Substituting the Oberflächenwellenmagnitude (MS) and the Raumwellenmagnitude (mb) correlated to earthquakes can easily be of explosion sources differ (for example, a nuclear bomb ): In nuclear explosions is the ratio between the measured, weak, surface waves, and the significantly stronger bumps exceptionally high.