Earthquake scale
An earthquake scale is used to compare the strength or impact of earthquakes .
Two fundamentally different types of scales are used in seismology to describe earthquakes : The energy released at the location of the earthquake is classified on a magnitude scale , while the effects of the vibrations at a point on the earth's surface affected by the earthquake are described using an intensity scale .
overview
The severity of an earthquake is described in terms of both magnitude and intensity . These two often confused terms refer to different but related observations. Magnitude is usually abbreviated with Arabic numbers and characterizes the strength of an earthquake by indirectly measuring the energy released. In contrast, the intensity is usually indicated by Roman numerals and describes the local effects and the damage potential in relation to people, animals, buildings and natural objects such as bodies of water or mountains . An earthquake has a magnitude but many intensities, as the effects vary depending on external circumstances such as the distance from the epicenter , the structure of the soil and the construction of buildings. In practice, different magnitudes can be specified for an earthquake, as there are a number of different magnitude scales. These are based on different measurement methods, so that the magnitude values can sometimes differ significantly from one another.
Charles Francis Richter , the creator of the Richter scale , distinguished intensity and magnitude as follows:
“I like to use the analogy with radio broadcasts. It applies to seismology, because seismographs pick up elastic waves emitted by the earthquake source, just as radio receivers pick up radio waves emitted by a transmitter. Magnitude can be compared with the power in kilowatts that a transmitter emits. The local intensity according to the Mercalli scale is then comparable with the signal strength at the receiver at a certain location or with the signal quality. The intensity, like the signal strength in general, will decrease with the distance from the source, although like this it depends on the local conditions such as the propagation path between the earthquake source and the particular location. "
Intensity scales
The first simple classification of earthquakes by intensity was developed by Domenico Pignataro in the 1880s. The first intensity scale in today's sense can only be the scale designed by PNG Egen in 1828, which was ahead of its time with it. Only the Rossi-Forel scale , which was introduced in the late 19th century, found general recognition . Since then, numerous intensity scales have been developed. The scale used differs from country to country: the United States uses the Modified Mercalli Scale (MM), while the European Union uses the European Macroseismic Scale (EMS), Japan uses the JMA scale, and India , Israel and Russia the Medvedev-Sponheuer-Kárník scale (MSK-64). Almost all of these scales have 12 degrees of intensity, which roughly correspond in terms of values, but differ in terms of formulation and depth of detail.
Magnitude scales
The first attempt to describe earthquakes by a single absolute value used the term magnitude based on the magnitude scale that is used for the brightness of stars.
Local magnitude (Richter scale)
The local magnitude scale (M L ), also commonly known as the Richter scale , is a quantitative logarithmic scale. In the 1930s, California seismologist Charles F. Richter designed a simple numerical scale to describe the relative magnitude of earthquakes in Southern California . The term “Richter Scale” was coined by journalists and is not used in specialist literature. M L is determined by measuring the maximum deflection on a seismogram recorded on a Wood-Anderson torsion seismometer (or another type of seismometer calibrated on a Wood-Anderson torsion seismometer).
Other recent Magnitudenmessungen include the measurement of the Raumwellenmagnitude ( body-wave magnitude ) m B , the Oberflächenwellenmagnitude ( surface wave magnitude ) M S or decay or Codamagnitude ( duration magnitude ) M D . Each of these alternative magnitudes is scaled to give values similar to the local magnitude scale. However, because they are based on the measurement of different areas of the seismogram, they do not always reflect the overall strength of the quake. In particular, strong or very strong earthquakes can become saturated, which means that the strength of the earthquake is systematically underestimated. With the local magnitude scale, this problem occurs at a strength of around 6, while in the case of the surface wave magnitude it only begins at around 8. Despite the limitations of the older magnitude scales, they are still widely used:
- because they can be calculated quickly
- because catalogs of such magnitudes go back decades and are therefore easy to compare
- because they are sufficient for the majority of the observed seismic events
- because the public is familiar with it.
Moment magnitude
Because of the fundamental limitation of the magnitude scales 1977, a uniformly applicable magnitude scale of was Hiroo Kanamori designed the moment magnitude scale ( moment magnitude ) M W . Especially for strong and very strong earthquakes, the moment magnitude scale provides the most reliable information about the strength of an earthquake. This is related to the fact that it uses the concept of seismic moment, which is derived from the concept of moment in physics. It allows conclusions to be drawn about the size of an earthquake - in particular the size of the underlying fault fracture and the offset on the fault surface - as well as the physical energy that it releases. This concept makes it possible not only to derive the seismic moment from the seismogram, but also to calculate it backwards, so to speak, by recording geological details such as the size of the fault fracture and the offset.
The values for earthquakes registered so far span more than 15 magnitudes.
Individual evidence
- ↑ a b Magnitude and intensity of an earthquake. In: Prof. Dr. Peter Bormann, GeoForschungsZentrum Potsdam. March 19, 2007. Retrieved July 25, 2019 .