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Macroseismic intensities and magnitudes for earthquake scaling
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Workings on the systematic collection of ground motion effects triggered by earthquakes began in the 19th century. The degree of ground motion and its effects were classified in terms of intensity grades. The intensity scales developed for this purpose were improved and designed over the years to minimize effects caused by subjective errors. The macroseismic intensity I represents a classification of the magnitude of ground motion based on observed phenomena in a defined area, e.g. a town. Effects of ground motion on people, objects in houses as well as damages to buildings form the basis for the appraisal. Intensities are a robust measure of magnitude classification:
I not felt, II scarcely felt, III weak, IV largely observed, V strong, VI slighty damaging, VII damaging, VIII heavily damaging, IX destructive, X very destructive, XI devastating, XII completely devastating.
The following table provides a detailed description of the definitions of intensity based on the latest scale implementation. This scale is the European Macroseismic Scale EMS-98 (Grünthal, 1998) which is bindingly established in Europe and is also applied on every other continent, as shown in the following table.
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EMS Intensity
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Definition
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description of maximum effects (shortened)
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I
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not felt
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Not felt.
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II
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scarcely felt
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Felt sporadically only by persons at rest.
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III
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weak
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Felt by a few people indoors. Resting people feel a slight swinging or light trembling.
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IV
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largely observed
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Outside felt by few, indoors felt by many people. A few people are awakened. Glasses, windows and doors rattle.
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V
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strong
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Outside felt by few, indoors felt by most people. Many people are awakened. A few people are frightened. Whole buildings shake or rock. Hanging objects swing heavily, small objects maybe shifted. Doors and windows swing open and close.
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VI
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slighty damaging
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Many people become frightened and run outside. Some objects fall down. On houses, especially the ones in bad condition, slight damages arise, e.g. wall cracks or plaster falling down.
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VII
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damaging
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Most people become frightened and run outdoors. Furniture is shifted. Objects fall out from shelves in great numbers. Moderate damage on solidly built houses (small cracks in walls, plaster falling off, chimney parts falling down). Especially buildings in bad condition show larger cracks in walls and collapse of partition walls occurs.
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VIII
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heavily damaging
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Many people lose balance. Heavy damage to buildings of simple construction, i.e. gables and eaves mouldings are collapsing. Some whole buildings of very simple construction are collapsing.
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IX
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destructive
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General panic among afflicted people. Even well constructed usual buildings show very heavy damage and carrying constructional components are partially collapsing.
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X
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very destructive
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Many solidly built houses are destroyed or show serious damage.
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XI
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devastating
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Most buildings are destroyed, even the ones with an earthquake-safe construction.
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XII
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completely devastating
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Nearly every building and construction is destroyed.
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Table 1:Shortened form of the Macroseismic Scale of Intensity EMS-98. (This shortened form is very much simplified and as such not to be used to estimate the intensity of quakes)
The intensities are highest right above the hypocentres (maximum intensity, I0) and decrease with increasing distance from the epicentre depending on the depth of the quake's seismic centre. With the invention and increasingly widespread application of earthquake threat analysis, the macroseismic methods went through a time of revival, since the inclusion of data on historic earthquakes in seismically uneventful areas is of high significance for this purpose. The macroseismic method is the only possibility to classify the intensity of historic earthquakes. With regard to the parameterisation of earthquake hazard maps with an expressive parameter representing ground motion, the intensity also gained in significance. An additional modern field of application of macroseismics is the conversion of earthquake threat assessments to earthquake risk assessments, which can be directly realised based on the intensity. A both, classic and modern field of application of macroseismics, is the analysis and presentation of the spatial distribution of earthquakes' intensities. For a more detailed insight into the European Macroseismic Scale EMS-98 please visit http://seismohazard.gfz-potsdam.de/projects/en/ems/menue_ems_e.html. As a result of the inclusion of vulnerability data and clearly defined frequencies of specific degrees of damages occurring with particular intensities, the EMS-98 is increasingly used as a tool for the estimation of earthquake risk assessments, i.e. the assessment of expected monetary losses.
 Figure 1: Exemplary calculation of the Richter magnitude ML according to Bolt (1993).
The magnitude M is an instrumental measure of earthquake power and was invented by Charles Richter in 1953. The magnitude is determined by the logarithm of the maximum deflection of seismographs in consideration of the distance to the seismic focus. That way a magnitude of 4 corresponds to an earthquake which leads to a maximum deflection of 1cm on a seismogram recorded with a 2800-fold enlarging Wood-Anderson-seismograph in 100 km distance. This original definition by Richter is nowadays used for local earthquakes through the local earthquake magnitude ML. Figure 1 (after Bolt, 1993) shows a nomogram, which represents an exemplary calculation of magnitude ML=4.7 for an earthquake with a measured amplitude and a measured time difference between the P- and S-waves of a seismogram. Concerning the released energy, one unit of the magnitude scale is equivalent to a factor of 30. The magnitude scale has no upper and lower end. The smallest magnitudes of ML~-2 are given by the natural ground restlessness which is easily measured by the very sensitive seismographs. The upper magnitude boundary is determined by the geometry of fracture faults and the fracturing properties. With this it is to be considered that classic concepts of magnitude show different saturations tending towards high magnitudes. Only the instant magnitude is free of such saturations. The instant magnitude Mw is a physically based and calibrated measure of earthquake power based on a mechanic model of an abruptly activated fracture zone resulting from tension. The Chile earthquake in 1960 reached the largest yet determined instant magnitude of Mw=9.5. There are different scales for magnitudes in use resulting from different frequency characteristics of seismographs as well as different distances of earthquakes from the location of registration. Their calibration goes along with significant deviations. Additionally deviations in magnitude regulations of ±0.3 magnitude units result from the magnitude regulations at the various registration points with their different properties of the shallow and deeper underground. Thus, the errors of calculating the magnitudes are in the same order as those from intensity estimations. There exist empiric relationships between the intensity and the kinds of magnitude making it feasible to convert the different quantities into one another and also to classify historic earthquakes in terms of magnitudes.
Source:
The content of this page was originally published in:
G. Grünthal: Erdbeben und Erdbebengefährdung in Deutschland sowie im europäischen Kontext. Geographie und Schule 151 (2004), 14-23.
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