- Seismic scale
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**seismic scale**is used to measure and compare the severity ofearthquake s. (For a quick review, see the table of seismic scales at the end of this article.)Two fundamentally different but equally important types of scales are commonly used by seismologists to describe earthquakes. The original force or energy of an earthquake is measured on a "magnitude scale", while the intensity of shaking occurring at any given point on the Earth's surface is measured on an "intensity scale".

**Magnitude and Intensity**The severity of an earthquake is described by both

**magnitude**and**intensity**. These two frequently-confused terms refer to different, but related, observations. "Magnitude," usually expressed as anArabic numeral , characterizes the size of an earthquake by measuring indirectly the energy released. By contrast, "intensity" indicates the local effects and potential for damage produced by an earthquake on the Earth's surface as it affects humans, animals, structures, and natural objects such as bodies of water. Intensities are usually expressed inRoman numerals , and represent the severity of the shaking resulting from an earthquake. Ideally, any given earthquake can be described by only one "magnitude", but many "intensities" since the earthquake effects vary with circumstances such as distance from theepicenter and local soil conditions. In practise, the same earthquake might have magnitude estimates typically differing by few tenths of a unit, depending on which magnitude scale is used and which data are included in the analysis.Charles Richter , the creator of theRichter magnitude scale , distinguished "intensity" and "magnitude" as follows: "I like to use the analogy withradio transmission s. It applies inseismology becauseseismograph s, or the receivers, record the waves of elastic disturbance, or radio waves, that are radiated from the earthquake source, or the broadcasting station. Magnitude can be compared to the power output inkilowatt s of a broadcasting station. Local intensity on the Mercalli scale is then comparable to the signal strength on a receiver at a given locality; in effect, the quality of the signal. Intensity, like signal strength, will generally fall off with distance from the source, although it also depends on the local conditions and the pathway from the source to the point."**eismic intensity scales**The first simple classification of earthquake intensity was devised by Domenico Pignataro in the 1780s. However, the first recognisable intensity scale in the modern sense of the word was drawn up by P.N.G. Egen in 1828; it was ahead of its time. The first widely adopted intensity scale, the

, was introduced in the late 19th century. Since then numerous intensity scales have been developed and are used in different parts of the world: the scale currently used in the United States is theRossi-Forel scale **Modified Mercalli scale (MM)**, while theis used in Europe, theEuropean Macroseismic Scale (EMS-94)**Shindo scale**is used inJapan , the**MSK-64 scale**is used inIndia ,Israel ,Russia and throughout the CIS, and the**Liedu scale**(GB/T 17742-1999) is used in mainland China;Hong Kong , on the other hand, uses the MM scale,cite web

url=http://www.weather.gov.hk/gts/equake/mag_and_int_e.htm

title=Magnitude and Intensity of an Earthquake

publisher=Hong Kong Observatory

accessdate=2008-09-15] and Taiwan uses the Shindo scale. Most of these scales have twelve degrees of intensity, which are roughly equivalent to one another in values but vary in the degree of sophistication employed in their formulation.**Magnitude scales**The first attempt to qualitatively define a single, absolute value to describe the size of earthquakes was the magnitude scale (the name being taking from similarly formulated scales used to represent the brightness of stars).

**Local magnitude scale and related scales**The

**local magnitude scale (M**, also popularly known as the_{L})**Richter Scale**, is a quantitative logarithmic scale. In the 1930s, California seismologistCharles F. Richter devised a simple numerical scale to describe the relative sizes of earthquakes in Southern California. The name "Richter Scale" was coined by journalists and is not generally used by seismologists in technical literature. M_{L}is obtained by measuring the maximum amplitude of a recording on a Wood-Anderson torsionseismometer (or one calibrated to it) at a distance of 600 km from the earthquake. Other more recent magnitude measurements include:(body wave magnitude **m**),_{b}(surface wave magnitude **M**), and_{s}**duration magnitude**(**M**). Each of these is scaled to give values similar to those given by the local magnitude scale; but because each is based on a measurement of one aspect of the_{D}seismogram , they do not always capture the overall power of the source. Specifically, some can be affected by**saturation**at higher magnitude values—meaning that they systematically underestimate the magnitude of larger events. This problem sets in at around magnitude 6 for local magnitude; surface-wave magnitude saturates above 8. Despite the limitations of older magnitude scales, they are still in wide use, as they can be calculated rapidly, catalogues of them dating back many years are available, they are sufficient for the vast majority of observed events, and the public is familiar with them.**Moment magnitude scale**Because of the limitations of the magnitude scales, a new, more uniformly applicable extension of them, known as

**moment magnitude (M**scale for representing the size of earthquakes, was introduced by_{W})Hiroo Kanamori in 1977. In particular, for very large earthquakes moment magnitude gives the most reliable estimate of earthquake size. This is because seismic moment is derived from the concept of moment in physics and therefore provides clues to the physical size of an earthquake—the size of fault rupture and accompanying slip displacement — as well as the amount of energy released. So while seismic moment, too, is calculated fromseismogram s, it can also be obtained by working backwards from geologic estimates of the size of the fault rupture and displacement. The values of moments for observed earthquakes range over more than 15 orders of magnitude, and because they are not influenced by variables such as local circumstances, the results obtained make it easy to objectively compare the sizes of different earthquakes.**Notes and References****ee also***

Earthquake

*Earthquake engineering

*Seismic performance

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