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Encyclopedia > Moment magnitude scale

The moment magnitude scale was introduced in 1979 by Thomas C. Hanks and Hiroo Kanamori as a successor to the Richter scale and is used by seismologists to compare the energy released by earthquakes.[1] The moment magnitude Mw is a dimensionless number defined by Hiroo Kanamori (金森 博雄 Kanamori Hiroo; October 17, 1936—) is a seismologist who has made fundamental contributions to understanding the physics of earthquakes and the tectonic processes that cause them. ... The Richter magnitude scale, or more correctly local magnitude ML scale, assigns a single number to quantify the amount of seismic energy released by an earthquake. ... Seismology (from the Greek seismos = earthquake and logos = word) is the scientific study of earthquakes and the movement of waves through the Earth. ... This article is about the natural seismic phenomenon. ... In dimensional analysis, a dimensionless number (or more precisely, a number with the dimensions of 1) is a pure number without any physical units. ...

M_mathrm{w} = {2 over 3}left(log_{10} frac{M_0}{mathrm{N}cdot mathrm{m}} - 9.1right) = {2 over 3}left(log_{10} frac{M_0}{mathrm{dyn}cdot mathrm{cm}} - 16.1right)

where M0 is the seismic moment (using one newton metre [N·m] as the reference moment).[citation needed] Seismic moment is a quantity used by earthquake seismologists to measure the size of an earthquake. ... Newton metre is the unit of moment (torque) in the SI system. ...


An increase of 1 step on this logarithmic scale corresponds to a 101.5 = 31.6 times increase in the amount of energy released, and an increase of 2 steps corresponds to a 10³ = 1000 times increase in energy. A logarithmic scale is a scale of measurement that uses the logarithm of a physical quantity instead of the quantity itself. ...


The constants in the equation are chosen so that estimates of moment magnitude roughly agree with estimates using other scales, such as the Local Magnitude scale, ML, commonly called the Richter magnitude scale. One advantage of the moment magnitude scale is that, unlike other magnitude scales, it does not saturate at the upper end. That is, there is no particular value beyond which all large earthquakes have about the same magnitude. For this reason, moment magnitude is now the most often used estimate of large earthquake magnitudes.[2] The symbol for the moment magnitude scale is Mw, with the subscript w meaning mechanical work accomplished. The USGS does not use this scale for earthquakes with a magnitude of less than 3.5. The Richter magnitude scale, or more correctly local magnitude ML scale, assigns a single number to quantify the amount of seismic energy released by an earthquake. ... In physics, mechanical work is the amount of energy transferred by a force. ... The United States Geological Survey (USGS) is a scientific agency of the United States government. ... This article is about the natural seismic phenomenon. ...

Contents

Radiated seismic energy

Potential energy is stored in the crust in the form of built-up stress. During an earthquake, this stored energy is transformed and results in Stress is a measure of force per unit area within a body. ...

  • cracks and deformation in rocks,
  • heat,
  • radiated seismic energy Es.

The seismic moment M0 is a measure of the total amount of energy that is transformed during an earthquake. Only a small fraction of the seismic moment M0 is converted into radiated seismic energy Es, which is what seismographs register. Using the estimate Seismographs (in Greek seismos = earthquake and graphein = write) are used by seismologists to record seismic waves. ...

E_mathrm{s} = M_0 cdot 10^{-4.8} = M_0 cdot 1.6times 10^{-5}

Choy and Boatwright defined in 1995 the energy magnitude

M_mathrm{e} = {2 over 3}log_{10} frac{E_mathrm{s}}{mathrm{N}cdot mathrm{m}} - 2.9

Nuclear explosions

The energy released by nuclear weapons is traditionally expressed in terms of the energy stored in a kiloton or megaton of the conventional explosive trinitrotoluene (TNT). The often quoted rule of thumb that a 1 kt TNT explosion is roughly equivalent to a magnitude 4 earthquake leads to the equation The mushroom cloud of the atomic bombing of Nagasaki, Japan, 1945, rose some 18 kilometers (11 mi) above the hypocenter A nuclear weapon derives its destructive force from nuclear reactions of fusion or fission. ... A megaton or megatonne is a unit of mass equal to 1,000,000 metric tons, i. ... A megaton or megatonne is a unit of mass equal to 1,000,000 metric tons, i. ... R-phrases S-phrases Related Compounds Related compounds picric acid hexanitrobenzene Except where noted otherwise, data are given for materials in their standard state (at 25 Â°C, 100 kPa) Infobox disclaimer and references Trinitrotoluene (TNT) is a chemical compound with the formula C6H2(NO2)3CH3. ... A rule of thumb is an easily learned and easily applied procedure for approximately calculating or recalling some value, or for making some determination. ...

M_mathrm{n} = {2 over 3}log_{10} frac{m_{mathrm{TNT}}}{mbox{kg}} = {2 over 3}log_{10} frac{m_{mathrm{TNT}}}{mbox{kt}} + 4 = {2 over 3}log_{10} frac{m_{mathrm{TNT}}}{mbox{Mt}} + 6.

where mTNT is the mass of the explosive TNT that is quoted for comparison.


Such comparison figures are not very meaningful. As with earthquakes, during an underground explosion of a nuclear weapon, only a small fraction of the total amount of energy transformed ends up being radiated as seismic waves. Therefore a seismic efficiency has to be chosen for a bomb that is quoted as a comparison. Using the conventional specific energy of TNT (4.184 MJ/kg), the above formula implies the assumption that about 0.5% of the bomb's energy is converted into radiated seismic energy Es. For real underground nuclear tests, the actual seismic efficiency achieved varies significantly and depends on the site and design parameters of the test. This article or section does not adequately cite its references or sources. ... The fuel value or relative energy density is the quantity of potential energy in fuel, food or other substance. ...


See also

‹ The template below has been proposed for deletion. ... The following is a list of major earthquakes. ... A seismic scale is used to measure and compare the relative severity of earthquakes. ...

External links

  • USGS: What is moment magnitude?

References

  1. ^ Hanks, Thomas C.; Kanamori, Hiroo (05/1979). "Moment magnitude scale". Journal of Geophysical Research 84 (B5): 2348-2350. Retrieved on 2007-10-06. 
  2. ^ Boyle, Alan (May 12, 2008). Quakes by the numbers. MSNBC. Retrieved on 2008-05-12. “That original scale has been tweaked through the decades, and nowadays calling it the "Richter scale" is an anachronism. The most common measure is known simply as the moment magnitude scale.”

Year 2007 (MMVII) was a common year starting on Monday of the Gregorian calendar in the 21st century. ... is the 279th day of the year (280th in leap years) in the Gregorian calendar. ... For the news website, see msnbc. ... 2008 (MMVIII) is the current year, a leap year that started on Tuesday of the Anno Domini (or common era), in accordance to the Gregorian calendar. ... is the 132nd day of the year (133rd in leap years) in the Gregorian calendar. ...

Sources

  • Hanks TC, Kanamori H (1979). "A moment magnitude scale". Journal of Geophysical Research 84 (B5): 2348-50. 
  • Choy GL, Boatwright JL (1995). "Global patterns of radiated seismic energy and apparent stress". Journal of Geophysical Research 100 (B9): 18205-28. 
  • Utsu,T., 2002, Relationships between magnitude scales, in: Lee, W.H.K, Kanamori, H., Jennings, P.C., and Kisslinger, C., editors, International Handbook of Earthquake and Engineering Seismology: Academic Press, a division of Elsevier, two volumes, International Geophysics, vol. 81-A, pages 733-746.. "{{{title}}}". 
Seismic scales
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Modern scales
Intensity scales
European Macroseismic Scale (EMS) | INQUA | Medvedev-Sponheuer-Karnik (MSK) | Modified Mercalli (MM) | Shindo
Magnitude scales
Local magnitude (Richter scale) | Moment magnitude
Historical scales
Mercalli-Cancani-Sieberg (MCS) | Mercalli-Wood-Neuman (MWN) | Omori | Rossi-Forel
Journal of Geophysical Research is a publication of the American Geophysical Union. ... Journal of Geophysical Research is a publication of the American Geophysical Union. ... The European Macroseismic Scale (EMS) is the basis for evaluation of seismic intensity in European countries. ... The Medvedev-Sponheuer-Karnik scale (MSK-64) is a macroseismic intensity scale used to measure the effects of earthquakes on humans, objects of nature, and structures. ... The Mercalli intensity scale is a scale used for measuring the intensity of an earthquake. ... The Japan Meteorological Agency seismic intensity scale (震度 shindo) is a measure used in Japan to indicate the strength of earthquakes. ... The Richter magnitude scale, or more correctly local magnitude ML scale, assigns a single number to quantify the amount of seismic energy released by an earthquake. ... The Mercalli intensity scale is a scale used for measuring the intensity of an earthquake. ... The Mercalli intensity scale is a scale used for measuring the intensity of an earthquake. ... The Rossi-Forel scale was one of the first seismic scales to reflect earthquake intensities. ...

  Results from FactBites:
 
Page 19 -- Measuring Earthquakes (545 words)
Seismic Moment: Not actually a magnitude scale, seismic moment is an estimate of the energy of an earthquake, and as such, is typically given in units of Newton-meters (Nm).
Moment Magnitude: The moment magnitude scale is a way of rating the seismic moment of an earthquake with a simple, logarithmic numerical scale similar to the original Richter magnitude scale.
Surface-wave Magnitude: Surface-wave magnitude is calculated using the amplitude, on a long-period vertical seismometer, of surface waves with a 20-second period.
  More results at FactBites »

 
 

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