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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 (é‡‘æ£® åšé›„ Kanamori Hiroo; October 17, 1936â€”) 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

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. ...

â€¹ 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. ...

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.
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