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Topic: Moment magnitude

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Moment magnitude scale

Seismic moment

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Great Chilean Earthquake

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1556 Shaanxi earthquake

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1994 Northridge earthquake

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Reseda, Los Angeles, California

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	SCSN - The Southern California Seismic Network
		Unlike other methods of earthquake magnitude, it is directly related to the size of rupture and the amount of slip on the rupture surface, and not based solely on empirical studies of the observed instrumental recordings.
		Local magnitude, as practiced today, is derived directly from Charles Richter's original magnitude scale: the magnitude is the logarithm (base 10) of the amplitude in microns on a Wood-Anderson torsion seismometer located 100 km from the earthquake (in other words, if the amplitude is 1.0 mm at this distance, the quake is a 3.0).
		Magnitude estimate is computed for each available horizontal component, and the mean or median of the values is taken as the earthquake magnitude.
	www.scsn.org /magnitude.html (1112 words)

	Moment magnitude scale - Wikipedia, the free encyclopedia
		The moment magnitude scale was introduced in 1979 by Tom Hanks and Hiroo Kanamori as a successor to the Richter scale and is used by seismologists to compare the energy released by earthquakes.
		The constants in the equation are chosen so that estimates of moment magnitude roughly agree with estimates using other scales such as 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.
	en.wikipedia.org /wiki/Moment_magnitude_scale (428 words)

	Moment (physics) - Wikipedia, the free encyclopedia
		In physics, the moment of force (often just moment, though there are other quantities of that name such as moment of inertia) is a quantity that represents the magnitude of force applied to a rotational system at a distance from the axis of rotation.
		The SI unit for moment is the newton meter (Nm).
		The principle of moments is derived from Archimedes' discovery of the operating principle of the lever.
	en.wikipedia.org /wiki/Moment_(physics) (394 words)

	Encyclopedia :: encyclopedia : Orders of magnitude (Site not responding. Last check: )
		An order of magnitude is the class of scale or magnitude of any amount, where each class contains values of a fixed ratio to the class preceding it.
		More precisely, the order of magnitude of a number can be defined in terms of the decimal logarithm, usually as the integer part of the logarithm.
		For example, an order of magnitude estimate for a variable between about 3 billion and 30 billion (such as the human population of the Earth) is 10 billion.
	www.hallencyclopedia.com /Orders_of_magnitude (621 words)

	Page 19 -- Measuring Earthquakes
		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.
	www.data.scec.org /Module/sec3pg19.html (545 words)

	Measurement
		Magnitude does not depend on population and effects to ground structures, but rather on wave amplitude and distance.
		These magnitude scales differ by the type of wave amplitude that is measured from the seismogram and the mathematical formula used to determine the magnitude.
		Using the seismic waves and field measurements that describe the fault area, the moment, a parameter related to the angular leverage of the forces that produce slip on a fault, can be measured.
	scign.jpl.nasa.gov /learn/eq8.htm (492 words)

	Earthquake Hazards Program: NEIC Earthquake Search
		MAGNITUDES: Magnitude, a logarithmic measure of the "size" of an earthquake, is related to the energy released as seismic waves at the focus of an earthquake.
		Surface-wave (Ms): Magnitudes are computed for earthquakes that are located at distances between 20 and 160 geocentric degrees from the recording station, seismic-wave period between 18 and 22 seconds, and depth is less than 50 km (generally Ms magnitudes are not computed for depths greater than 50 km.
		Moment Magnitude (Mw): The magnitude is computed from a long-period body- and mantle-wave moment tensor inversion method; it is also related to the product of the area of the earthquake fault, multiplied by the average fault slip over that area and by the shear modulus of the fault rocks.
	wwwneic.cr.usgs.gov /neis/epic/code_magnitude.html (923 words)

	Earthquake Size
		The magnitude is the most often cited measure of an earthquake's size, but it is not the only measure, and in fact, there are different types of earthquake magnitude.
		The body-wave magnitude for these two earthquakes is much closer because the rule for estimating body-wave magnitude is to use the maximum amplitude in the first five seconds of shaking.
		Usually we measure the moment directly from seismograms, since the size of the very long-period waves generated by an earthquake is proportional to the seismic moment.
	eqseis.geosc.psu.edu /~cammon/HTML/Classes/IntroQuakes/Notes/earthquake_size.html (1780 words)

	The San Andreas Fault - VIII. "Magnitude" and "Intensity"
		Magnitude is a measure of the size of an earthquake.
		An earthquake of magnitude 2 is the smallest earthquake normally felt by humans.
		On the moment magnitude scale, the San Francisco earthquake is estimated at magnitude 7.7 compared to an estimated Richter magnitude of 8.3.
	pubs.usgs.gov /gip/earthq3/magnitude.html (332 words)

	Wallace Creek Trail Guide \| Glossary
		All magnitude scales are calibrated to yield approximately the same value for any given earthquake; however, since the moment magnitude scale is not based on the same measurements as Richter (local or surface-wave) magnitudes, the different magnitudes do not always agree, particularly for very large quakes.
		Surface-wave magnitude is calculated using the amplitude, on a long-period vertical seismometer, of surface waves with a 20-second period.
		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.
	www.scec.org /wallacecreek/misc/defs.html (2649 words)

	ABAG On Shaky Ground - Magnitude (Site not responding. Last check: )
		Magnitude is a measure of overall earthquake size.
		Thus, one principal factor in determining shaking hazard is the magnitude of the earthquake.
		The moment magnitude is proportional to the area of the fault surface that has slipped.
	www.abag.ca.gov /bayarea/eqmaps/doc/big.html (185 words)

	UALR Arkansas Earthquake Center - The Moment Magnitude Scale - Modified Mercalli Scale, MM scale, measuring earthquake ...
		Moment magnitude is the measurement and term generally prefered by scientists and seismologists to the Richter scale because moment magnitude is more precise.
		Moment Magnitude is not based on instrumental recordings of a quake, but on the area of the fault that ruptured in the quake.
		Moment Magnitude is calculated in part by multiplying the area of the fault's rupture surface by the distance the earth moves along the fault
	quake.ualr.edu /public/moment.htm (140 words)

	Plate Tectonics - Discover Our Earth
		Modern seismologists prefer a magnitude scale that reflects the actual area of rupture along a fault caused by an earthquake, this is the moment magnitude.
		This moment magnitude scale is also logarithmic, and has been adjusted to look similar to the familiar Richter Scale.
		Earthquakes of magnitude 5 and above are those that cause damage to structures and loss of human life, and these are the quakes reported in the news.
	atlas.geo.cornell.edu /education/student/tectonics/e_magnitude_i.html (185 words)

	Richter Magnitude
		The uncertainty in an estimate of the magnitude is about plus or minus 0.3 units, and seismologists often revise magnitude estimates as they obtain and analyze additional data.
		Both the magnitude and the seismic moment are related to the amount of energy that is radiated by an earthquake.
		For this moment is in units of dyne-cm, and energy is in units of ergs.
	www.seismo.unr.edu /ftp/pub/louie/class/100/magnitude.html (1477 words)

	The Severity of an Earthquake — Infoplease.com
		Magnitude is related to the amount of seismic energy released at the hypocenter of the earthquake.
		The magnitude of an earthquake is determined from the logarithm of the amplitude of waves recorded by seismographs.
		Adjustments are included in the magnitude formula to compensate for the variation in the distance between the various seismographs and the epicenter of the earthquakes.
	www.infoplease.com /ipa/A0763386.html (1125 words)

	Magnitude - Voyager, the free encyclopedia (Site not responding. Last check: )
		In physics, the magnitude of a vector is a scalar in the physical sense, i.e.
		In astronomy, magnitude refers to the logarithmic measure of the brightness of an object, measured in a specific wavelength or passband, usually in optical or near-infrared wavelengths: see apparent magnitude and absolute magnitude.
		In seismology, the magnitude is a logarithmic measure of the energy released during an earthquake.
	www.voyager.in /Magnitude (216 words)

	Plate Tectonics
		The Richter magnitude is based on the size of the largest waves arriving at a particular type of instrument, the Wood Anderson seismograph.
		Until January, the USGS used the surface wave magnitude scale to determine the official magnitudes of earthquakes.
		So when the more accurate moment magnitude scale came along, San Francisco was downgraded to 7.8, and the Alaskan quake was boosted to 9.2.
	www.platetectonics.com /article.asp?a=74&c=1 (1347 words)

	Earthquakes!
		Different methods of measuring magnitude have superseded the Richter Scale, including surface-wave magnitude, which measures the seismic waves crackling around Earth's surface, and moment magnitude, the newest method, which is based on the size of the fault on which an earthquake occurs and the amount the Earth slips.
		Moment magnitude is the most uniformly applicable scale.
		It is the intensity of the 1906 San Francisco earthquake that is remembered, not its magnitude.
	www.factmonster.com /spot/earthquake1.html (527 words)

	International Forum: Development of a New Ankle-Foot Orthosis with Dorsiflexion Assist, Part 1: Desirable ...
		The springs anterior to the ankle joint generate a dorsiflexion assist moment in accordance with the degree of plantarflexion induced by the rotation of the uprights, and the posterior springs generate a plantarflexion assist moment in accordance with the degree of dorsiflexion.
		The magnitude of the dorsiflexion assist moment was selected from among those generated by the combinations of springs shown in Figure 4.
		The magnitude of the dorsiflexion assist moment and the initial ankle angle appropriate for each patient are influenced by many factors, including the patient's gender, degree of disability, muscle strength and weight.
	www.oandp.org /jpo/library/1997_04_174.asp (2766 words)

	So Long, Old Magnitude, Alaska Science Forum
		She went on to explain that Charles Richter's 1930s-era method of measuring earthquake magnitude was accurate for typical California seismic events because California's earthquakes usually are medium-sized and are caused by breaks in the comparatively shallow crustal rocks.
		The Richter magnitude can be calculated fairly quickly by measuring the extent of those waves as they are represented by the jagged lines on a seismogram, and normalizing the readings to a specific distance from the epicenter.
		Nowadays, the magnitude of the 1964 earthquake is generally given as 9.2, often followed by a Mw notation.
	www.gi.alaska.edu /ScienceForum/ASF11/1183.html (642 words)

	Details of the Duvall Earthquake Magnitude
		This is known as a body wave magnitude and is based on the amplitude or height of the initial signal (the P wave) on a seismogram.
		A magnitude of 5.2 was determined by Oregon State University a couple of hours later by analyzing the complete wave forms of the earthquake as recorded by broadband seismographs.
		This is known as a moment magnitude and is considered by seismologists the best type of magnitude to measure the relative amount of energy released by the earthquake.
	www.geophys.washington.edu /SEIS/EQ_Special/Duvall/duval.mag.html (343 words)

	FINAL EXAM REVIEW
		A magnitude scale corrects for distance from the focus and is thus a true measure of the earthquake's size.
		The moment is equal to the area of the fault times the amount the fault moves (slip) times the strength of the rock in which the fault is located.
		The moment is measured using all of the data in seismograms from the earthquake.
	www.trinity.edu /gkroeger/GEOS1307/final_exam_review.htm (2211 words)

	Reference.com/Encyclopedia/Richter magnitude scale
		The diminution of amplitude due to distance between the earthquake epicenter and the seismometer is corrected for by subtracting the logarithm of the expected amplitude of a magnitude 0 event at that distance.
		Richter arbitrarily chose a magnitude 0 event to be an earthquake that would show a maximum combined horizontal displacement of 1 micrometre on a seismogram recorded using a Wood-Anderson torsion seismometer 100 km from the earthquake epicenter.
		By the beginning of the 21st century, most seismologists considered the traditional magnitude scales to be largely obsolete, being replaced by a more physically meaningful measurement called the seismic moment which is more directly relatable to the physical parameters, such as the dimension of the earthquake rupture, and the energy released from the earthquake.
	www.reference.com /browse/wiki/Richter_scale (700 words)

	[No title]
		Earthquake ground shaking and damage are related to the size (magnitude) of the earthquake, the distance from the epicenter, the local geological conditions and the characteristics of buildings.
		Because fault geometry and observer azimuth are a part of the computation, moment is a more consistent measure of earthquake size than is magnitude, and more importantly, moment does not have an intrinsic upper bound.
		One whole unit of magnitude represents approximately 32 times (actually 101.5 times) the energy, based on a long-standing empirical formula that says log(E) is proportional to 1.5M, where E is energy and M is magnitude**.
	web.ics.purdue.edu /~braile/edumod/eqhazard/eqhazard1.htm (2681 words)

	Richter magnitude scale - Free net encyclopedia (Site not responding. Last check: )
		His motivation for creating the local magnitude scale was to separate the vastly larger number of smaller earthquakes from the few larger earthquakes observed in California at the time.
		His inspiration for the technique was the stellar magnitude scale used in astronomy to describe the brightness of stars and other celestial objects.
		The diminution of amplitude due to distance between the earthquake epicenter and the seismometer is corrected for by subtracting the common logarithm of the expected amplitude of a magnitude 0 event at that distance.
	www.netipedia.com /index.php/Richter_scale (829 words)

	Regional Moment Tensors as Discriminants?
		Patton (1988) examined long-period surface waveforms recorded at regional distances from the NTS Harzer explosion and concluded that the isotropic component of the moment tensor is not resolvable at long-periods due to the similarity in the basis Green’s functions, particularly for shallow source depth.
		We performed moment tensor inversions for cases in which the isotropic component was restrained to zero and where the 6-degree of freedom moment tensor is unrestrained.
		The moment tensor results are remarkable in the consistency of radiation patterns for the five nuclear explosions.
	www.seismo.berkeley.edu /~dreger/nukepage/nukemt.html (5708 words)

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