## Moment magnitude scale |

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The **moment magnitude scale** (**MMS**; denoted as ** M_{w}** or

The scale was developed in the 1970s to succeed the 1930s-era *M*_{L}). Even though the formulas are different, the new scale retains a continuum of magnitude values similar to that defined by the older one. Under suitable assumptions, as with the Richter magnitude scale, an increase of one step on this ^{1.5} (about 32) times increase in the amount of energy released, and an increase of two steps corresponds to a 10^{3} (1,000) times increase in energy. Thus, an earthquake of *M*_{w} of 7.0 releases about 32 times as much energy as one of 6.0 and nearly 1,000 times that of 5.0.

The moment magnitude is based on the ^{[2]}

Since January 2002, the MMS has been the scale used by the ^{[3]}

Popular press reports of earthquake magnitude usually fail to distinguish between magnitude scales, and are often reported as "Richter magnitudes" when the reported magnitude is a moment magnitude (or a surface-wave or body-wave magnitude). Because the scales are intended to report the same results within their applicable conditions, the confusion is minor.^{[citation needed]}

- history
- definition
- relations between seismic moment, potential energy released and radiated energy
- comparative energy released by two earthquakes
- comparison with richter scale
- subtypes of m
_{w} - see also
- notes
- sources
- external links

In 1935, _{L}) scale (popularly known as the ^{[3]} Because of this, there is an upper limit on the highest measurable magnitude, and all large earthquakes will tend to have a local magnitude of around 7.^{[4]} Further, the magnitude becomes unreliable for measurements taken at a distance of more than about 600 kilometres (370 mi) from the epicenter. Since this M_{L} scale was simple to use and corresponded well with the damage which was observed, it was extremely useful for engineering earthquake-resistant structures, and gained common acceptance.^{[5]}

The Richter scale was not effective for characterizing some classes of quakes. As a result, _{s}). Gutenberg also combined compressional _{s}).^{[5]}

The Richter scale, as modified, was successfully applied to characterize localities. This enabled local building codes to establish standards for buildings which were earthquake resistant. However a series of quakes were poorly handled by the modified Richter scale. This series of "great earthquakes", included faults that broke along a line of up to 1000 km. Examples include the _{s} scale was unable to characterize these "great earthquakes" accurately.^{[5]}

The difficulties with use of M_{s} in characterizing the quake resulted from the size of these earthquakes. Great quakes produced 20 s waves such that M_{s} was comparable to normal quakes, but also produced very long period waves (more than 200 s) which carried large amounts of energy. As a result, use of the modified Richter scale methodology to estimate earthquake energy was deficient at high energies.^{[5]}

The concept of ^{[6]} by _{w} scale was designed. The reason for the delay was that the necessary spectra of seismic signals had to be derived by hand at first, which required personal attention to every event. Faster computers than those available in the 1960s were necessary and seismologists had to develop methods to process earthquake signals automatically. In the mid-1970s Dziewonski^{[7]} started the ^{[8]} After this advance, it was possible to introduce M_{w} and estimate it for large numbers of earthquakes.
Hence the moment magnitude scale represented a major step forward in characterizing earthquakes.^{[9]}

Most earthquake magnitude scales suffered from the fact that they only provided a comparison of the amplitude of waves produced at a standard distance and frequency band; it was difficult to relate these magnitudes to a physical property of the earthquake. Gutenberg and Richter suggested that radiated energy E_{s} could be estimated as

(in Joules). Unfortunately, the duration of many very large earthquakes was longer than 20 seconds, the period of the surface waves used in the measurement of M_{s}. This meant that giant earthquakes such as the 1960 Chilean earthquake (M 9.5) were only assigned an M_{s} 8.2. ^{[10]} recognized this deficiency and he took the simple, but important, step of defining a magnitude based on estimates of radiated energy, M_{w}, where the "w" stood for work (energy):

Kanamori recognized that measurement of radiated energy is technically difficult since it involves integration of wave energy over the entire frequency band. To simplify this calculation, he noted that the lowest frequency parts of the spectrum can often be used to estimate the rest of the spectrum. The lowest frequency *M*_{0}. Using an approximate relation between radiated energy and seismic moment (which assumes stress drop is complete and ignores fracture energy),

(where **E** is in Joules and *M*_{0} is in N-m), Kanamori approximated M_{w} by

The formula above made it much easier to estimate the energy-based magnitude M_{w}, but it changed the fundamental nature of the scale into a moment magnitude scale.
_{w} scale was very similar to a relationship between M_{L} and *M*_{0} that was reported by Thatcher & Hanks (1973)

Hanks & Kanamori (1979) combined their work to define a new magnitude scale based on estimates of seismic moment

Although the formal definition of moment magnitude is given by this paper and is designated by **M**, it has been common for many authors to refer to M_{w} as moment magnitude. In most of these cases, they are actually referring to moment magnitude **M** as defined above.

Moment magnitude is now the most common measure of earthquake size for medium to large earthquake magnitudes,^{[11]} but in practice seismic moment, the seismological parameter it is based on, is not measured routinely for smaller quakes. For example, the

Current practice in official earthquake reports is to adopt moment magnitude as the preferred magnitude, i.e. M_{w} is the official magnitude reported whenever it can be computed. Because seismic moment (*M*_{0}, the quantity needed to compute M_{w}) is not measured if the earthquake is too small, the reported magnitude for earthquakes smaller than M 4 is often Richter's M_{L}.

Popular press reports most often deal with significant earthquakes larger than M ~ 4. For these events, the official magnitude is the moment magnitude M_{w}, not Richter's local magnitude M_{L}.

Other Languages

Afrikaans: Moment-magnitude-skaal

العربية: مقياس درجة العزم

čeština: Momentová škála

Cymraeg: Graddfa maint moment

dansk: Momentmagnitude-skalaen

Deutsch: Momenten-Magnituden-Skala

Esperanto: Momant-magnituda skalo

euskara: Momentu magnitude eskala

فارسی: مقیاس بزرگای گشتاوری

français: Échelle de magnitude de moment

한국어: 모멘트 규모

Հայերեն: Երկրաշարժի մագնիտուդ

हिन्दी: आघूर्ण परिमाण मापक्रम

Bahasa Indonesia: Skala kekuatan momen

italiano: Scala di magnitudo del momento sismico

עברית: סולם מגניטודה לפי מומנט

ქართული: მაგნიტუდა

Kreyòl ayisyen: Echèl valè MMS

македонски: Скала на моментна магнитуда

მარგალური: მაგნიტუდა

Bahasa Melayu: Skala magnitud momen

Nederlands: Momentmagnitudeschaal

日本語: モーメント・マグニチュード

norsk: Momentmagnitude

norsk nynorsk: Momentmagnitude

polski: Magnituda

português: Escala de magnitude de momento

русский: Магнитуда землетрясения

Scots: Moment magnitude scale

Simple English: Moment magnitude scale

slovenčina: Momentové magnitúdo

slovenščina: Momentna magnitudna lestvica

suomi: Momenttimagnitudi

svenska: Momentmagnitudskalan

தமிழ்: உந்தத்திறன் ஒப்பளவு

ไทย: มาตราขนาดโมเมนต์

Türkçe: Moment magnitüd ölçeği

українська: Магнітуда землетрусу

Tiếng Việt: Thang độ lớn mô men

中文: 矩震級