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{{Short description|Measure of an earthquake's strength}}
{{about|the earthquake magnitude scale introduced by Charles Richter in 1935|a review of earthquake magnitude scales|seismic magnitude scales|the musical scale used for tuning harmonicas|Richter tuning|the single by EPMD|Richter Scale (song)}}
{{use mdy dates|date=February 2013}}
{{Earthquakes}}
The '''Richter scale'''<ref>{{Harvnb|Kanamori|1978|p=411}}. {{Harvtxt|Hough|2007|pp=122–126}} discusses the name at some length.</ref> ({{IPAc-en|ˈ|r|ɪ|k|t|ər}}),
Because of various shortcomings of the original {{M|L}} scale, most seismological authorities now use other similar scales such as the [[moment magnitude scale]] ({{M|w}}) to report earthquake magnitudes, but much of the news media still erroneously refers to these as "Richter" magnitudes. All magnitude scales retain the [[logarithm]]ic character of the original and are scaled to have roughly comparable numeric values (typically in the middle of the scale). Due to the variance in earthquakes, it is essential to understand the Richter scale uses
==Richter magnitudes==
[[Image:Earthquake_severity.jpg|thumb| ]]
The Richter magnitude of an earthquake is determined from the [[logarithm]] of the [[amplitude]] of waves recorded by seismographs.
|chapter-url=http://www.johnmartin.com/earthquakes/eqsafs/safs_693.htm
|chapter=The Richter Scale ML
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|id=Professional Paper 1515
|page=177
|archive-date=April 25, 2016
|archive-url=https://web.archive.org/web/20160425121745/http://www.johnmartin.com/earthquakes/eqsafs/safs_693.htm
|url-status=dead
}}</ref>
:<math display="block">M_\mathrm{L} = \log_{10} A - \log_{10} A_\mathrm{0}(\delta) = \log_{10} [A / A_\mathrm{0}(\delta)],\ </math>
where {{mvar|A}} is the maximum excursion of the
Because of the logarithmic basis of the scale, each whole number increase in magnitude represents a tenfold increase in measured amplitude. In terms of energy, each whole number increase corresponds to an increase of about 31.6 times the amount of energy released, and each increase of 0.2 corresponds to approximately a doubling of the energy released.
Events with magnitudes greater than 4.5 are strong enough to be recorded by a seismograph anywhere in the world, so long as its sensors are not located in the earthquake's [[Shadow zone|shadow]].<ref>{{Cite journal|last=Brush|first=Stephen G.|date=September 1980|title=Discovery of the Earth's core|url=http://aapt.scitation.org/doi/10.1119/1.12026|journal=American Journal of Physics|language=en|volume=48|issue=9|pages=705–724|doi=10.1119/1.12026|issn=0002-9505}}</ref><ref>{{Cite book |title=A dictionary of earth sciences.|date=2008|author=Michael Allaby|isbn=978-0-19-921194-4|edition=3rd |location=Oxford|oclc=177509121}}</ref><ref>{{Cite journal|last=Einarsson|first=P.|date=September 1978|title=S-wave shadows in the Krafla Caldera in NE-Iceland, evidence for a magma chamber in the crust|url=http://dx.doi.org/10.1007/bf02597222|journal=Bulletin Volcanologique|volume=41|issue=3|pages=187–195|doi=10.1007/bf02597222|issn=0258-8900|hdl=20.500.11815/4200|hdl-access=free}}</ref>
The following describes the typical effects of earthquakes of various magnitudes near the epicenter.<ref name="GNSScience1">{{cite web | url=https://www.gns.cri.nz/Home/Learning/Science-Topics/Earthquakes/Monitoring-Earthquakes/Other-earthquake-questions/What-is-the-Richter-Magnitude-Scale | title=What is the Richter Magnitude Scale? | publisher=[[GNS Science]] | access-date=3 August 2021 | url-status=dead | archive-url=https://web.archive.org/web/20210803200647/https://www.gns.cri.nz/Home/Learning/Science-Topics/Earthquakes/Monitoring-Earthquakes/Other-earthquake-questions/What-is-the-Richter-Magnitude-Scale |archive-date=3 August 2021}}</ref> The values are typical and may not be exact in a future event because intensity and ground effects depend not only on the magnitude but also on (1) the distance to the epicenter, (2) the depth of the earthquake's focus beneath the epicenter, (3) the location of the epicenter, and (4) [[Seismic site effects|geological conditions]].
{| class="wikitable"
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!Magnitude
!Description
!Typical maximum [[Modified Mercalli intensity scale|
!Average earthquake effects
!Average frequency of occurrence globally (estimated)
|-
|style="background:lightskyblue;"|1.0–1.9
|[[Microearthquake|Micro]]
|I
|Microearthquakes, not felt
|Continual/several million per year
|-
|style="background:paleturquoise;"|2.0–2.9
|rowspan="
|I
|Felt slightly by some people. No damage to buildings.
|Over one million per year
|-
|style="background:palegreen;"|3.0–3.9
|rowspawn "1"|Slight
|II to III
|Often felt by people, but very rarely causes damage. Shaking of indoor objects can be noticeable.
|Over 100,000 per year
|-
|style="background:greenyellow;"|4.0–4.9
|Light
|IV to V
|Noticeable shaking of indoor objects and rattling noises. Felt by most people in the affected area. Slightly felt outside. Generally causes zero to minimal damage. Moderate to significant damage is very unlikely. Some objects may fall off shelves or be knocked over.
|10,000 to 15,000 per year
|-
|style="background:yellow;"|5.0–5.9
|Moderate
|VI to VII
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|1,000 to 1,500 per year
|-
|style="background:gold;"|6.0–6.9
|Strong
|VII to IX
|Damage to a moderate number of well-built structures in populated areas. [[Earthquake-resistant structures]] survive with slight to moderate damage. Poorly designed structures receive moderate to severe damage. Felt in wider areas; up to hundreds of kilometers from the epicenter. Strong to violent shaking in the epicentral area.
|100 to 150 per year
|-
|style="color:white; background:darkorange;"|7.0–7.9
|Major
|rowspan="3"| VIII or higher
|Causes damage to most buildings, some to partially or completely collapse or receive severe damage. Well-designed structures are likely to receive damage. Felt across great distances with major damage mostly limited to 250 km from the epicenter.
|10 to 20 per year
|-
|style="color:white; background:red;"|8.0–8.9
|rowspan="
|Major damage to buildings, and structures likely to be destroyed. Will cause moderate to heavy damage to sturdy or earthquake-resistant buildings. Damaging in large areas. Felt in extremely large regions.
|One per year
|-
|style="color:white; background:maroon;"|9.0–9.9
|rowspawn"1"|Extreme
|Near total destruction – severe damage or collapse to all buildings. Heavy damage and shaking extend to distant locations. Permanent changes in ground topography.
|One to three per century<ref name="McCaffrey2008">{{cite journal | title=Global frequency of magnitude 9 earthquakes | first=R. | last=McCaffrey | journal=Geology | date=2008 | volume=36 | issue=3 | pages=263–266 | doi=10.1130/G24402A.1}}</ref>
|}
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Millions of minor earthquakes occur every year worldwide, equating to hundreds every hour every day.<ref name="IRIS">{{cite web|title=How Often Do Earthquakes Occur|url=http://www.mgs.md.gov/seismic/education/no3.pdf}}</ref> On the other hand, earthquakes of magnitude ≥8.0 occur about once a year, on average.<ref name="IRIS" /> The largest recorded earthquake was the [[1960 Valdivia earthquake|Great Chilean earthquake]] of May 22, 1960, which had a magnitude of 9.5 on the [[moment magnitude scale]].<ref>{{cite web |url=https://earthquake.usgs.gov/regional/world/10_largest_world.php |title=Largest Earthquakes in the World Since 1900 |date=30 November 2012 |access-date=18 December 2013 |url-status=dead |archive-url=https://web.archive.org/web/20091007163455/http://earthquake.usgs.gov/regional/world/10_largest_world.php |archive-date=October 7, 2009 |df=mdy-all }}</ref>
Seismologist Susan Hough has suggested that a magnitude 10 quake may represent a very approximate upper limit for what the Earth's tectonic zones are capable of, which would be the result of the largest known continuous belt of faults rupturing together (along the Pacific coast of the Americas).<ref>{{cite book|last1=Silver|first1=Nate|title=The signal and the noise : the art and science of prediction|date=2013|publisher=Penguin|location=London|isbn=9780141975658}}</ref> A research at the [[Tohoku University]] in Japan found that a magnitude 10 earthquake was theoretically possible if a combined {{convert|3,000|km|mi}} of faults from the [[Japan Trench]] to the [[Kuril–Kamchatka Trench]] ruptured together and moved by {{convert|60|m|ft}} (or if a similar large-scale rupture occurred elsewhere). Such an earthquake would cause ground motions for up to an hour, with tsunamis hitting shores while the ground is still shaking, and if this kind of earthquake occurred, it would probably be a 1-in-10,000
== Development ==
[[File:CharlesRichter.jpg|thumb|upright|[[Charles Francis Richter]] (circa 1970)]]
Prior to the development of the magnitude scale, the only measure of an earthquake's strength or "size" was a subjective assessment of the intensity of shaking observed near the [[epicenter]] of the earthquake, categorized by various [[seismic intensity scales]] such as the [[
In 1931, [[Kiyoo Wadati]] showed how he had measured, for several strong earthquakes in Japan, the amplitude of the shaking observed at various distances from the epicenter. He then plotted the logarithm of the amplitude against the distance and found a series of curves that showed a rough correlation with the estimated magnitudes of the earthquakes.<ref>{{Harvnb|Richter|1935|p=2}}.</ref> Richter resolved some difficulties with this method<ref>{{Harvnb|Richter|1935|pp=1–5}}.</ref> and then, using data collected by his colleague [[Beno Gutenberg]], he produced similar curves, confirming that they could be used to compare the relative magnitudes of different earthquakes.<ref>{{Harvnb|Richter|1935|pp=2–3}}.</ref>
When Richter presented the resulting scale in 1935, he called it (at the suggestion of Harry Wood) simply a "magnitude" scale.<ref>{{Harvnb|Richter|1935|p=1}}. His article is titled: "An Instrumental Earthquake Magnitude Scale".</ref> "Richter magnitude" appears to have originated when [[Perry Byerly]] told the press that the scale was Richter's and "should be referred to as such."<ref>{{Harvnb|Hough|2007|pp=123–124}}.</ref> In 1956, Gutenberg and Richter, while still referring to "magnitude scale", labelled it "local magnitude", with the symbol {{M|L}}, to distinguish it from two other scales they had developed, the [[surface wave magnitude]] (M<sub>S</sub>) and [[body wave magnitude]] (M<sub>B</sub>)<!-- These are non-standard: do not use the "M" template here. --> scales.<ref>{{Harvnb|Gutenberg|Richter|1956b|p=30}}.</ref>
==Details==
[[File:How-the-Richter-Magnitude-Scale-is-determined.jpg|thumb|
How Richter Magnitude Scale is determined - the larger the value on the log graph, the higher the damage caused.]]
The Richter scale was defined in 1935 for particular circumstances and instruments; the particular circumstances refer to it being defined for Southern California and "implicitly incorporates the [[attenuation|attenuative]] properties of Southern California crust and mantle."<ref>{{cite web |url=https://earthquake.usgs.gov/earthquakes/eqarchives/mineblast/definitions.php |title=Explanation of Bulletin Listings, USGS}}</ref> The particular instrument used would become saturated by strong earthquakes and unable to record high values. The scale was replaced in the 1970s by the [[moment magnitude scale]] (MMS, symbol {{M|w}}); for earthquakes adequately measured by the Richter scale, numerical values are approximately the same. Although values measured for earthquakes now are {{M|w}}, they are frequently reported by the press as Richter values, even for earthquakes of magnitude over 8, when the Richter scale becomes meaningless.
The Richter and MMS scales measure the energy released by an earthquake; another scale, the [[Mercalli intensity scale]], classifies earthquakes by their ''effects'', from detectable by instruments but not noticeable, to catastrophic. The energy and effects are not necessarily strongly correlated; a shallow earthquake in a populated area with soil of certain types can be far more intense in
Several scales have
All magnitude scales have been designed to give numerically similar results. This goal has been achieved well for {{M|L}}, {{M|s}}, and {{M|w}}.<ref>{{Harvnb|Richter|1935}}.</ref><ref>Richter, C.F., "Elementary Seismology", ed, Vol., W. H. Freeman and Co., San Francisco, 1956.</ref> The {{M|b}} scale gives somewhat different values than the other scales. The reason for so many different ways to measure the same thing is that at different distances, for different [[hypocenter|hypocentral]] depths, and for different earthquake sizes, the amplitudes of different types of elastic waves must be measured.
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{{M|L}} is the scale used for the majority of earthquakes reported (tens of thousands) by local and regional seismological observatories. For large earthquakes worldwide, the moment magnitude scale (MMS) is most common, although {{M|s}} is also reported frequently.
The [[seismic moment]], '''''{{M|0}}''''', is proportional to the area of the rupture times the average slip that took place in the earthquake, thus it measures the physical size of the event. {{M|w}} is derived from it empirically as a quantity without units, just a number designed to conform to the {{M|s}} scale.<ref>{{cite journal | last1 = Hanks | first1 = T. C. | last2 = Kanamori | first2 = H. | year = 1979 | title = Moment magnitude scale | journal = Journal of Geophysical Research | volume = 84 | issue = B5| page = 2348 | doi=10.1029/jb084ib05p02348 | bibcode = 1979JGR....84.2348H}}</ref> A spectral analysis is required to obtain {{M|0}}
All scales, except {{M|w}}, saturate for large earthquakes, meaning they are based on the amplitudes of waves
New techniques to avoid the saturation problem and to measure magnitudes rapidly for very large earthquakes are being developed. One of these is based on the long-period P-wave;<ref>Di Giacomo, D., Parolai, S., Saul, J., Grosser, H., Bormann, P., Wang, R. & Zschau, J., 2008. "Rapid determination of the energy magnitude Me," in ''European Seismological Commission 31st General Assembly,'' Hersonissos.</ref>
The [[energy]] release of an earthquake,<ref>{{cite journal | last1 = Vassiliou | first1 = Marius | last2 = Kanamori | first2 = Hiroo | year = 1982 | title = The Energy Release in Earthquakes | journal = Bull. Seismol. Soc. Am. | volume = 72 | pages = 371–387 }}</ref> which closely correlates to its destructive power, scales with the {{frac|3|2}} power of the shaking amplitude (see [[
==Magnitude empirical formulae==
These formulae for Richter magnitude <math>\ M_\mathsf{
:<math>\ M_\
: where <math>\ A\ </math> is the amplitude (maximum ground displacement) of the [[P-wave]], in [[micrometre|micrometers (μm)]], measured at 0.8 Hz.
:<math>\ M_\mathsf{L} = \log_{10} A + 1.6\ \log_{10} D - 0.15\ ,</math>
: where
:: <math>\ A\ </math> is [[seismograph]] signal amplitude in [[millimetre|mm]] and
:: <math>\ D\ </math> is in [[kilometre|km]], for distances under 200 km .
and
: <math>\ M_\mathsf{L} = \log_{10} A + 3.0\ \log_{10} D - 3.38\ ;</math>
: where <math>\ D\ </math> is in [[kilometre|km]], for distances between 200 km and 600 km .
The '''Bisztricsany empirical formula''' (1958) for epicentre distances between 4° and 160° is:<ref name=al-Arifi>{{cite journal |last1=al-Arifi |first1=Nassir S. |last2=al-Humidan |first2=Saad |date=July 2012 |title=Local and regional earthquake magnitude calibration of Tabuk analog sub-network, Northwest of Saudi Arabia |journal=Journal of King Saud University – Science |volume=24 |issue=3 |pages=257–263 |doi=10.1016/j.jksus.2011.04.001 |doi-access=free}}</ref>
: <math>\ M_\
: where
:: <math>\ \tau\ </math> is the duration of the surface wave in seconds, and
:: <math>\ M_\mathsf{L}\ </math> is mainly between 5 and 8.
: <math>\ M_\
: where
:: <math>\ F - P\ </math> is the total duration of oscillation in seconds.
:: <math>\ M_\mathsf{L}\ </math> mainly takes on values between 3 and 5.
:<math>\ M_\
: where <math>
==See also==
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* [[1935 in science]]
* [[Seismic intensity scales]]
* [[Seismic magnitude scales]]
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{{div col|colwidth=30em}} {{refbegin}}
*{{Citation |last1=Bolt |first1=B. A. |year=1993 |title=Earthquakes and geological discovery |publisher=Scientific American Library |isbn=0-7167-5040-6 |url-access=registration |url=https://archive.org/details/earthquakesgeolo0000bolt}}.
*{{Citation |first1=D. M. |last1=Boore |date=September 1989 |title=The Richter scale: its development and use for determining earthquake source parameter |journal=Tectonophysics |volume=166 |issue=1–3 |pages=1–14 |doi=10.1016/0040-1951(89)90200-x |url=http://w.daveboore.com/pubs_online/richter_scale_tectonophysics_1989.pdf}}
*{{Citation |first1=D. H. |last1=Chung |first2=D. L. |last2=Bernreuter |date=1980 |title=Regional Relationships Among Earthquake Magnitude Scales. |url=https://www.osti.gov/scitech/servlets/purl/5073993/}}, NUREG/CR-1457.
*{{Citation |first1=B. |last1=Gutenberg |first2=C. F. |last2=Richter |date=21 February 1936 |title=Discussion: Magnitude and energy of earthquakes |journal=Science |volume=83 |issue=2147 |pages=183–185 |doi=10.1126/science.83.2147.183 |url=https://www.science.org/doi/pdf/10.1126/science.83.2147.183 |pmid=17770563 |bibcode=1936Sci....83..183G}}.
*{{Citation |last1=Gutenberg |first1=B. |last2=Richter |first2=C. F. |year=1956b |title=Earthquake magnitude, intensity, energy, and acceleration (Second Paper) |journal=Bulletin of the Seismological Society of America |volume=46 |issue=2 |pages=105–145}}.
*{{Citation |last=Hough |first=S. E. |year=2007 |title=Richter's scale: measure of an earthquake, measure of a man |publisher=Princeton University Press |isbn=978-0-691-12807-8 |url=https://books.google.com/books?id=rvmDeAxEiO8C}}.
*{{Citation |first1=L. K. |last1=Hutton |first2=David M. |last2=Boore |date=December 1987 |title=The ''M''<sub>L</sub> scale in Southern California |journal=Nature |volume=271 |pages=411–414 |doi=10.1038/271411a0 |url=http://gps-prod-storage.cloud.caltech.edu.s3.amazonaws.com/people_personal_assets/kanamori/HKnat78.pdf |bibcode=1978Natur.271..411K}}.
*{{Citation |first1=Hiroo |last1=Kanamori |date=February 2, 1978 |title=Quantification of Earthquakes |journal=Nature |volume=271 |issue=5644 |pages=411–414 |doi=10.1038/271411a0 |url=http://gps-prod-storage.cloud.caltech.edu.s3.amazonaws.com/people_personal_assets/kanamori/HKnat78.pdf |bibcode=1978Natur.271..411K}}.
*{{Citation |first1=C. F. |last1=Richter |author-link=Charles F. Richter |date=January 1935 |title=An Instrumental Earthquake Magnitude Scale |journal=Bulletin of the Seismological Society of America |volume=25 |issue=1 |pages=1–32 |url=http://authors.library.caltech.edu/47921/1/1.full%20(1).pdf |access-date=March 14, 2018 |archive-date=July 10, 2018 |archive-url=https://web.archive.org/web/20180710125050/https://authors.library.caltech.edu/47921/1/1.full%20(1).pdf |url-status=dead}}.
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