Richter scale: Difference between revisions

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}}), also called the '''Richter magnitude scale''', '''Richter's magnitude scale''', and the '''Gutenberg–Richter scale''',<ref>{{cite book |last1=McPhee |first1=John |title=Annals of the Former World |date=1998 |publisher=Farrar, Straus and Giroux |page=608}}</ref> is a measure of the strength of [[earthquake]]s, developed by [[Charles Francis Richter]] in collaboration with [[Beno Gutenberg]], and presented in Richter's landmark 1935 paper, where he called it the "magnitude scale".<ref>{{Harvnb|Kanamori|1978|p=411}}; {{Harvnb|Richter|1935}}.</ref> This was later revised and renamed the '''local magnitude scale''', denoted as ML or {{M|L}}.<ref>{{Harvnb|Gutenberg|Richter|1956b|p=30}}.</ref>

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 logarithms[[common logarithm]]s simply to make the measurements manageable (i.e., a magnitude 3 quake factors 10³ while a magnitude 5 quake factors 10⁵ and has seismometer readings 100 times larger).<ref>{{Cite web|title=Discovery Project 17: Orders of Magnitude|url=https://www.stewartmath.com/precalc_7e_dp/precalc_7e_dp17.html|access-date=2022-02-24|website=www.stewartmath.com}}</ref>

Events with magnitudes greater than 4.5 are bla bla bal 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>

!Typical maximum [[Modified Mercalli intensity scale|Modifiedmodified Mercalli Intensityintensity]]<ref>{{cite web|title=Magnitude / Intensity Comparison|url=http://earthquake.usgs.gov/learn/topics/mag_vs_int.php|url-status=dead|archive-url=https://web.archive.org/web/20110623113247/http://earthquake.usgs.gov/learn/topics/mag_vs_int.php|archive-date=2011-06-23}}</ref>

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 [[Rossi-ForelRossi–Forel scale]]. ("Size" is used in the sense of the quantity of energy released, not the size of the area affected by shaking, though higher-energy earthquakes do tend to affect a wider area, depending on the local geology.) In 1883, [[John Milne]] surmised that the shaking of large earthquakes might generate waves detectable around the globe, and in 1899 E. Von Rehbur Paschvitz observed in Germany seismic waves attributable to an earthquake in [[Tokyo]].<ref>{{Harvnb|Bolt|1993|p=47}}.</ref> In the 1920s, [[Harry O. Wood]] and [[John August Anderson|John A. Anderson]] developed the [[Wood–Anderson seismograph]], one of the first practical instruments for recording seismic waves.<ref>{{Harvnb|Hough|2007|p=}};</ref> Wood then built, under the auspices of the [[California Institute of Technology]] and the [[Carnegie Institution for Science|Carnegie Institute]], a network of seismographs stretching across [[Southern California]].<ref>{{Harvnb|Hough|2007|p=57}}.</ref> He also recruited the young and unknown Charles Richter to measure the seismograms and locate the earthquakes generating the seismic waves.<ref>{{Harvnb|Hough|2007|pp=57, 116}}.</ref>

Additional developments were required to produce a practical method of assigning an absolute measure of magnitude . First, to span the wide range of possible values, Richter adopted Gutenberg's suggestion of a [[logarithm]]ic scale, where each step represents a tenfold increase of magnitude, similar to the magnitude scale used by astronomers [[Apparent magnitude|for star brightness]].<ref>[pending]</ref> Second, he wanted a magnitude of zero to be around the limit of human perceptibility.<ref>{{Harvnb|Richter|1935|p=14}}: {{Harvnb|Gutenberg|Richter|1936|p=183}}.</ref> Third, he specified the Wood–Anderson seismograph as the standard instrument for producing seismograms. Magnitude was then defined as "the logarithm of the maximum trace amplitude, expressed in [[microns]]", measured at a distance of {{Convert|100|km|mi|abbr=on}}. The scale was calibrated by defining a magnitude 0 shock as one that produces (at a distance of {{Convert|100|km|mi|abbr=on}}) a maximum amplitude of 1&nbsp;micron (1&nbsp;µmμm, or 0.001&nbsp;millimeters) on a seismogram recorded by a Wood-Anderson torsion seismometer.<ref>{{Harvnb|Richter|1935|p=5}}. See also {{Harvnb|Hutton|Boore|1987|p=1}}; {{Harvnb|Chung|Bernreuter|1980|p=10}}.</ref> Finally, Richter calculated a table of distance corrections,<ref>{{Harvnb|Richter|1935|p=6}}, Table I.</ref> in that for distances less than 200 kilometers<ref>{{Harvnb|Richter|1935|p=32}}.</ref> the attenuation is strongly affected by the structure and properties of the regional geology.<ref>{{Harvnb|Chung|Bernreuter|1980|p=5}}.</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_S) and [[body wave magnitude]] (M_B)<!-- These are non-standard: do not use the "M" template here. --> scales.<ref>{{Harvnb|Gutenberg|Richter|1956b|p=30}}.</ref>

: where <math>\ A\ </math> is the amplitude (maximum ground displacement) of the [[P-wave]], in [[micrometre|micrometers (µmμm)]], measured at 0.8&nbsp;Hz.

: where <math>\ A\ </math> is the amplitude in [[micrometre|µmμm]].