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The Richter magnitude scale (often shortened to Richter scale) was developed to assign a single number to quantify the energy released during an earthquake.
The scale is a base-10 logarithmic scale. The magnitude is defined as the logarithm of the ratio of the amplitude of waves measured by a seismograph to an arbitrary small amplitude. An earthquake that measures 5.0 on the Richter scale has a shaking amplitude 10 times larger than one that measures 4.0, and corresponds to a 31.6 times larger release of energy.
Since the mid-20th century, the use of the Richter magnitude scale has largely been supplanted by the moment magnitude scale in many countries. However, the Richter scale is still widely used in Russia and other CIS countries. Also worth noting is that earthquake measurements under the moment magnitude scale in the United States—3.5 and up, on the MMS scale—are still usually erroneously referred to as being measured under the Richter scale in the general public, as well as the media, due to the familiarity with earthquakes being measured by the Richter scale instead of the MMS scale.
Developed in 1935 by Charles Francis Richter in partnership with Beno Gutenberg, both from the California Institute of Technology, the scale was firstly intended to be used only in a particular study area in California, and on seismograms recorded on a particular instrument, the Wood-Anderson torsion seismograph. Richter originally reported values to the nearest quarter of a unit, but values were later reported with one decimal place. His motivation for creating the local magnitude scale was to compare the size of different earthquakes. Richter, who since childhood had aspirations in astronomy, drew inspiration from the apparent magnitude scale used to account for the brightness of stars lost due to distance. Richter arbitrarily chose a magnitude 0 event to be an earthquake that would show a maximum combined horizontal displacement of 1 µm (0.00004 in) on a seismogram recorded using a Wood-Anderson torsion seismograph 100 km (62 mi) from the earthquake epicenter. This choice was intended to prevent negative magnitudes from being assigned. The smallest earthquakes that could be recorded and located at the time were around magnitude 3. However, the Richter scale has no lower limit, and sensitive modern seismographs now routinely record quakes with negative magnitudes.
ML (local magnitude) was not designed to be applied to data with distances to the hypocenter of the earthquake greater than 600 km (373 mi). For national and local seismological observatories the standard magnitude scale is today still ML. Unfortunately this scale saturates[clarification needed] at around ML = 7, because the high frequency waves recorded locally have wavelengths shorter than the rupture lengths[clarification needed] of large earthquakes.
To express the size of earthquakes around the globe, Gutenberg and Richter later developed a magnitude scale based on surface waves, surface wave magnitude Ms; and another based on body waves, body wave magnitude mb. These are types of waves that are recorded at teleseismic distances. The two scales were adjusted such that they were consistent with the ML scale. This succeeded better with the Ms scale than with the mb scale. Both of these scales saturate when the earthquake is bigger than magnitude 8 and therefore the moment magnitude scale, Mw, was invented.
These older magnitude scales have been superseded by methods for estimating the seismic moment, creating the moment magnitude scale, although the older scales are still widely used because they can be calculated quickly.
The Richter scale proper was defined in 1935 for particular circumstances and instruments; the instrument used saturated for strong earthquakes. The scale was replaced by the moment magnitude scale (MMS); for earthquakes adequately measured by the Richter scale, numerical values are approximately the same. Although values measured for earthquakes now are actually (MMS), they are frequently reported as Richter values, even for earthquakes of magnitude over 8, where the Richter scale becomes meaningless. Anything above 5 is classified as a risk by the USGS.
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 than a much more energetic deep earthquake in an isolated area.
There are several scales which have historically been described as the "Richter scale," especially the local magnitude and the surface wave scale. In addition, the body wave magnitude, , and the moment magnitude, , abbreviated MMS, have been widely used for decades, and a couple of new techniques to measure magnitude are in the development stage.
All magnitude scales have been designed to give numerically similar results. This goal has been achieved well for , , and . The 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 hypocentral depths, and for different earthquake sizes, the amplitudes of different types of elastic waves must be measured.
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 is most common, although is also reported frequently.
The seismic moment, , 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. is derived from it empirically as a quantity without units, just a number designed to conform to the scale. A spectral analysis is required to obtain , whereas the other magnitudes are derived from a simple measurement of the amplitude of a specifically defined wave.
All scales, except , saturate for large earthquakes, meaning they are based on the amplitudes of waves which have a wavelength shorter than the rupture length of the earthquakes. These short waves (high frequency waves) are too short a yardstick to measure the extent of the event. The resulting effective upper limit of measurement for is about 7 and about 8.5 for .
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, the other is based on a recently discovered channel wave.
The energy release of an earthquake, which closely correlates to its destructive power, scales with the 3⁄2 power of the shaking amplitude. Thus, a difference in magnitude of 1.0 is equivalent to a factor of 31.6 () in the energy released; a difference in magnitude of 2.0 is equivalent to a factor of 1000 ( ) in the energy released. The elastic energy radiated is best derived from an integration of the radiated spectrum, but one can base an estimate on because most energy is carried by the high frequency waves.
The Richter magnitude of an earthquake is determined from the logarithm of the amplitude of waves recorded by seismographs (adjustments are included to compensate for the variation in the distance between the various seismographs and the epicenter of the earthquake). The original formula is:
where A is the maximum excursion of the Wood-Anderson seismograph, the empirical function A0 depends only on the epicentral distance of the station, . In practice, readings from all observing stations are averaged after adjustment with station-specific corrections to obtain the ML value.
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 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.
The following describes the typical effects of earthquakes of various magnitudes near the epicenter. The values are typical only and should be taken with extreme caution, since intensity and thus ground effects depend not only on the magnitude, but also on the distance to the epicenter, the depth of the earthquake's focus beneath the epicenter, the location of the epicenter and geological conditions (certain terrains can amplify seismic signals).
|Magnitude||Description||Mercalli intensity||Average earthquake effects||Average frequency of occurrence (estimated)|
|Less than 2.0||Micro||I||Microearthquakes, not felt, or felt rarely by sensitive people. Recorded by seismographs.||Continual/several million per year|
|2.0–2.9||Minor||I to II||Felt slightly by some people. No damage to buildings.||Over one million per year|
|3.0–3.9||II to IV||Often felt by people, but very rarely causes damage. Shaking of indoor objects can be noticeable.||Over 100,000 per year|
|4.0–4.9||Light||IV to VI||Noticeable shaking of indoor objects and rattling noises. Felt by most people in the affected area. Slightly felt outside. Generally causes none to minimal damage. Moderate to significant damage very unlikely. Some objects may fall off shelves or be knocked over.||10,000 to 15,000 per year|
|5.0–5.9||Moderate||VI to VIII||Can cause damage of varying severity to poorly constructed buildings. At most, none to slight damage to all other buildings. Felt by everyone. Casualties range from none to a few.||1,000 to 1,500 per year|
|6.0–6.9||Strong||VII to X||Damage to many buildings 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 miles/kilometers from the epicenter. Damage can be caused far from the epicenter. Strong to violent shaking in epicentral area. Death toll ranges from none to 25,000.||100 to 150 per year|
|7.0–7.9||Major||VIII or greater||Causes damage to most buildings, some to partially or completely collapse or receive severe damage. Well-designed structures are likely to receive damage. Felt in enormous areas. Death toll ranges from none to 250,000.||10 to 20 per year|
|8.0–8.9||Great||Major damage to buildings, structures likely to be destroyed. Will cause moderate to heavy damage to sturdy or earthquake-resistant buildings. Damaging in large areas, some totally destroyed. Felt in extremely large regions. Death toll ranges from 100 to 1 million.||One per year|
|9.0 and greater||Near or at total destruction - severe damage or collapse to all buildings. Damage and shaking extends to distant locations. Permanent changes in ground topography. Death toll ranges from 1,000 to several million.||One per 10 to 50 years|
(Based on U.S. Geological Survey documents.)
The intensity and death toll depend on several factors (earthquake depth, epicenter location, population density, to name a few) and can vary widely.
Minor earthquakes occur every day and hour. On the other hand, great earthquakes occur once a year, on average. The largest recorded earthquake was the Great Chilean Earthquake of May 22, 1960, which had a magnitude of 9.5 on the moment magnitude scale. The larger the magnitude, the less frequent the earthquake happens.
The following table lists the approximate energy equivalents in terms of TNT explosive force – though note that the earthquake energy is released underground rather than overground. Most energy from an earthquake is not transmitted to and through the surface; instead, it dissipates into the crust and other subsurface structures. In contrast, a small atomic bomb blast (see nuclear weapon yield) will not simply cause light shaking of indoor items, since its energy is released above ground.
31.6227 to the power of 0 equals 1, 31.6227 to the power of 1 equals 31.6227 and 31.6227 to the power of 2 equals 1000. Therefore, an 8.0 on the Richter scale releases 31.6227 times more energy than a 7.0 and a 9.0 on the Richter scale releases 1000 times more energy than a 7.0. Thus,
|Approximate Magnitude||Approximate TNT for
Seismic Energy Yield
|0.0||15 g||63 kJ|
|0.2||30 g||130 kJ||Large hand grenade|
|0.5||85 g||360 kJ|
|1.0||480 g||2.0 MJ|
|1.2||1.1 kg||4.9 MJ||Single stick of dynamite [DynoMax Pro]|
|1.4||2.2 kg||9.8 MJ||Seismic impact of typical small construction blast|
|1.5||2.7 kg||11 MJ|
|2.0||15 kg||63 MJ|
|2.1||21 kg||89 MJ||West fertilizer plant explosion|
|2.5||85 kg||360 MJ|
|3.0||480 kg||2.0 GJ|
|3.5||2.7 metric tons||11 GJ||PEPCON fuel plant explosion, Henderson, Nevada, 1988|
|3.87||9.5 metric tons||40 GJ||Explosion at Chernobyl nuclear power plant, 1986|
|3.91||11 metric tons||46 GJ||Massive Ordnance Air Blast bomb|
|4.0||15 metric tons||63 GJ||Maine/New England, October 16, 2012|
|4.3||43 metric tons||180 GJ||Kent Earthquake (Britain), 2007
Eastern Kentucky earthquake, November 2012
|5.0||480 metric tons||2.0 TJ||Lincolnshire earthquake (UK), 2008
|5.5||2.7 kilotons||11 TJ||Little Skull Mtn. earthquake (Nevada, USA), 1992
|5.6||3.8 kilotons||16 TJ||Newcastle, Australia, 1989
|6.0||15 kilotons||63 TJ||Double Spring Flat earthquake (Nevada, USA), 1994|
|6.3||43 kilotons||180 TJ|| Rhodes earthquake (Greece), 2008
|6.4||60 kilotons||250 TJ||Kaohsiung earthquake (Taiwan), 2010|
|6.5||85 kilotons||360 TJ|| Caracas earthquake (Venezuela), 1967
|6.6||120 kilotons||500 TJ||San Fernando earthquake (California, USA), 1971|
|6.7||170 kilotons||710 TJ||Northridge earthquake (California, USA), 1994|
|6.8||240 kilotons||1.0 PJ|| Nisqually earthquake (Anderson Island, WA), 2001
|6.9||340 kilotons||1.4 PJ|| San Francisco Bay Area earthquake (California, USA), 1989
|7.0||480 kilotons||2.0 PJ|| Java earthquake (Indonesia), 2009
|7.1||680 kilotons||2.8 PJ|| Messina earthquake (Italy), 1908
|7.2||950 kilotons||4.0 PJ||Vrancea earthquake (Romania), 1977
|7.5||2.7 megatons||11 PJ|| Kashmir earthquake (Pakistan), 2005
|7.6||3.8 megatons||16 PJ|| Nicoya earthquake (Costa Rica), 2012
|7.7||5.4 megatons||22 PJ|| Sumatra earthquake (Indonesia), 2010
|7.8||7.6 megatons||32 PJ|| Tangshan earthquake (China), 1976
|7.9||10-15 megatons||42-63 PJ||Tunguska event
1802 Vrancea earthquake
|8.0||15 megatons||63 PJ|| Mino-Owari earthquake (Japan), 1891
San Juan earthquake (Argentina), 1894
|8.1||21 megatons||89 PJ||México City earthquake (Mexico), 1985
Guam earthquake, August 8, 1993
|8.35||50 megatons||210 PJ||Tsar Bomba - Largest thermonuclear weapon ever tested|
|8.5||85 megatons||360 PJ||Sumatra earthquake (Indonesia), 2007|
|8.6||-||-||Sumatra earthquake (Indonesia), 2012|
|8.7||170 megatons||710 PJ||Sumatra earthquake (Indonesia), 2005|
|8.75||200 megatons||840 PJ||Krakatoa 1883|
|8.8||240 megatons.||1.0 EJ||Chile earthquake, 2010,|
|9.0||480 megatons||2.0 EJ|| Lisbon earthquake (Portugal), All Saints Day, 1755
The Great Japan earthquake, March 2011
|9.15||800 megatons||3.3 EJ||Toba eruption 75,000 years ago; among the largest known volcanic events.|
|9.2||950 megatons||4.0 EJ|| Anchorage earthquake (Alaska, USA), 1964
Sumatra-Andaman earthquake and tsunami (Indonesia), 2004
|9.5||2.7 gigatons||11 EJ||Valdivia earthquake (Chile), 1960|
|10.0||15 gigatons||63 EJ||Never recorded, equivalent to an earthquake rupturing a very large, lengthy fault, or an extremely rare/impossible mega-earthquake, shown in science fiction[clarification needed]|
|12.55||100 teratons||420 ZJ||Yucatán Peninsula impact (creating Chicxulub crater) 65 Ma ago (108 megatons; over 4x1030 ergs = 400 ZJ).|
|22.88 or 32||310 yottatons||1.3×1039 J||Approximate magnitude of the starquake on the magnetar SGR 1806-20, registered on December 27, 2004.[clarification needed]|
These formulae are an alternative method to calculate Richter magnitude instead of using Richter correlation tables based on Richter standard seismic event (=0, A=0.001mm, D=100 km).
The Lillie empirical formula:
For distance less than 200 km:
For distance between 200 km and 600 km:
where A is seismograph signal amplitude in mm, D distance in km.
The Bisztricsany (1958) empirical formula for epicentral distances between 4˚ to 160˚:
The Tsumura empirical formula:
The Tsuboi, University of Tokio, empirical formula:
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