Richter scale: Difference between revisions
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|[[1755 Lisbon earthquake|Lisbon Earthquake (Portugal), All Saints Day, 1755]]</br>[[2011 Sendai earthquake and tsunami|Sendai earthquake (Japan), 2011]]<ref>[http://www.dailymail.co.uk/news/article-1365229/Japan-earthquake-tsunami-Fears-massive-death-toll.html Fires light up Japanese night after mega-quake and huge tsunami which wiped out towns, killed over 1,000 and cut power to millions] Daily Mail Saturday, Mar 12 2011</ref> |
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Revision as of 16:38, 12 March 2011
The Richter magnitude scale, also known as the local magnitude () scale, assigns a single number to quantify the amount of seismic energy released by an earthquake. It is a base-10 logarithmic scale obtained by calculating the logarithm of the combined horizontal amplitude (shaking amplitude) of the largest displacement from zero on a particular type of seismometer (Wood–Anderson torsion). So, for example, an earthquake that measures 5.0 on the Richter scale has a shaking amplitude 10 times larger than one that measures 4.0. The effective upper limit of measurement for local magnitude is just below 9 for local magnitudes and just below 10 for moment magnitude when applied to large earthquakes.[1]
The Richter scale has been superseded by the moment magnitude scale, which is calibrated to give generally similar values for medium-sized earthquakes (magnitudes between 3 and 7). Unlike the Richter scale, the moment magnitude scale reports a fundamental property of the earthquake derived from instrument data, rather than reporting instrument data which is not always comparable across earthquakes, and does not saturate in the high-magnitude range. Since the Moment Magnitude scale generally yields very similar results to the Richter scale, magnitudes of earthquakes reported in the mass media are usually reported without indicating which scale is being used.
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.[2]
Development
Developed in 1935 by Charles Richter in partnership with Beno Gutenberg, both of 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 seismometer. 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 separate the vastly larger number of smaller earthquakes from the few larger earthquakes observed in California at the time.
His inspiration was the apparent magnitude scale used in astronomy to describe the brightness of stars and other celestial objects. 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 seismograph recorded using a Wood-Anderson torsion seismometer 100 km (62 mi) from the earthquake epicenter. This choice was intended to prevent negative magnitudes from being assigned. However, the Richter scale has no actual lower limit, and sensitive modern seismographs now routinely record quakes with negative magnitudes.
Because ML was not designed to to be applied to data with distances to the hypocenter of the earthquake greater than 600 km, its values become unreliable when the earthquake is larger than 7 and Richter's original method is no longer applied.[3]
To overcome this shortcoming, 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.[4] MS and mb can still saturate when the earthquake is big enough.
These older magnitude scales have been superseded by the implementation of methods for estimating the seismic moment, creating the moment magnitude scale, although the former are still widely used because they can be calculated quickly.
Richter magnitudes
![](http://upload.wikimedia.org/wikipedia/commons/thumb/3/3f/RichterFreqEnergyGraph-en.svg/300px-RichterFreqEnergyGraph-en.svg.png)
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:[5]
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 of about 4.6 or greater are strong enough to be recorded by any of the seismographs in the world, given that the seismograph's sensors are not located in an 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, and geological conditions (certain terrains can amplify seismic signals).
Richter magnitudes | Description | Earthquake effects | Frequency of occurrence |
---|---|---|---|
Less than 2.0 | Micro | Micro earthquakes, not felt.[6] | About 8,000 per day |
2.0–2.9 | Minor | Generally not felt, but recorded. | About 1,000 per day |
3.0–3.9 | Often felt, but rarely causes damage. | 49,000 per year (est.) | |
4.0–4.9 | Light | Noticeable shaking of indoor items, rattling noises. Significant damage unlikely. | 6,200 per year (est.) |
5.0–5.9 | Moderate | Can cause major damage to poorly constructed buildings over small regions. At most slight damage to well-designed buildings. | 800 per year |
6.0–6.9 | Strong | Can be destructive in areas up to about 160 kilometres (100 mi) across in populated areas. | 120 per year |
7.0–7.9 | Major | Can cause serious damage over larger areas. | 18 per year |
8.0–8.9 | Great | Can cause serious damage in areas several hundred miles across. | 1 per year |
9.0–9.9 | Devastating in areas several thousand miles across. |
1 per 20 years | |
10.0+ | Epic[citation needed] | Never recorded; see below for equivalent seismic energy yield. |
Extremely rare (Unknown) |
(Based on U.S. Geological Survey documents.)[7]
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 (MW) of 9.5.[8]
Richter magnitudes examples
The following table lists the approximate energy equivalents in terms of TNT explosive force[9] – though note that the energy is that released underground (i.e. a small atomic bomb blast will not simply cause light shaking of indoor items) rather than the overground energy release. Most energy from an earthquake is not transmitted to and through the surface; instead, it dissipates into the crust and other subsurface structures.
See also
- Earthquake
- Seismic scale
- Seismite
- Moment magnitude scale
- Japan Meteorological Agency seismic intensity scale
- Order of magnitude
- Rohn Emergency Scale for measuring the magnitude (intensity) of any emergency
References
- ^ "Richter scale". Glossary. USGS. March 31, 2010.
- ^ USGS: The Richter Magnitude Scale
- ^ "USGS Earthquake Magnitude Policy". USGS. March 29, 2010.
- ^ William L. Ellsworth (1991). "SURFACE-WAVE MAGNITUDE (Ms) AND BODY-WAVE MAGNITUDE (mb)". USGS. Retrieved 2008-09-14.
{{cite journal}}
: Cite journal requires|journal=
(help) [dead link] - ^ Ellsworth, William L. (1991). "The Richter Scale ML, from The San Andreas Fault System, California (Professional Paper 1515)". USGS: c6, p177. Retrieved 2008-09-14.
{{cite journal}}
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(help); More than one of|author=
and|last=
specified (help) [dead link] - ^ This is what Richter thought. But recent evidence shows that earthquakes with negative magnitudes (down to −0.7) can also be felt, especially when the focus is very shallow (a few hundred metres). See: Thouvenot, F.; Bouchon, M. (2008). What is the lowest magnitude threshold at which an earthquake can be felt or heard, or objects thrown into the air?, in Fréchet, J., Meghraoui, M. & Stucchi, M. (eds), Modern Approaches in Solid Earth Sciences (vol. 2), Historical Seismology: Interdisciplinary Studies of Past and Recent Earthquakes, Springer, Dordrecht, 313–326.
- ^ USGS: FAQ- Measuring Earthquakes
- ^ USGS: List of World's Largest Earthquakes
- ^ FAQs – Measuring Earthquakes
- ^ "Magnitude 5.0 – Ontario-Quebec border region, Canada". earthquake.usgs.gov. Retrieved 2010-06-23.
- ^ "Moderate 5.0 earthquake shakes Toronto, Eastern Canada and U.S." nationalpost.com. Retrieved 2010-06-23.
- ^ "M8.1 South End of Island August 8, 1993". eeri.org. Retrieved 2011-03-11..
{{cite web}}
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(help) - ^ Petraglia, M.; R. Korisettar, N. Boivin, C. Clarkson,4 P. Ditchfield,5 S. Jones,6 J. Koshy,7 M.M. Lahr,8 C. Oppenheimer,9 D. Pyle,10 R. Roberts,11 J.-C. Schwenninger,12 L. Arnold,13 K. White. (6 July 2007). "Middle Paleolithic Assemblages from the Indian Subcontinent Before and After the Toba Super-eruption". Science 317 (5834): 114–116. doi:10.1126/science.1141564. PMID 17615356.
- ^ Bralower, Timothy J. (1998). "The Cretaceous-Tertiary boundary cocktail: Chicxulub impact triggers margin collapse and extensive sediment gravity flows" (PDF). Geology. 26: 331–334. doi:10.1130/0091-7613(1998)026<0331:TCTBCC>2.3.CO;2. ISSN 0091-7613. Retrieved 2009-09-03.
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ignored (|author=
suggested) (help) - ^ Klaus, Adam; Norris, Richard D.; Kroon, Dick; Smit, Jan (2000). "Impact-induced mass wasting at the K-T boundary: Blake Nose, western North Atlantic". Geology. 28: 319–322. doi:10.1130/0091-7613(2000)28<319:IMWATK>2.0.CO;2. ISSN 0091-7613.
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ignored (help) - ^ Busby, Cathy J. (2002). "Coastal landsliding and catastrophic sedimentation triggered by Cretaceous-Tertiary bolide impact: A Pacific margin example?". Geology. 30: 687–690. doi:10.1130/0091-7613(2002)030<0687:CLACST>2.0.CO;2. ISSN 0091-7613.
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suggested) (help) - ^ Simms, Michael J. (2003). "Uniquely extensive seismite from the latest Triassic of the United Kingdom: Evidence for bolide impact?". Geology. 31: 557–560. doi:10.1130/0091-7613(2003)031<0557:UESFTL>2.0.CO;2. ISSN 0091-7613.
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:|access-date=
requires|url=
(help) - ^ Simkin, Tom (2006). "This dynamic planet. World map of volcanoes, earthquakes, impact craters, and plate tectonics. Inset VI. Impacting extraterrestrials scar planetary surfaces" (PDF). U.S. Geological Survey. Retrieved 2009-09-03.
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suggested) (help) - ^ Phil Plait (2009). "Anniversary of a cosmic blast". discovermagazine.com. Retrieved 2010-11-26.