Richter magnitude scale
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 limit of measurement for local magnitude is about 6.8.
The Richter scale has been superseded by the moment magnitude scale, which is calibrated to give generally similar values for medium-sized earthquakes (magnitudes between 3 and 7).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 is built on sound seismological principles, and does not saturate in the high-magnitude range. Magnitudes are still widely stated on the Richter scale in the mass media, although usually moment magnitudes—numerically about the same—are actually given.
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.[1]
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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 is derived from measurements taken from a single, band-limited seismograph, its values saturate when the earthquake is larger than 6.8, and do not increase for more powerful earthquakes.[2] To overcome this shortcoming, Gutenberg and Richter later developed a magnitude scales based on surface waves, surface wave magnitude MS, and another based on body waves, body wave magnitude mb.[3] 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
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:[4]
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 | Microearthquakes, not felt. | 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 | Never recorded; see below for equivalent seismic energy yield. |
Extremely rare (Unknown) |
(Based on U.S. Geological Survey documents.)[5]
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.[6]
The following table lists the approximate energy equivalents in terms of TNT explosive force[7] – 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.
| Richter Approximate Magnitude |
Approximate TNT for Seismic Energy Yield |
Joule equivalent | Example |
|---|---|---|---|
| 0.0 | 15.0 g (0.529 oz) | 63.1 kJ | |
| 0.5 | 84.4 g (2.98 oz) | 355 kJ | Large hand grenade |
| 1.0 | 474 g (1.05 lb) | 2.00 MJ | Construction site blast |
| 1.5 | 2.67 kg (5.88 lb) | 11.2 MJ | World War II conventional bombs |
| 2.0 | 15.0 kg (33.1 lb) | 63.1 MJ | Late World War II conventional bombs |
| 2.5 | 84.4 kg (186 lb) | 355 MJ | World War II blockbuster bomb |
| 3.0 | 474 kg (1050 lb) | 2.00 GJ | Massive Ordnance Air Blast bomb |
| 3.5 | 2.67 metric tons | 11.2 GJ | Chernobyl nuclear disaster, 1986 |
| 4.0 | 15.0 metric tons | 63.1 GJ | Small atomic bomb |
| 4.5 | 84.4 metric tons | 355 GJ | |
| 5.0 | 474 metric tons | 2.00 TJ | Seismic yield of Nagasaki atomic bomb (Total yield including air yield 21 kT, 88 TJ) Lincolnshire earthquake (UK), 2008 2010 Central Canada earthquake[8][9] |
| 5.5 | 2.67 kilotons | 11.2 TJ | Little Skull Mtn. earthquake (Nevada, USA), 1992 Alum Rock earthquake (California, USA), 2007 2008 Chino Hills earthquake (Los Angeles, USA) |
| 6.0 | 15.0 kilotons | 63.1 TJ | Double Spring Flat earthquake (Nevada, USA), 1994 |
| 6.5 | 84.4 kilotons | 355 TJ | Caracas (Venezuela), 1967 Rhodes (Greece), 2008 Eureka Earthquake (Humboldt County, California, USA), 2010 Southeast of Taiwan (270 km), 2010 |
| 6.7 | 168 kilotons | 708 TJ | Northridge earthquake (California, USA), 1994 |
| 6.9 | 336 kilotons | 1.41 PJ | San Francisco Bay Area earthquake (California, USA), 1989 |
| 7.0 | 474 kilotons | 2.00 PJ | Java earthquake (Indonesia), 2009 2010 Haiti earthquake |
| 7.1 | 670 kilotons | 2.82 PJ | 1944 San Juan earthquake 2010 Canterbury earthquake (New Zealand) |
| 7.2 | 938 kilotons | 3.94 PJ | 1977 Vrancea earthquake |
| 7.5 | 2.67 megatons | 11.2 PJ | Kashmir earthquake (Pakistan), 2005 Antofagasta earthquake (Chile), 2007 |
| 7.8 | 7.52 megatons | 31.6 PJ | Tangshan earthquake (China), 1976 Hawke's Bay earthquake (New Zealand), 1931) April 2010 Sumatra earthquake (Indonesia) |
| 8.0 | 15.0 megatons | 63.1 PJ | San Francisco earthquake (California, USA), 1906 Queen Charlotte Islands earthquake (Britsh Columbia, Canada), 1949 México City earthquake (Mexico), 1985 Gujarat earthquake (India), 2001 Chincha Alta earthquake (Peru), 2007 Sichuan earthquake (China), 2008 1894 San Juan earthquake |
| 8.5 | 84.4 megatons | 355 PJ | Energy released is larger than that of the Tsar Bomba (50 megatons, 210 PJ), the largest thermonuclear weapon ever tested Toba eruption 75,000 years ago; among the largest known volcanic events.[10] Sumatra earthquake (Indonesia), 2007 |
| 8.8 | 238 megatons | 1.00 EJ | Chile earthquake, 2010 |
| 9.0 | 474 megatons | 2.00 EJ | Lisbon Earthquake (Lisbon, Portugal), All Saints Day, 1755 |
| 9.2 | 946 megatons | 3.98 EJ | Anchorage earthquake (Alaska, USA), 1964 |
| 9.3 | 1.34 gigatons | 5.62 EJ | Indian Ocean earthquake, 2004 |
| 9.5 | 2.67 gigatons | 11.2 EJ | Valdivia earthquake (Chile), 1960 |
| 10.0 | 15.0 gigatons | 63.1 EJ | Never recorded by humans |
| 12.55 | 100 teratons | 422 ZJ | Yucatán Peninsula impact (creating Chicxulub crater) 65 Ma ago (108 megatons; over 4x1030 ergs = 400 ZJ).[11][12][13][14][15] |
See also
- Earthquake
- Seismic scale
- Seismite
- Moment magnitude scale
- Japan Meteorological Agency seismic intensity scale
- Order of magnitude
References
- ↑ USGS: The Richter Magnitude Scale
- ↑ William L. Ellsworth (1991). The Richter Scale (ML). USGS. http://www.johnmartin.com/earthquakes/eqsafs/safs_693.htm. Retrieved 2008-09-14.
- ↑ William L. Ellsworth (1991). SURFACE-WAVE MAGNITUDE (Ms) AND BODY-WAVE MAGNITUDE (mb). USGS. http://www.johnmartin.com/earthquakes/eqsafs/safs_694.htm. Retrieved 2008-09-14.
- ↑ Ellsworth, William L. (1991). The Richter Scale ML, from The San Andreas Fault System, California (Professional Paper 1515). USGS. pp. c6, p177. http://www.johnmartin.com/earthquakes/eqsafs/safs_693.htm. Retrieved 2008-09-14.
- ↑ 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. http://earthquake.usgs.gov/earthquakes/recenteqsww/Quakes/us2010xwa7.php#details. Retrieved 2010-06-23.
- ↑ "Moderate 5.0 earthquake shakes Toronto, Eastern Canada and U.S.". nationalpost.com. http://news.nationalpost.com/2010/06/23/tremors-felt-in-toronto-ottawa-reports/. Retrieved 2010-06-23.
- ↑ 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.; Charles K. Paull; R. Mark Leckie (1998). "The Cretaceous-Tertiary boundary cocktail: Chicxulub impact triggers margin collapse and extensive sediment gravity flows". Geology 26: 331–334. doi:10.1130/0091-7613(1998)026<0331:TCTBCC>2.3.CO;2. ISSN 0091-7613. http://www.geosc.psu.edu/people/faculty/personalpages/tbralower/Braloweretal1998.pdf. Retrieved 2009-09-03.
- ↑ 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.
- ↑ Busby, Cathy J.; Grant Yip; Lars Blikra; Paul Renne (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.
- ↑ 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.
- ↑ Simkin, Tom; Robert I. Tilling; Peter R. Vogt; Stephen H. Kirby; Paul Kimberly; David B. Stewart (2006). "This dynamic planet. World map of volcanoes, earthquakes, impact craters, and plate tectonics. Inset VI. Impacting extraterrestrials scar planetary surfaces". U.S. Geological Survey. http://mineralsciences.si.edu/tdpmap/pdfs/impact.pdf. Retrieved 2009-09-03.
External links
- IRIS Real-time Seismic Monitor of the Earth
- USGS: magnitude and intensity comparison
- USGS: Earthquake Magnitude Policy
- USGS: 2000-2006 Earthquakes worldwide
- USGS: 1990-1999 Earthquakes worldwide
- Alaska Railroad Earthquake with a table of yield-to-magnitude relations.
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