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Richter magnitude scale

The Richter magnitude scale, also known as the local magnitude (ML) 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 of the largest displacement from zero on a Wood–Anderson torsion seismometer output. 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 ML is about 6.8.

Though still widely used, the Richter scale has been superseded by the moment magnitude scale, which gives generally similar values.

The energy release of an earthquake, which closely correlates to its destructive power, scales with the 32 power of the shaking amplitude. Thus, a difference in magnitude of 1.0 is equivalent to a factor of 31.6 ( = (101.0)(3 / 2)) in the energy released; a difference of magnitude of 2.0 is equivalent to a factor of 1000 ( = (102.0)(3 / 2) ) in the energy released. [1]

Contents

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 decimal numbers were used later. 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 one micrometre on a seismograph recorded using a Wood-Anderson torsion seismometer 100 kilometres (62 mi) from the earthquake epicenter. This choice was intended to prevent negative magnitudes from being assigned. However, the Richter scale has no upper or 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.[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 traditional magnitude scales have been superseded by the implementation of methods for estimating the seismic moment and its associated moment magnitude scale, although 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]

ML = log10A − log10A0(delta),

where A is the maximum excursion of the Wood-Anderson seismograph, the empirical function A0 depends only on the epicentral distance of the station, delta. 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.

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. This table 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 here is that of the underground energy release (ie a small atomic bomb blast will not simply cause light shaking of indoor items) rather than the overground energy release; the majority of energy transmission of an earthquake is not transmitted to and through the surface, but is instead dissipated into the crust and other subsurface structures.

Richter
Approximate Magnitude
Approximate TNT for
Seismic Energy Yield
Joule equivalent Example
0.0 1 kg (2.2 lb) 4.2 MJ
0.5 5.6 kg (12.4 lb) 23.5 MJ Large hand grenade
1.0 32 kg (70 lb) 134.4 MJ Construction site blast
1.5 178 kg (392 lb) 747.6 MJ WWII conventional bombs
2.0 1 metric ton 4.2 GJ Late WWII conventional bombs
2.5 5.6 metric tons 23.5 GJ WWII blockbuster bomb
3.0 32 metric tons 134.4 GJ Massive Ordnance Air Blast bomb
3.5 178 metric tons 747.6 GJ Chernobyl nuclear disaster, 1986
4.0 1 kiloton 4.2 TJ Small atomic bomb
4.5 5.6 kilotons 23.5 TJ
5.0 32 kilotons 134.4 TJ Nagasaki atomic bomb (actual seismic yield was negligible since it detonated in the atmosphere)
Lincolnshire earthquake (UK), 2008
5.4 150 kilotons 625 TJ 2008 Chino Hills earthquake (Los Angeles, United States)
5.5 178 kilotons 747.6 TJ Little Skull Mtn. earthquake (NV, USA), 1992
Alum Rock earthquake (CA, USA), 2007
6.0 1 megaton 4.2 PJ Double Spring Flat earthquake (NV, USA), 1994
6.5 5.6 megatons 23.5 PJ Rhodes (Greece), 2008
6.7 16.2 megatons 67.9 PJ Northridge earthquake (CA, USA), 1994
6.9 26.8 megatons 112.2 PJ San Francisco Bay Area earthquake (CA, USA), 1989
7.0 32 megatons 134.4 PJ
7.1 50 megatons 210 PJ Energy released is equivalent to that of Tsar Bomba, the largest thermonuclear weapon ever tested.
7.5 178 megatons 747.6 PJ Kashmir earthquake (Pakistan), 2005
Antofagasta earthquake (Chile), 2007
7.8 600 megatons 2.4 EJ Tangshan earthquake (China), 1976
8.0 1 gigaton 4.2 EJ Toba eruption 75,000 years ago; which, according to the Toba catastrophe theory, affected modern human evolution
San Francisco earthquake (CA, USA), 1906
Queen Charlotte earthquake (BC, Canada), 1949
México City earthquake (Mexico), 1985
Gujarat earthquake (India), 2001
Chincha Alta earthquake (Peru), 2007
Sichuan earthquake (China), 2008 (initial estimate: 7.8)
8.5 5.6 gigatons 23.5 EJ Sumatra earthquake (Indonesia), 2007
9.0 32 gigatons 134.4 EJ Lisbon Earthquake (Lisbon, Portugal), All Saints Day, 1755
9.2 90.7 gigatons 379.7 EJ Anchorage earthquake (AK, USA), 1964
9.3 114 gigatons 477 EJ Indian Ocean earthquake, 2004 (40 ZJ in this case)
9.5 178 gigatons 747.6 EJ Valdivia earthquake (Chile), 1960 (251 ZJ in this case)
10.0 1 teraton 4.2 ZJ Never recorded by man.
12.0 1 petaton 4.2 YJ Yucatan impact (Chicxulub crater) 65 Ma ago.

Related

  • Earthquake
  • Seismic scale
  • Moment magnitude scale
  • Japan Meteorological Agency seismic intensity scale
  • Order of magnitude

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