The expression Richter Magnitude Scale refers to a number of ways to assign a single number to quantify the energy contained in an earthquake.

In all cases, the magnitude is a base-10 logarithmic scale obtained by calculating the logarithm of the amplitude of waves measured by a seismograph. An earthquake that measures 5.0 on the Richter scale has a shaking amplitude 10 times larger and corresponds to an energy release of √1000 ≈ 31.6 times greater than one that measures 4.0.^[1]

Since the 1970s the use of the Richter Magnitude Scale has largely been supplanted by the moment magnitude scale in many countries. However, the Richter Magnitude Scale is still widely used in Russia and other CIS countries.

Development[link]

Charles Richter, c. 1970

Developed in 1935 by Charles 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.^[1] His inspiration was the apparent magnitude scale used in astronomy to describe the brightness of stars and other celestial objects.^[2] 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 of magnitude 3, approximately. However, the Richter scale has no lower limit, and sensitive modern seismographs now routinely record quakes with negative magnitudes.

M_L (local magnitude) was not designed to be applied to data with distances to the hypocenter of the earthquake greater than 600 km^[3] (373 mi). For national and local seismological observatories the standard magnitude scale is today still M_L. Unfortunately this scale saturates at M6.5, approximately, because the high frequency waves recorded locally have wavelengths shorter than the rupture lengths of large earthquakes.

To be able to measure the size of earthquakes around the globe, Gutenberg and Richter later developed a magnitude scale based on surface waves, surface wave magnitude M_S; and another based on body waves, body wave magnitude m_b.^[4] These are types of waves that are recorded at teleseismic distances. The two scales were adjusted such that they were consistent with the M_L scale. This succeeded better with the M_s scale than with the m_b scale. Both of these scales saturate when the earthquake is bigger than magnitude 8 and therefore the moment magnitude scale, M_w, was invented.^[5]

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.

Details[link]

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 Failed to parse (Missing texvc executable; please see math/README to configure.): M_w

(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 whom?]}

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 Failed to parse (Missing texvc executable; please see math/README to configure.): M_L and the surface wave Failed to parse (Missing texvc executable; please see math/README to configure.): M_s scale. In addition, the body wave magnitude, Failed to parse (Missing texvc executable; please see math/README to configure.): m_b , and the moment magnitude, Failed to parse (Missing texvc executable; please see math/README to configure.): M_w , 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 Failed to parse (Missing texvc executable; please see math/README to configure.): M_L , Failed to parse (Missing texvc executable; please see math/README to configure.): M_s , and Failed to parse (Missing texvc executable; please see math/README to configure.): M_w .^[6][7] The Failed to parse (Missing texvc executable; please see math/README to configure.): m_b 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.

Failed to parse (Missing texvc executable; please see math/README to configure.): M_L 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 Failed to parse (Missing texvc executable; please see math/README to configure.): M_s is also reported frequently.

The seismic moment, Failed to parse (Missing texvc executable; please see math/README to configure.): M_o , 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. Failed to parse (Missing texvc executable; please see math/README to configure.): M_w is derived from it empirically as a quantity without units, just a number designed to conform to the Failed to parse (Missing texvc executable; please see math/README to configure.): M_s scale.^[8] A spectral analysis is required to obtain Failed to parse (Missing texvc executable; please see math/README to configure.): M_o , whereas the other magnitudes are derived from a simple measurement of the amplitude of a specifically defined wave.

All scales, except Failed to parse (Missing texvc executable; please see math/README to configure.): M_w , 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 Failed to parse (Missing texvc executable; please see math/README to configure.): M_L

is about 6.5 and about 8 for Failed to parse (Missing texvc executable; please see math/README to configure.): M_s

.^[9]

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,^[10] the other is based on a recently discovered channel wave.^[11]

The energy release of an earthquake,^[12] 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 (Failed to parse (Missing texvc executable; please see math/README to configure.): =({10^{1.0}})^{(3/2)} ) in the energy released; a difference in magnitude of 2.0 is equivalent to a factor of 1000 (Failed to parse (Missing texvc executable; please see math/README to configure.): =({10^{2.0}})^{(3/2)}

) in the energy released.[13] The elastic energy radiated is best derived from an integration of the radiated spectrum, but one can base an estimate on Failed to parse (Missing texvc executable; please see math/README to configure.): m_b

because most energy is carried by the high frequency waves.

Richter magnitudes[link]

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:^[14]

Failed to parse (Missing texvc executable; please see math/README to configure.): M_\mathrm{L} = \log_{10} A - \log_{10} A_\mathrm{0}(\delta) = \log_{10} [A / A_\mathrm{0}(\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, Failed to parse (Missing texvc executable; please see math/README to configure.): \delta . In practice, readings from all observing stations are averaged after adjustment with station-specific corrections to obtain the M_L 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 about 4.6 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, and geological conditions (certain terrains can amplify seismic signals).

(Based on U.S. Geological Survey documents.)^[16]

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.^[17]

Examples[link]

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.^[18] 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.

Following, 31.623 to the power of 0 equals 1, 31.623 to the power of 1 equals 31.623 and 31.623 to the power of 2 equals 1000. Therefore, an 8.0 on the Richter scale releases 31.623 times more energy than a 7.0 and a 9.0 on the Richter scale releases 1000 times more energy than a 7.0.

• Quakes using the more modern magnitude scales will denote their abbreviations: Failed to parse (Missing texvc executable; please see math/README to configure.): M_W and Failed to parse (Missing texvc executable; please see math/README to configure.): M_S . Those that have no denoted prefix are Failed to parse (Missing texvc executable; please see math/README to configure.): M_L . Please be advised that the magnitude "number" (example 7.0) displayed for those quakes on this table may represent a significantly greater or lesser release in energy than by the correctly given magnitude (example Failed to parse (Missing texvc executable; please see math/README to configure.): M_W ).

Magnitude empirical formulae[link]

These formulae are an alternative method to calculate Richter magnitude instead of using Richter correlation tables based on Richter standard seismic event (Failed to parse (Missing texvc executable; please see math/README to configure.): M_\mathrm{L} =0, A=0.001mm, D=100km).

The Lillie empirical formula:

Failed to parse (Missing texvc executable; please see math/README to configure.): M_\mathrm{L} = \log_{10}A - 2.48+ 2.76\log_{10}\Delta

Where:

• A is the amplitude (maximum ground displacement) of the P-wave, in micrometers, measured at 0.8 Hz.
• Failed to parse (Missing texvc executable; please see math/README to configure.): \Delta

is the epicentral distance, in km.

For distance less than 200km:

Failed to parse (Missing texvc executable; please see math/README to configure.): M_\mathrm{L} = \log_{10} A + 1.6\log_{10} D - 0.15

For distance between 200km and 600km:

Failed to parse (Missing texvc executable; please see math/README to configure.): M_\mathrm{L} = \log_{10} A + 3.0\log_{10} D - 3.38

where A is seismograph signal amplitude in mm, D distance in km.

The Bisztricsany (1958) empirical formula for epicentral distances between 4˚ to 160˚:

Failed to parse (Missing texvc executable; please see math/README to configure.): M_\mathrm{L} = 2.92 + 2.25 \log_{10} (\tau) - 0.001 \Delta^{\circ}

Where:

• Failed to parse (Missing texvc executable; please see math/README to configure.): M_\mathrm{L}

is magnitude (mainly in the range of 5 to 8)

• Failed to parse (Missing texvc executable; please see math/README to configure.): \tau

is the duration of the surface wave in seconds

• Failed to parse (Missing texvc executable; please see math/README to configure.): \Delta

is the epicentral distance in degrees.

The Tsumura empirical formula:

Failed to parse (Missing texvc executable; please see math/README to configure.): M_\mathrm{L} = -2.53 + 2.85 \log_{10} (F-P) + 0.0014 \Delta^{\circ}

Where:

• Failed to parse (Missing texvc executable; please see math/README to configure.): M_\mathrm{L}

is the magnitude (mainly in the range of 3 to 5).

• Failed to parse (Missing texvc executable; please see math/README to configure.): F-P

is the total duration of oscillation in seconds.

• Failed to parse (Missing texvc executable; please see math/README to configure.): \Delta

is the epicentral distance in kilometers.

The Tsuboi, University of Tokio, empirical formula:

Failed to parse (Missing texvc executable; please see math/README to configure.): M_\mathrm{L} = \log_{10}A + 1.73\log_{10}\Delta - 0.83

Where:

• Failed to parse (Missing texvc executable; please see math/README to configure.): M_\mathrm{L}

is the magnitude.

• Failed to parse (Missing texvc executable; please see math/README to configure.): A

is the amplitude in um.

• Failed to parse (Missing texvc executable; please see math/README to configure.): \Delta

is the epicentral distance in kilometers.

See also[link]

• Earthquake
• Largest earthquakes by magnitude
• Seismic scale
• Seismite
• Mercalli intensity scale
• Moment magnitude scale
• Japan Meteorological Agency seismic intensity scale
• Order of magnitude
• Rohn Emergency Scale for measuring the magnitude (intensity) of any emergency

References[link]

External links[link]

• 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.

Look up magnitude in Wiktionary, the free dictionary.

Magnitude may refer to:

In mathematics:

• Magnitude (mathematics), the relative size of a mathematical object
• Magnitude (vector), a term for the size or length of a vector
• Scalar (mathematics), a quantity defined only by its magnitude
• Euclidean vector, a quantity defined by both its magnitude and its direction
• Order of magnitude, the class of scale having a fixed value ratio to the preceding class

In astronomy:

• Magnitude (astronomy), a measure of brightness and brightness differences used in astronomy
  • Apparent magnitude, the apparent brightness of a celestial object as observed from Earth
  • Absolute magnitude, the brightness of a celestial object corrected to a standard luminosity distance
  • Photographic magnitude, the brightness of a celestial object corrected for photographic sensitivity, symbol m_pg
• Magnitude of eclipse or geometric magnitude, the size of the eclipsed part of the Sun during a solar eclipse or the Moon during a lunar eclipse

As an earthquake unit of measure:

• Richter magnitude scale, a measure of the energy of an earthquake
• Moment magnitude scale, an alternative to the Richter scale for measuring earthquakes
• Surface wave magnitude

In popular culture:

• Magnitude, a recurring character from the television series Community

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