Seismic magnitude scales are used to describe the overall strength or "size" of an earthquake. These are distinguished from seismic intensity scales that categorize the intensity or severity of ground shaking (quaking) caused by an earthquake at a given location. Magnitudes are usually determined from measurements of an earthquake's seismic waves as recorded on a seismogram. Magnitude scales vary based on what aspect of the seismic waves are measured and how they are measured. Different magnitude scales are necessary because of differences in earthquakes, the information available, and the purposes for which the magnitudes are used.
Earthquake magnitude and ground-shaking intensity
The Earth's crust is stressed by tectonic forces. When this stress becomes great enough to rupture the crust, or to overcome the friction that prevents one block of crust from slipping past another, energy is released, some of it in the form of various kinds of seismic waves that cause ground-shaking, or quaking.
Magnitude is an estimate of the relative "size" or strength of an earthquake, and thus its potential for causing ground-shaking. It is "approximately related to the released seismic energy".. The relationship between magnitude and the energy released is complicated. See §3.1.2.5 and §3.3.3 for details.
Intensity refers to the strength or force of shaking at a given location, and can be related to the peak ground velocity. With an isoseismal map of the observed intensities (see illustration) an earthquake's magnitude can be estimated from both the maximum intensity observed (usually but not always near the epicenter), and from the extent of the area where the earthquake was felt..
The intensity of local ground-shaking depends on several factors besides the magnitude of the earthquake,. one of the most important being soil conditions. For instance, thick layers of soft soil (such as fill) can amplify seismic waves, often at a considerable distance from the source, while sedimentary basins will often resonate, increasing the duration of shaking. This is why, in the 1989 Loma Prieta earthquake, the Marina district of San Francisco was one of the most damaged areas, though it was nearly 100 km from the epicenter.. Geological structures were also significant, such as where seismic waves passing under the south end of San Francisco Bay reflected off the base of the Earth's crust towards San Francisco and Oakland. A similar effect channeled seismic waves between the other major faults in the area..
Magnitude scales
An earthquake radiates energy in the form of different kinds of seismic waves, whose characteristics reflect the nature of both the rupture and the earth's crust the waves travel through.See , Chapters 2 and 3, for a very readable explanation of these waves and their interpretation. J. R. Kayal's description of seismic waves can be found here . Determination of an earthquake's magnitude generally involves identifying specific kinds of these waves on a seismogram, and then measuring one or more characteristics of a wave, such as its timing, orientation, amplitude, frequency, or duration.See , pp. 20–21, for a short explanation, or MNSOP-2 for a technical description. Additional adjustments are made for distance, kind of crust, and the characteristics of the seismograph that recorded the seismogram.
The various magnitude scales represent different ways of deriving magnitude from such information as is available. All magnitude scales retain the logarithmic scale as devised by Charles Richter, and are adjusted so the mid-range approximately correlates with the original "Richter" scale..
Most magnitude scales are based on measurements of only part of an earthquake's seismic wave-train, and therefore are incomplete. This results in systematic underestimation of magnitude in certain cases, a condition called saturation..
Since 2005 the International Association of Seismology and Physics of the Earth's Interior (IASPEI) has standardized the measurement procedures and equations for the principal magnitude scales, , , , and .IASPEI .
"Richter" magnitude scale
The first scale for measuring earthquake magnitudes, developed in 1935 by Charles F. Richter and popularly known as the "Richter" scale, is actually the , label ML or ML.. Richter established two features common to all magnitude scales. First, the scale is logarithmic, so that each unit represents a tenfold increase in the amplitude of the seismic waves.. As the energy of a wave is proportional to A1.5, where A denotes the amplitude, each unit of magnitude represents a 101.5 ≈ 32-fold increase in the seismic energy (strength) of an earthquake.. Second, Richter arbitrarily defined the zero point of the scale to be where an earthquake at a distance of 100 km makes a maximum horizontal displacement of 0.001 mm (1 μm, or 0.00004 in.) on a seismogram recorded with a Wood-Anderson torsion seismograph.; . Subsequently redefined by as 10 mm of motion by an quake at 17 km. Subsequent magnitude scales are calibrated to be approximately in accord with the original "Richter" (local) scale around magnitude 6.; , figure 2.
All "Local" (ML) magnitudes are based on the maximum amplitude of the ground shaking, without distinguishing the different seismic waves. They underestimate the strength: of distant earthquakes (over ~600 km) because of attenuation of the S waves, of deep earthquakes because the surface waves are smaller, and of strong earthquakes (over M ~7) because they do not take into account the duration of shaking. The original "Richter" scale, developed in the geological context of Southern California and Nevada, was later found to be inaccurate for earthquakes in the central and eastern parts of the North American continent (everywhere east of the Rocky Mountains) because of differences in the continental crust.. All these problems prompted the development of other scales.
Most seismological authorities, such as the United States Geological Survey, report earthquake magnitudes above 4.0 as moment magnitude , which the press describes as "Richter magnitude".The "USGS Earthquake Magnitude Policy" for reporting earthquake magnitudes to the public as formulated by the USGS Earthquake Magnitude Working Group was implemented January 18, 2002, and posted at https://earthquake.usgs.gov/aboutus/docs/020204mag_policy.php. It has since been removed; a copy is archived at the , and the essential part can be found here .
Other "local" magnitude scales Richter's original "local" scale has been adapted for other localities. These may be labelled "ML", or with a lowercase "l", either Ml, or Ml., p. 59. (Not to be confused with the Russian surface-wave MLH scale..) Whether the values are comparable depends on whether the local conditions have been adequately determined and the formula suitably adjusted.See Datasheet 3.1 in NMSOP-2 for a partial compilation and references.
Japan Meteorological Agency magnitude scale
In Japan, for shallow (depth < 60 km) earthquakes within 600 km, the Japanese Meteorological Agency calculates; , p. 78; . a magnitude labeled MJMA, MJMA, or MJ. (These should not be confused with moment magnitudes JMA calculates, which are labeled Mw(JMA) or M(JMA), nor with the Shindo intensity scale.) JMA magnitudes are based (as typical with local scales) on the maximum amplitude of the ground motion; they agree "rather well". with the seismic moment magnitude in the range of 4.5 to 7.5,See also figure 3.70 in NMSOP-2. but underestimate larger magnitudes.
Body-wave magnitude scales
Body-waves consist of P waves that are the first to arrive (see seismogram), or S waves, or reflections of either. Body-waves travel through rock directly..
mB scale The original "body-wave magnitude" – mB or mB (uppercase "B") – was developed by and ; . See also . to overcome the distance and magnitude limitations of the scale inherent in the use of surface waves. is based on the P and S waves, measured over a longer period, and does not saturate until around M 8. However, it is not sensitive to events smaller than about M = 5.5.. Use of as originally defined has been largely abandoned,. replaced by the standardized scale.MNSOP-2/IASPEI , pp. 15–16.
mb scale The mb or mb scale (lowercase "m" and "b") is similar to , but uses only P waves measured in the first few seconds on a specific model of short-period seismograph.; . It was introduced in the 1960s with the establishment of the World-Wide Standardized Seismograph Network (WWSSN); the short period improves detection of smaller events, and better discriminates between tectonic earthquakes and underground nuclear explosions.; examines this at length.
Measurement of has changed several times.. As originally defined by mb was based on the maximum amplitude of waves in the first 10 seconds or more. However, the length of the period influences the magnitude obtained. Early USGS/NEIC practice was to measure on the first second (just the first few P wavesIASPEI .), but since 1978 they measure the first twenty seconds.; . The modern practice is to measure short-period scale at less than three seconds, while the broadband scale is measured at periods of up to 30 seconds.IASPEI/NMSOP-2 .
mbLg scale
The regional mbLg scale – also denoted mb_Lg, mbLg, MLg (USGS), Mn, and mN – was developed by for a problem the original ML scale could not handle: all of North America east of the Rocky Mountains. The ML scale was developed in southern California, which lies on blocks of oceanic crust, typically basalt or sedimentary rock, which have been accreted to the continent. East of the Rockies the continent is a craton, a thick and largely stable mass of continental crust that is largely granite, a harder rock with different seismic characteristics. In this area the ML scale gives anomalous results for earthquakes that by other measures seemed equivalent to quakes in California.
Nuttli resolved this by measuring the amplitude of short-period (~ 1 second) Lg waves,. The "g" subscript refers to the granitic layer through which Lg waves propagate.. See also J. R. Kayal, "Seismic Waves and Earthquake Location", here , page 5. a complex form of the Love wave that, although a surface wave, he found provided a result more closely related to the scale than the scale.. Lg waves attenuate quickly along any oceanic path, but propagate well through the granitic continental crust, and MbLg is often used in areas of stable continental crust; it is especially useful for detecting underground nuclear explosions..
Surface-wave magnitude scales
Surface waves propagate along the Earth's surface, and are principally either Rayleigh waves or Love waves.. See especially figure 1-10. For shallow earthquakes the surface waves carry most of the energy of the earthquake, and are the most destructive. Deeper earthquakes, having less interaction with the surface, produce weaker surface waves.
The surface-wave magnitude scale, variously denoted as Ms, MS, and Ms, is based on a procedure developed by Beno Gutenberg in 1942; based on work by . for measuring shallow earthquakes stronger or more distant than Richter's original scale could handle. Notably, it measured the amplitude of surface waves (which generally produce the largest amplitudes) for a period of "about 20 seconds".. The scale approximately agrees with at ~6, then diverges by as much as half a magnitude. A revision by , sometimes labeled MSn,. measures only waves of the first second.
A modification – the "Moscow-Prague formula" – was proposed in 1962, and recommended by the IASPEI in 1967; this is the basis of the standardized Ms20 scale (Ms_20, Ms(20)).. A "broad-band" variant (Ms_BB, Ms(BB)) measures the largest velocity amplitude in the Rayleigh-wave train for periods up to 60 seconds.MNSOP-2 . The MS7 scale used in China is a variant of Ms calibrated for use with the Chinese-made "type 763" long-period seismograph..