Seismic magnitude scales
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."[1]
Intensity refers to the strength or force of
shaking at a given location, and can be related
Isoseismal map for the 1968 Illinois earthquake. The
irregular distribution of shaking arises from variations of
to the peak ground velocity. With an
geology and ground conditions.
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.[2]
The intensity of local ground-shaking depends on several factors besides the magnitude of the
earthquake,[3] 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.[4] 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.[5]
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.[6] 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
[7]
duration.
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.[8]
Typical seismogram. The compressive P waves (following the red lines) –
essentially sound passing through rock – are the fastest seismic waves,
and arrive first, typically in about 10 seconds for an earthquake around
50 km away. The sideways-shaking S waves (following the green lines)
arrive some seconds later, traveling a little over half the speed of the P
waves; the delay is a direct indication of the distance to the quake. S
waves may take an hour to reach a point 1000 km away. Both of these
are body-waves, that pass directly through the earth's crust. Following the
S waves are various kinds of surface-waves – Love waves and Rayleigh
waves – that travel only at the earth's surface. Surface waves are smaller
for deep earthquakes, which have less interaction with the surface. For
shallow earthquakes – less than roughly 60 km deep – the surface waves
are stronger, and may last several minutes; these carry most of the
energy of the quake, and cause the most severe damage.
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.[9]
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, ML , Ms , mb ,
mB and mbLg .[10]
"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 Local magnitude scale, label ML or ML.[11]
Richter established two features now common to all magnitude scales.
1. First, the scale is logarithmic, so that each unit represents a ten-fold increase in the
amplitude of the seismic waves.[12] 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.[13]
2. 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 millimeters (1 μm,
or 0.00004 in.) on a seismogram recorded with a Wood-Anderson torsion seismograph.[14]
Subsequent magnitude scales are calibrated to be approximately in accord with the original
"Richter" (local) scale around magnitude 6.[15]
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 continent (everywhere
east of the Rocky Mountains) because of differences in the continental crust.[16] 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 (below), which the press describes as "Richter
magnitude".[17]
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.[18] (Not to be confused with the Russian surface-wave MLH
scale.[19]) Whether the values are comparable depends on whether the local conditions have been
adequately determined and the formula suitably adjusted.[20]
Japan Meteorological Agency magnitude scale
In Japan, for shallow (depth < 60 km) earthquakes within 600 km, the Japanese Meteorological Agency
calculates[21] 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"[22] with the seismic moment magnitude Mw in the range of 4.5 to
7.5,[23] 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.[24]
mB scale
The original "body-wave magnitude" – mB or mB (uppercase "B") – was developed by Gutenberg 1945c
and Gutenberg & Richter 1956[25] to overcome the distance and magnitude limitations of the ML scale
inherent in the use of surface waves. mB 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.[26] Use of mB as originally defined has been largely abandoned,[27] now replaced by the
standardized mBBB scale.[28]
mb scale
The mb or mb scale (lowercase "m" and "b") is similar to mB , but uses only P waves measured in the
first few seconds on a specific model of short-period seismograph.[29] 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.[30]
Measurement of mb has changed several times.[31] As originally defined by Gutenberg (1945c) 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 mb on the first
second (just the first few P waves[32]), but since 1978 they measure the first twenty seconds.[33] The
modern practice is to measure short-period mb scale at less than three seconds, while the broadband
mBBB scale is measured at periods of up to 30 seconds.[34]
mbLg scale
The regional mbLg scale – also denoted mb_Lg, mbLg, MLg (USGS), Mn, and mN – was developed by
Nuttli (1973) 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 which by other measures seemed equivalent to quakes in California.
Nuttli resolved this by measuring the amplitude of short-period (~1 sec.) Lg waves,[35] a complex form of
the Love wave which, although a surface wave, he found provided a result more closely related to the mb
scale than the Ms scale.[36] 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.[37]
Surface-wave magnitude
scales
Surface waves propagate along the
Earth's surface, and are principally
either Rayleigh waves or Love
waves.[38] 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.
Differences in the crust underlying North America east of the
Rocky Mountains makes that area more sensitive to earthquakes.
Shown here: the 1895 New Madrid earthquake, M ~6, was felt
through most of the central U.S., while the 1994 Northridge quake,
though almost ten times stronger at M 6.7, was felt only in
southern California. From USGS Fact Sheet 017–03.
The surface-wave magnitude scale, variously denoted as Ms, MS, and Ms, is based on a procedure
developed by Beno Gutenberg in 1942[39] 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".[40] The Ms scale
approximately agrees with ML at ~6, then diverges by as much as half a magnitude.[41] A revision by
Nuttli (1983), sometimes labeled MSn,[42] 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)).[43] A "broad-band"
variant (Ms_BB, Ms(BB)) measures the largest velocity amplitude in the Rayleigh-wave train for periods
up to 60 seconds.[44] The MS7 scale used in China is a variant of Ms calibrated for use with the Chinesemade "type 763" long-period seismograph.[45]
The MLH scale used in some parts of Russia is actually a surface-wave magnitude.[46]
Moment magnitude and energy magnitude scales
Other magnitude scales are based on aspects of seismic waves that only indirectly and incompletely
reflect the force of an earthquake, involve other factors, and are generally limited in some respect of
magnitude, focal depth, or distance. The moment magnitude scale – Mw or Mw – developed by
seismologists Thomas C. Hanks and Hiroo Kanamori,[47] is based on an earthquake's seismic moment,
M0, a measure of how much work an earthquake does in sliding one patch of rock past another patch of
rock.[48] Seismic moment is measured in Newton-meters (Nm or N·m) in the SI system of measurement,
or dyne-centimeters (dyn-cm; 1 dyn-cm = 10−7 Nm) in the older CGS system. In the simplest case the
moment can be calculated knowing only the amount of slip, the area of the surface ruptured or slipped,
and a factor for the resistance or friction encountered. These factors can be estimated for an existing fault
to determine the magnitude of past earthquakes, or what might be anticipated for the future.[49]
An earthquake's seismic moment can be estimated in various ways, which are the bases of the Mwb, Mwr,
Mwc, Mww, Mwp, Mi, and Mwpd scales, all subtypes of the generic Mw scale. See Moment magnitude
scale § Subtypes for details.
Seismic moment is considered the most objective measure of an earthquake's "size" in regard of total
energy.[50] However, it is based on a simple model of rupture, and on certain simplifying assumptions; it
does not account for the fact that the proportion of energy radiated as seismic waves varies among
earthquakes.[51]
Much of an earthquake's total energy as measured by Mw is dissipated as friction (resulting in heating of
the crust).[52] An earthquake's potential to cause strong ground shaking depends on the comparatively
small fraction of energy radiated as seismic waves, and is better measured on the energy magnitude scale,
Me.[53] The proportion of total energy radiated as seismic waves varies greatly depending on focal
mechanism and tectonic environment;[54] Me and Mw for very similar earthquakes can differ by as much
as 1.4 units.[55]
Despite the usefulness of the Me scale, it is not generally used due to difficulties in estimating the
radiated seismic energy.[56]
Two earthquakes differing greatly in the damage done
In 1997 there were two large earthquakes off the coast of Chile. The magnitude of the
first, in July, was estimated at Mw 6.9, but was barely felt, and only in three places. In
October a Mw 7.1 quake in nearly the same location, but twice as deep and on a different
kind of fault, was felt over a broad area, injured over 300 people, and destroyed or
seriously damaged over 10,000 houses. As can be seen in the table below, this disparity of
damage done is not reflected in either the moment magnitude (Mw ) nor the surface-wave
magnitude (Ms ). Only when the magnitude is measured on the basis of the body-wave
(mb ) or the seismic energy (Me ) is there a difference comparable to the difference in
damage.
Date
ISC #
Lat.
Long.
Depth
Damage
Ms
Mw
mb
Me
Type of
fault
6 July
1997
1035633
(http://w
ww.isc.a
c.uk/cgibin/For
matBibp
rint.pl?e
vid=103
5633)
−30.06
−71.87
23 km
Barely
felt
6.5
6.9
5.8
6.1
interplatethrust
15 Oct.
1997
1047434
(http://w
ww.isc.a
c.uk/cgibin/For
matBibp
rint.pl?e
vid=104
7434)
−30.93
−71.22
58 km
Extensive
6.8
7.1
6.8
7.5
intraslabnormal
0.3
0.2
1.0
1.4
Difference:
Rearranged and adapted from Table 1 in Choy, Boatwright & Kirby 2001, p. 13. Seen also
in IS 3.6 2012, p. 7.
Energy class (K-class) scale
K (from the Russian word класс, 'class', in the sense of a category[57]) is a measure of earthquake
magnitude in the energy class or K-class system, developed in 1955 by Soviet seismologists in the remote
Garm (Tajikistan) region of Central Asia; in revised form it is still used for local and regional quakes in
many states formerly aligned with the Soviet Union (including Cuba). Based on seismic energy (K = log
ES, in Joules), difficulty in implementing it using the technology of the time led to revisions in 1958 and
1960. Adaptation to local conditions has led to various regional K scales, such as KF and KS.[58]
K values are logarithmic, similar to Richter-style magnitudes, but have a different scaling and zero point.
K values in the range of 12 to 15 correspond approximately to M 4.5 to 6.[59] M(K), M(K), or possibly
MK indicates a magnitude M calculated from an energy class K.[60]
Tsunami magnitude scales
Earthquakes that generate tsunamis generally rupture relatively slowly, delivering more energy at longer
periods (lower frequencies) than generally used for measuring magnitudes. Any skew in the spectral
distribution can result in larger, or smaller, tsunamis than expected for a nominal magnitude.[61] The
tsunami magnitude scale, Mt, is based on a correlation by Katsuyuki Abe of earthquake seismic moment
(M0 ) with the amplitude of tsunami waves as measured by tidal gauges.[62] Originally intended for
estimating the magnitude of historic earthquakes where seismic data is lacking but tidal data exist, the
correlation can be reversed to predict tidal height from earthquake magnitude.[63] (Not to be confused
with the height of a tidal wave, or run-up, which is an intensity effect controlled by local topography.)
Under low-noise conditions, tsunami waves as little as 5 cm can be predicted, corresponding to an
earthquake of M ~6.5.[64]
Another scale of particular importance for tsunami warnings is the mantle magnitude scale, Mm.[65] This
is based on Rayleigh waves that penetrate into the Earth's mantle, and can be determined quickly, and
without complete knowledge of other parameters such as the earthquake's depth.
Duration and coda magnitude scales
Md designates various scales that estimate magnitude from the duration or length of some part of the
seismic wave-train. This is especially useful for measuring local or regional earthquakes, both powerful
earthquakes that might drive the seismometer off-scale (a problem with the analog instruments formerly
used) and preventing measurement of the maximum wave amplitude, and weak earthquakes, whose
maximum amplitude is not accurately measured. Even for distant earthquakes, measuring the duration of
the shaking (as well as the amplitude) provides a better measure of the earthquake's total energy.
Measurement of duration is incorporated in some modern scales, such as Mwpd and mBc .[66]
Mc scales usually measure the duration or amplitude of a part of the seismic wave, the coda.[67] For short
distances (less than ~100 km) these can provide a quick estimate of magnitude before the quake's exact
location is known.[68]
Macroseismic magnitude scales
Magnitude scales generally are based on instrumental measurement of some aspect of the seismic wave
as recorded on a seismogram. Where such records do not exist, magnitudes can be estimated from reports
of the macroseismic events such as described by intensity scales.[69]
One approach for doing this (developed by Beno Gutenberg and Charles Richter in 1942[70]) relates the
maximum intensity observed (presumably this is over the epicenter), denoted I0 (capital I with a
subscripted zero), to the magnitude. It has been recommended that magnitudes calculated on this basis be
labeled Mw(I0),[71] but are sometimes labeled with a more generic Mms.
Another approach is to make an isoseismal map showing the area over which a given level of intensity
was felt. The size of the "felt area" can also be related to the magnitude (based on the work of Frankel
1994 and Johnston 1996). While the recommended label for magnitudes derived in this way is
M0(An),[72] the more commonly seen label is Mfa.[73] A variant, MLa, adapted to California and Hawaii,
derives the local magnitude (ML) from the size of the area affected by a given intensity.[74] MI (uppercase letter "I", distinguished from the lower-case letter in Mi) has been used for moment magnitudes
estimated from isoseismal intensities calculated per Johnston 1996.[75]
Peak ground velocity (PGV) and peak ground acceleration (PGA) are measures of the force that causes
destructive ground shaking.[76] In Japan, a network of strong-motion accelerometers provides PGA data
that permits site-specific correlation with different magnitude earthquakes. This correlation can be
inverted to estimate the ground shaking at that site due to an earthquake of a given magnitude at a given
distance. From this a map showing areas of likely damage can be prepared within minutes of an actual
earthquake.[77]
Other magnitude scales
Many earthquake magnitude scales have been developed or proposed, with some never gaining broad
acceptance and remaining only as obscure references in historical catalogs of earthquakes. Other scales
have been used without a definite name, often referred to as "the method of Smith (1965)" (or similar
language), with the authors often revising their method. On top of this, seismological networks vary on
how they measure seismograms. Where the details of how a magnitude has been determined are
unknown, catalogs will specify the scale as "unknown" (variously Unk, Ukn, or UK). In such cases, the
magnitude is considered generic and approximate.
An Mh ("magnitude determined by hand") label has been used where the magnitude is too small or the
data too poor (typically from analog equipment) to determine a Local magnitude, or multiple shocks or
cultural noise complicates the records. The Southern California Seismic Network uses this "magnitude"
where the data fail the quality criteria.[78]
A special case is the Seismicity of the Earth catalog of Gutenberg & Richter (1954). Hailed as a milestone
as a comprehensive global catalog of earthquakes with uniformly calculated magnitudes,[79] they never
published the full details of how they determined those magnitudes.[80] Consequently, while some
catalogs identify these magnitudes as MGR, others use UK (meaning "computational method
unknown").[81] Subsequent study found many of the Ms values to be "considerably overestimated".[82]
Further study has found that most of the MGR magnitudes "are basically Ms for large shocks shallower
than 40 km, but are basically mB for large shocks at depths of 40–60 km."[83] Gutenberg and Richter also
used an italic, non-bold M without subscript[84] – also used as a generic magnitude, and not to be
confused with the bold, non-italic M used for moment magnitude – and a "unified magnitude" m (bolding
added).[85] While these terms (with various adjustments) were used in scientific articles into the
1970s,[86] they are now only of historical interest. An ordinary (non-italic, non-bold) capital "M" without
subscript is often used to refer to magnitude generically, where an exact value or the specific scale used is
not important.
See also
Magnitude of completeness
Epicentral distance
Citations
1. Bormann, Wendt & Di Giacomo 2013, p. 37. The relationship between magnitude and the
energy released is complicated. See §3.1.2.5 and §3.3.3 for details.
2. Bormann, Wendt & Di Giacomo 2013, §3.1.2.1.
3. Bolt 1993, p. 164 et seq..
4. Bolt 1993, pp. 170–171.
5. Bolt 1993, p. 170.
6. See Bolt 1993, 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 (https://escweb.
wr.usgs.gov/share/mooney/SriL.II2.pdf).
7. See Havskov & Ottemöller 2009, §1.4, pp. 20–21, for a short explanation, or MNSOP-2 EX
3.1 2012 for a technical description.
8. Chung & Bernreuter 1980, p. 1.
9. Bormann, Wendt & Di Giacomo 2013, p. 18.
10. IASPEI IS 3.3 2014, pp. 2–3.
11. Kanamori 1983, p. 187.
12. Richter 1935, p. 7.
13. Spence, Sipkin & Choy 1989, p. 61.
14. Richter 1935, pp. 5; Chung & Bernreuter 1980, p. 10. Subsequently redefined by Hutton &
Boore 1987 as 10 mm of motion by an ML 3 quake at 17 km.
15. Chung & Bernreuter 1980, p. 1; Kanamori 1983, p. 187, figure 2.
16. Chung & Bernreuter 1980, p. ix.
17. 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 Wayback Machine (https://web.archive.org/web/2016042
8095841/http://earthquake.usgs.gov:80/aboutus/docs/020204mag_policy.php), and the
essential part can be found here (http://dapgeol.tripod.com/usgsearthquakemagnitudepolicy.
htm).
18. Bormann, Wendt & Di Giacomo 2013, §3.2.4, p. 59.
19. Rautian & Leith 2002, pp. 158, 162.
20. See Datasheet 3.1 in NMSOP-2 (http://bib.telegrafenberg.de/publizieren/vertrieb/nmsop/)
Archived (https://web.archive.org/web/20190804200233/http://bib.telegrafenberg.de/publizie
ren/vertrieb/nmsop/) 2019-08-04 at the Wayback Machine for a partial compilation and
references.
21. Katsumata 1996; Bormann, Wendt & Di Giacomo 2013, §3.2.4.7, p. 78; Doi 2010.
22. Bormann & Saul 2009, p. 2478.
23. See also figure 3.70 in NMSOP-2.
24. Havskov & Ottemöller 2009, p. 17.
25. Bormann, Wendt & Di Giacomo 2013, p. 37; Havskov & Ottemöller 2009, §6.5. See also
Abe 1981.
26. Havskov & Ottemöller 2009, p. 191.
27. Bormann & Saul 2009, p. 2482.
28. MNSOP-2/IASPEI IS 3.3 2014, §4.2, pp. 15–16.
29. Kanamori 1983, pp. 189, 196; Chung & Bernreuter 1980, p. 5.
30. Bormann, Wendt & Di Giacomo 2013, pp. 37, 39; Bolt (1993, pp. 88–93) examines this at
length.
31. Bormann, Wendt & Di Giacomo 2013, p. 103.
32. IASPEI IS 3.3 2014, p. 18.
33. Nuttli 1983, p. 104; Bormann, Wendt & Di Giacomo 2013, p. 103.
34. IASPEI/NMSOP-2 IS 3.2 2013, p. 8.
35. Bormann, Wendt & Di Giacomo 2013, §3.2.4.4. The "g" subscript refers to the granitic layer
through which Lg waves propagate.Chen & Pomeroy 1980, p. 4. See also J. R. Kayal,
"Seismic Waves and Earthquake Location", here (https://escweb.wr.usgs.gov/share/moone
y/SriL.II2.pdf), page 5.
36. Nuttli 1973, p. 881.
37. Bormann, Wendt & Di Giacomo 2013, §3.2.4.4.
38. Havskov & Ottemöller 2009, pp. 17–19. See especially figure 1-10.
39. Gutenberg 1945a; based on work by Gutenberg & Richter 1936.
40. Gutenberg 1945a.
41. Kanamori 1983, p. 187.
42. Stover & Coffman 1993, p. 3.
43. Bormann, Wendt & Di Giacomo 2013, pp. 81–84.
44. MNSOP-2 DS 3.1 2012, p. 8.
45. Bormann et al. 2007, p. 118.
46. Rautian & Leith 2002, pp. 162, 164.
47. Hanks, Thomas (1979). "A moment magnitude scale" (https://agupubs.onlinelibrary.wiley.co
m/doi/abs/10.1029/JB084ib05p02348?casa_token=Ob3hlS8_Ur4AAAAA:V-crV2WsODI0_C
ZP662OUwD-t4uyd0-r3_RYD0dyHcjfF7S6OwvqVhY6GfdevVhtjmsqIERTz57y8cC7).
Journal of Geophysical Research.
48. The IASPEI standard formula for deriving moment magnitude from seismic moment is
Mw = (2/3) (log M0 – 9.1). Formula 3.68 in Bormann, Wendt & Di Giacomo 2013, p. 125.
49. Anderson 2003, p. 944.
50. Havskov & Ottemöller 2009, p. 198
51. Havskov & Ottemöller 2009, p. 198; Bormann, Wendt & Di Giacomo 2013, p. 22.
52. Bormann, Wendt & Di Giacomo 2013, p. 23
53. NMSOP-2 IS 3.6 2012, §7.
54. See Bormann, Wendt & Di Giacomo 2013, §3.2.7.2 for an extended discussion.
55. NMSOP-2 IS 3.6 2012, §5.
56. Bormann, Wendt & Di Giacomo 2013, p. 131.
57. Rautian et al. 2007, p. 581.
58. Rautian et al. 2007; NMSOP-2 IS 3.7 2012; Bormann, Wendt & Di Giacomo 2013, §3.2.4.6.
59. Bindi et al. 2011, p. 330. Additional regression formulas for various regions can be found in
Rautian et al. 2007, Tables 1 and 2. See also IS 3.7 2012, p. 17.
60. Rautian & Leith 2002, p. 164.
61. Bormann, Wendt & Di Giacomo 2013, §3.2.6.7, p. 124.
62. Abe 1979; Abe 1989, p. 28. More precisely, Mt is based on far-field tsunami wave
amplitudes in order to avoid some complications that happen near the source. Abe 1979,
p. 1566.
63. Blackford 1984, p. 29.
64. Abe 1989, p. 28.
65. Bormann, Wendt & Di Giacomo 2013, §3.2.8.5.
66. Bormann, Wendt & Di Giacomo 2013, §3.2.4.5.
67. Havskov & Ottemöller 2009, §6.3.
68. Bormann, Wendt & Di Giacomo 2013, §3.2.4.5, pp. 71–72.
69. Musson & Cecić 2012, p. 2.
70. Gutenberg & Richter 1942.
71. Grünthal 2011, p. 240.
72. Grünthal 2011, p. 240.
73. "Magnitude Types | U.S. Geological Survey" (https://www.usgs.gov/programs/earthquake-ha
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74. Stover & Coffman 1993, p. 3.
75. Engdahl & Villaseñor 2002.
76. Makris & Black 2004, p. 1032.
77. Doi 2010.
78. Hutton, Woessner & Haukson 2010, pp. 431, 433.
79. NMSOP-2 IS 3.2 2013, pp. 1–2.
80. Abe 1981, p. 74; Engdahl & Villaseñor 2002, p. 667.
81. Engdahl & Villaseñor 2002, p. 688.
82. Abe & Noguchi 1983.
83. Abe 1981, p. 72.
84. Defined as "a weighted mean between MB and MS." Gutenberg & Richter 1956, p. 1.
85. "At Pasadena, a weighted mean is taken between mS as found directly from body waves,
and mS, the corresponding value derived from MS ...." Gutenberg & Richter 1956, p. 2.
86. E.g., Kanamori 1977.
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External links
Perspective: a graphical comparison of earthquake energy release (https://www.youtube.co
m/watch?v=YXMKSOsv3QA) – Pacific Tsunami Warning Center
USGS ShakeMap (https://earthquake.usgs.gov/data/shakemap/) Providing near-real-time
maps of ground motion and shaking intensity following significant earthquakes.
Retrieved from "https://en.wikipedia.org/w/index.php?title=Seismic_magnitude_scales&oldid=1263822054"