Seismic intensity scale used to quantify the degree of shaking during earthquakes
"Mercalli" redirects here. For the scientist whom the scale is named after, see
Giuseppe Mercalli
.
The
Modified Mercalli intensity scale
(
MM
,
MMI
, or
MCS
) measures the effects of an
earthquake
at a given location. This is in contrast with the
seismic magnitude
usually reported for an earthquake.
Magnitude scales measure the inherent force or strength of an earthquake ? an event occurring at greater or lesser depth. (The "
" scale is widely used.) The MM scale measures intensity of shaking, at any particular location, on the surface. It was developed from
Giuseppe Mercalli
's
Mercalli intensity scale
of 1902.
While shaking experienced at the surface is caused by the
seismic energy
released by an earthquake, earthquakes differ in how much of their energy is radiated as seismic waves. They also differ in the depth at which they occur; deeper earthquakes have less interaction with the surface, their energy is spread throughout a larger volume, and the energy reaching the surface is spread across a larger area. Shaking intensity is localized. It generally diminishes with distance from the earthquake's
epicenter
, but it can be amplified in
sedimentary basins
and in certain kinds of unconsolidated soils.
Intensity scales
categorize intensity empirically, based on the effects reported by untrained observers, and are adapted for the effects that might be observed in a particular region.
[1]
By not requiring instrumental measurements, they are useful for estimating the magnitude and location of historical (preinstrumental) earthquakes: the greatest intensities generally correspond to the epicentral area, and their degree and extent (possibly augmented by knowledge of local geological conditions) can be compared with other local earthquakes to estimate the magnitude.
History
[
edit
]
Italian volcanologist
Giuseppe Mercalli
formulated his first intensity scale in 1883.
[2]
It had six degrees or categories, has been described as "merely an adaptation" of the then-standard
Rossi?Forel scale
of 10 degrees, and is now "more or less forgotten".
[3]
Mercalli's second scale, published in 1902, was also an adaptation of the Rossi?Forel scale, retaining the 10 degrees and expanding the descriptions of each degree.
[4]
This version "found favour with the users", and was adopted by the Italian Central Office of Meteorology and Geodynamics.
[5]
In 1904, Adolfo Cancani proposed adding two additional degrees for very strong earthquakes, "catastrophe" and "enormous catastrophe", thus creating a 12-degree scale.
[6]
His descriptions being deficient,
August Heinrich Sieberg
augmented them during 1912 and 1923, and indicated a
peak ground acceleration
for each degree.
[7]
[8]
This became known as the "Mercalli?Cancani scale, formulated by Sieberg", or the "Mercalli?Cancani?Sieberg scale", or simply "MCS",
[8]
and was used extensively in Europe and remains in use in Italy by the
National Institute of Geophysics and Volcanology
(INGV).
[9]
When
Harry O. Wood
and Frank Neumann translated this into English in 1931 (along with modification and condensation of the descriptions, and removal of the acceleration criteria), they named it the "modified Mercalli intensity scale of 1931" (MM31).
[10]
Some seismologists refer to this version the "Wood?Neumann scale".
[8]
Wood and Neumann also had an abridged version, with fewer criteria for assessing the degree of intensity.
The Wood?Neumann scale was revised in 1956 by
Charles Francis Richter
and published in his influential textbook
Elementary Seismology
.
[11]
Not wanting to have this intensity scale confused with the
Richter magnitude scale
he had developed, he proposed calling it the "modified Mercalli scale of 1956" (MM56).
[8]
In their 1993 compendium of historical seismicity in the United States,
[12]
Carl Stover and Jerry Coffman ignored Richter's revision, and assigned intensities according to their slightly modified interpretation of Wood and Neumann's 1931 scale,
[a]
effectively creating a new, but largely undocumented version of the scale.
[13]
The basis by which the
U.S. Geological Survey
(and other agencies) assigns intensities is nominally Wood and Neumann's MM31. However, this is generally interpreted with the modifications summarized by Stover and Coffman because in the decades since 1931, "some criteria are more reliable than others as indicators of the level of ground shaking".
[14]
Also, construction codes and methods have evolved, making much of built environment stronger; these make a given intensity of ground shaking seem weaker.
[15]
Also, some of the original criteria of the most intense degrees (X and above), such as bent rails, ground fissures, landslides, etc., are "related less to the level of ground shaking than to the presence of ground conditions susceptible to spectacular failure".
[14]
The categories "catastrophe" and "enormous catastrophe" added by Cancani (XI and XII) are used so infrequently that current USGS practice is to merge them into a single category "Extreme" abbreviated as "X+".
[16]
Scale values
[
edit
]
The lesser degrees of the MMI scale generally describe the manner in which the earthquake is felt by people. The greater numbers of the scale are based on observed structural damage.
This table gives MMIs that are typically observed at locations near the epicenter of the earthquake.
[17]
Scale level
|
Peak ground acceleration
(approx.)
[18]
|
Ground conditions
|
Notable Examples
|
I. Not felt
|
<0.0005
g
0
(0.0049 m/s
2
)
|
Not felt except by very few under especially favorable conditions.
|
II. Weak
|
0.003
g
0
(0.029 m/s
2
)
|
Felt only by a few people at rest, especially on upper floors of buildings. Delicately suspended objects may swing.
|
III. Weak
|
Felt quite noticeably by people indoors, especially on upper floors of buildings: Many people do not recognize it as an earthquake. Standing vehicles may rock slightly. Vibrations are similar to the passing of a truck, with duration estimated.
|
1992 Nicaragua earthquake
|
IV. Light
|
0.028
g
0
(0.27 m/s
2
)
|
Felt indoors by many, outdoors by few during the day: At night, some are awakened. Dishes, windows, and doors are disturbed; walls make cracking sounds. Sensations are like a heavy truck striking a building. Standing vehicles are rocked noticeably.
|
2006 Pangandaran earthquake and tsunami
|
V. Moderate
|
0.062
g
0
(0.61 m/s
2
)
|
Felt by nearly everyone; many awakened: Some dishes and windows are broken. Unstable objects are overturned.
Pendulum clocks
may stop.
|
2010 Mentawai earthquake and tsunami
|
VI. Strong
|
0.12
g
0
(1.2 m/s
2
)
|
Felt by all, and many are frightened. Some heavy furniture is moved; a few instances of fallen
plaster
occur. Damage is slight.
|
2021 West Sulawesi earthquake
|
VII. Very strong
|
0.22
g
0
(2.2 m/s
2
)
|
Damage is negligible in buildings of good design and construction; but slight to moderate in well-built ordinary structures; damage is considerable in poorly built or badly designed structures; some chimneys are broken. Noticed by motorists.
|
May 1998 Afghanistan earthquake
and
2002 Hindu Kush earthquakes
|
VIII. Severe
|
0.40
g
0
(3.9 m/s
2
)
|
Damage slight in specially designed structures; considerable damage in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned. Sand and mud ejected in small amounts. Changes in well water. Motorists are disturbed.
|
2009 Sumatra earthquakes
,
2021 Haiti earthquake
,
and
2023 Herat earthquakes
|
IX. Violent
|
0.75
g
0
(7.4 m/s
2
)
|
Damage is considerable in specially designed structures; well-designed frame structures are thrown off-kilter. Damage is great in substantial buildings, with partial collapse. Buildings are shifted off foundations.
Liquefaction
occurs. Underground pipes are broken.
|
2006 Yogyakarta earthquake
,
2011 T?hoku earthquake and tsunami
,
and
2023 Al Haouz earthquake
|
X. Extreme
|
>1.39
g
0
(13.6 m/s
2
)
|
Some well-built wooden structures are destroyed; most masonry and frame structures are destroyed with foundations. Rails are bent. Landslides considerable from river banks and steep slopes. Shifted sand and mud. Water splashed over banks.
|
2010 Haiti earthquake
,
April 2015 Nepal earthquake
,
and
2018 Sulawesi earthquake and tsunami
|
XI. Extreme
|
Few, if any, (masonry) structures remain standing. Bridges are destroyed. Broad fissures erupt in the ground. Underground pipelines are rendered completely out of service. Earth slumps and land slips in soft ground. Rails are bent greatly.
|
1995 Kobe earthquake
,
2005 Kashmir earthquake
,
and
2008 Sichuan earthquake
|
XII. Extreme
|
Damage is total. Waves are seen on ground surfaces. Lines of sight and level are distorted. Objects are thrown upward into the air.
|
1939 Erzincan earthquake
,
1960 Valdivia earthquake
,
and
2023 Turkey?Syria earthquakes
|
Correlation with magnitude
[
edit
]
Magnitude
|
Typical Maximum Modified Mercalli Intensity
|
1.0?3.0
|
I
|
3.0?3.9
|
II?III
|
4.0?4.9
|
IV?V
|
5.0?5.9
|
VI?VII
|
6.0?6.9
|
VII?IX
|
7.0 and higher
|
VIII
or higher
|
Magnitude/intensity comparison, USGS
|
Magnitude and intensity, while related, are very different concepts. Magnitude is a function of the energy liberated by an earthquake, while intensity is the degree of shaking experienced at a point on the surface, and varies from some maximum intensity at or near the epicenter, out to zero at distance. It depends upon many factors, including the depth of the
hypocenter
, terrain, distance from the epicenter, whether the underlying strata there amplify surface shaking, and any directionality due to the earthquake mechanism. For example, a magnitude 7.0 quake in
Salta
, Argentina, in 2011, that was 576.8 km deep, had a maximum felt intensity of V,
[19]
while a magnitude 2.2 event in
Barrow in Furness
, England, in 1865, about 1 km deep, had a maximum felt intensity of VIII.
[20]
The small table is a rough guide to the degrees of the MMI scale.
[17]
[21]
The colors and descriptive names shown here differ from those used on certain shake maps in other articles.
Estimating site intensity and its use in seismic hazard assessment
[
edit
]
Dozens of intensity-prediction equations
have been published to estimate the macroseismic intensity at a location given the magnitude, source-to-site distance, and perhaps other parameters (e.g. local site conditions). These are similar to
ground motion-prediction equations
for the estimation of instrumental strong-motion parameters such as
peak ground acceleration
. A summary of intensity prediction equations is available.
[23]
Such equations can be used to estimate the
seismic hazard
in terms of macroseismic intensity, which has the advantage of being related more closely to
seismic risk
than instrumental strong-motion parameters.
Correlation with physical quantities
[
edit
]
The MMI scale is not defined in terms of more rigorous, objectively quantifiable measurements such as shake amplitude, shake frequency, peak velocity, or peak acceleration. Human-perceived shaking and building damage are best correlated with peak acceleration for lower-intensity events, and with peak velocity for higher-intensity events.
[25]
Comparison to the moment magnitude scale
[
edit
]
The effects of any one earthquake can vary greatly from place to place, so many MMI values may be measured for the same earthquake. These values can be displayed best using a contoured map of equal intensity, known as an
isoseismal map
. However, each earthquake has only one magnitude.
See also
[
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]
References
[
edit
]
Notes
[
edit
]
- ^
Their modifications were mainly to degrees IV and V, with VI contingent on reports of damage to man-made structures, and VII considering only "damage to buildings or other man-made structures". See details at
Stover & Coffman 1993
, pp. 3?4.
Citations
[
edit
]
- ^
"The Severity of an Earthquake"
.
USGS
. USA.gov. November 5, 2021.
- ^
Davison 1921
, p. 103.
- ^
Musson, Grunthal & Stucchi 2010
, p. 414.
- ^
Davison 1921
, p. 108.
- ^
Musson, Grunthal & Stucchi 2010
, p. 415.
- ^
Davison 1921
, p. 112.
- ^
Davison 1921
, p. 114.
- ^
a
b
c
d
Musson, Grunthal & Stucchi 2010
, p. 416.
- ^
National Institute of Geophysics and Volcanology.
"Intensity evaluation method"
. Archived from
the original
on 2022-10-20
. Retrieved
2022-10-20
.
- ^
Wood & Neumann 1931
.
- ^
Richter 1958
;
Musson, Grunthal & Stucchi 2010
, p. 416.
- ^
Stover & Coffman 1993
- ^
Grunthal 2011
, p. 238. The most definitive exposition of the Stover and Coffman's effective scale is at
Musson & Ceci? 2012
, §12.2.2.
- ^
a
b
Dewey et al. 1995
, p. 5.
- ^
Davenport & Dowrick 2002
.
- ^
Musson, Grunthal & Stucchi 2010
, p. 423.
- ^
a
b
"Magnitude vs Intensity"
(PDF)
.
USGS
.
Archived
(PDF)
from the original on 2022-03-05
. Retrieved
2022-03-05
.
- ^
"3.5. Representing Macroseismic Intensity on Maps ? ShakeMap Documentation documentation"
.
usgs.github.io
. Retrieved
11 April
2024
.
- ^
United States Geological Survey
.
"M 7.0 ? 26 km NNE of El Hoyo, Argentina ? Impact"
.
ANSS Comprehensive Earthquake Catalog
.
- ^
British Geological Survey
.
"UK Historical Earthquake Database"
. Retrieved
2018-03-15
.
- ^
"Modified Mercalli Intensity Scale"
.
Association of Bay Area Governments
. Archived from
the original
on 2023-03-26
. Retrieved
2017-09-02
.
- ^
"Ground motion prediction equations (1964?2021) by John Douglas, University of Strathclyde, Glasgow, United Kingdom"
.
- ^
"ShakeMap Scientific Background"
.
USGS
. Archived from
the original
on 2009-08-25
. Retrieved
2017-09-02
.
Sources
[
edit
]
- Dewey, James W.; Reagor, B. Glen; Dengler, L.; Moley, K. (1995),
"Intensity Distribution and Isoseismal Maps for the Northridge, California, Earthquake of January 17, 1994"
(PDF)
,
U. S. Geological Survey
, Open-File Report 95-92
.
- Grunthal, Gottfried (2011),
"Earthquakes, Intensity"
, in Gupta, Harsh K. (ed.),
Encyclopedia of Solid Earth Geophysics
, Springer, pp. 237?242,
ISBN
978-90-481-8701-0
- Lee, William H.K.
; Jennings, Paul; Kisslinger, Carl;
Kanamori, Hiroo
, eds. (2002).
International Handbook of Earthquake & Engineering Seismology, Part A
. Elsevier.
ISBN
978-0-08-048922-3
.
OCLC
51272640
.
- Musson, R.M.W. (2000). "Intensity-based seismic risk assessment".
Soil Dynamics and Earthquake Engineering
.
20
(5?8): 353?360.
Bibcode
:
2000SDEE...20..353M
.
doi
:
10.1016/s0267-7261(00)00083-x
.
- Richter, Charles F. (1958),
Elementary Seismology
, W. H. Freeman,
ISBN
978-0716702115
,
LCCN
58-5970
- Satake, Kenji
;
Atwater, Brian F.
(May 2007).
"Long-Term Perspectives on Giant Earthquakes and Tsunamis at Subduction Zones"
.
Annual Review of Earth and Planetary Sciences
.
35
(1): 349?374.
Bibcode
:
2007AREPS..35..349S
.
doi
:
10.1146/annurev.earth.35.031306.140302
.
ISSN
0084-6597
.
- Schopf, James Morton; Oftedahl, Orrin G. (1976),
The Reinhardt Thiessen coal thin-section slide collection of the U.S. Geological Survey; catalog and notes
,
doi
:
10.3133/b1432
- Stover, Carl W.; Coffman, Jerry L. (1993),
"Seismicity of the United States, 1568 ? 1989 (Revised)"
(PDF)
,
U.S. Geological Survey
, Professional Paper 1527
.
- Wood, Harry O.; Neumann, Frank (1931),
"Modified Mercalli Intensity Scale of 1931"
(PDF)
,
Bulletin of the Seismological Society of America
,
21
(4): 277?283,
Bibcode
:
1931BuSSA..21..277W
,
doi
:
10.1785/BSSA0210040277
- Xu, Yueren; Liu-Zeng, Jing; Allen, Mark B.; Zhang, Weiheng; Du, Peng (March 2021).
"Landslides of the 1920 Haiyuan earthquake, northern China"
(PDF)
.
Landslides
.
18
(3): 935?953.
Bibcode
:
2021Lands..18..935X
.
doi
:
10.1007/s10346-020-01512-5
.
ISSN
1612-510X
.
S2CID
221568806
.
Further reading
[
edit
]
External links
[
edit
]
|
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Modern scales
| Intensity scales
| |
---|
Magnitude scales
| |
---|
|
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Historical scales
| |
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