Modified Mercalli intensity scale

{{Short description|Seismic intensity scale used to quantify the degree of shaking during earthquakes}}

{{Redirect|Mercalli|the scientist whom the scale is named after|Giuseppe Mercalli}}

{{Earthquakes}}

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 "{{m|w|link=y}}" 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.{{cite web|title=The Severity of an Earthquake|url=https://pubs.usgs.gov/gip/earthq4/severitygip.html|publisher=United States Geological Survey|date= November 5, 2021}} 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

Italian volcanologist Giuseppe Mercalli formulated his first intensity scale in 1883.{{Harvnb|Davison|1921|p=103}}. 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".{{Harvnb|Musson|Grünthal|Stucchi|2010|p=414}}. 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.{{Harvnb|Davison|1921|p=108}}. This version "found favour with the users", and was adopted by the Italian Central Office of Meteorology and Geodynamics.{{Harvnb|Musson|Grünthal|Stucchi|2010|p=415}}.

In 1904, Adolfo Cancani proposed adding two additional degrees for very strong earthquakes, "catastrophe" and "enormous catastrophe", thus creating a 12-degree scale.{{Harvnb|Davison|1921|p=112}}. His descriptions being deficient, August Heinrich Sieberg augmented them during 1912 and 1923, and indicated a peak ground acceleration for each degree.{{Harvnb|Davison|1921|p=114}}. This became known as the "Mercalli–Cancani scale, formulated by Sieberg", or the "Mercalli–Cancani–Sieberg scale", or simply "MCS",{{Harvnb|Musson|Grünthal|Stucchi|2010|p=416}}. and was used extensively in Europe and remains in use in Italy by the National Institute of Geophysics and Volcanology (INGV).{{Cite web |last=National Institute of Geophysics and Volcanology |title=Intensity evaluation method |url=http://legacy.ingv.it/roma/SITOINGLESE/activities/pererischio/macrosismica/macros/metod_val.html |access-date=2022-10-20 |archive-date=2022-10-20 |archive-url=https://web.archive.org/web/20221020112435/http://legacy.ingv.it/roma/SITOINGLESE/activities/pererischio/macrosismica/macros/metod_val.html |url-status=dead }}

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).{{Harvnb|Wood|Neumann|1931}}. Some seismologists refer to this version as the "Wood–Neumann scale". 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.{{Harvnb|Richter|1958}}; {{Harvnb|Musson|Grünthal|Stucchi|2010|p=416}}. Not wanting to have this intensity scale confused with the Richter scale he had developed, he proposed calling it the "modified Mercalli scale of 1956" (MM56).

In their 1993 compendium of historical seismicity in the United States,{{Harvnb|Stover|Coffman|1993}} 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,{{efn|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 {{Harvnb|Stover|Coffman|1993|pp=3–4}}.}} effectively creating a new, but largely undocumented version of the scale.{{Harvnb|Grünthal|2011|p=238}}. The most definitive exposition of the Stover and Coffman's effective scale is at {{Harvnb|Musson|Cecić|2012|loc=§12.2.2}}.

The basis by which the United States 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".{{Harvnb|Dewey|Reagor|Dengler|Moley|1995|p=5}}. Also, construction codes and methods have evolved, making much of built environment stronger; these make a given intensity of ground shaking seem weaker.{{Harvnb|Davenport|Dowrick|2002}}. 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".

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+".{{Harvnb|Musson|Grünthal|Stucchi|2010|p=423}}.

Scale values

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.{{cite web|title=Magnitude vs Intensity|url=https://prd-wret.s3.us-west-2.amazonaws.com/assets/palladium/production/atoms/files/Mag_vs_Int_Pkg_1.pdf|url-status=live|archive-url=https://web.archive.org/web/20220305084449/https://prd-wret.s3.us-west-2.amazonaws.com/assets/palladium/production/atoms/files/Mag_vs_Int_Pkg_1.pdf|archive-date=2022-03-05|access-date=2022-03-05|publisher=United States Geological Survey}}

class="wikitable"

! Scale level

! Peak ground acceleration (approx.){{cite web |title=3.5. Representing Macroseismic Intensity on Maps – ShakeMap Documentation documentation |url=https://usgs.github.io/shakemap/manual4_0/ug_intensity.html |website=usgs.github.io |access-date=11 April 2024}}

! Ground conditions

! Examples

style="{{shindo/color|mmi|1}}width:180px;text-align:left" id="mmi-1" | I. Not felt

| <{{cvt|0.0005|g0|m/s2}}

| Not felt except by very few under especially favorable conditions.

style="{{shindo/color|mmi|2}}text-align:left" id="mmi-2" | II. Weak

| rowspan="2" | {{cvt|0.003|g0|m/s2}}

| Felt only by a few people at rest, especially on upper floors of buildings. Delicately suspended objects may swing.

style="{{shindo/color|mmi|3}}text-align:left" id="mmi-3" | 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 slightly rock. Vibrations are similar to the passing of a truck, with duration estimated.

| 1992 Nicaragua earthquake

style="{{shindo/color|mmi|4}}text-align:left" id="mmi-4" | IV. Light

| {{cvt|0.028|g0|m/s2}}

| 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

style="{{shindo/color|mmi|5}}text-align:left" id="mmi-5" | V. Moderate

| {{cvt|0.062|g0|m/s2}}

| Felt by nearly everyone; many awakened. Some dishes and windows are broken. Unstable objects are overturned. Pendulum clocks may stop.

| 2010 Mentawai earthquake

style="{{shindo/color|mmi|6}}text-align:left" id="mmi-6" | VI. Strong

| {{cvt|0.12|g0|m/s2}}

| 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
2000 Enggano earthquake

style="{{shindo/color|mmi|7}}text-align:left" id="mmi-7" | VII. Very strong

| {{cvt|0.22|g0|m/s2}}

| 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
2002 Hindu Kush earthquakes
2009 Sumatra earthquakes

style="{{shindo/color|mmi|8}}text-align:left" id="mmi-8" | VIII. Severe

| {{cvt|0.40|g0|m/s2}}

| Damage is slight in specially designed structures; considerable damage in ordinary substantial buildings with partial collapse. Damage is great in poorly built structures. The fall of chimneys, factory stacks, columns, monuments, and walls occur. Heavy furniture is overturned. Sand and mud is ejected in small amounts. Changes occur in well water. Motorists are disturbed.

| 2005 Nias–Simeulue earthquake

style="{{shindo/color|mmi|9}}text-align:left" id="mmi-9" | IX. Violent

| {{cvt|0.75|g0|m/s2}}

| 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.

| 1977 Vrancea earthquake
2010 Yushu earthquake
2023 Al Haouz earthquake

style="{{shindo/color|mmi|10}}text-align:left" id="mmi-10" | X. Extreme

| rowspan=3 | >{{cvt|1.39|g0|m/s2}}

| Some well-built wooden structures are destroyed; most masonry and frame structures are destroyed with foundations. Rails are bent. Landslides are considerable from river banks and steep slopes. Sand and mud is shifted. Water is splashed over banks.

|
April 2015 Nepal earthquake
2018 Sulawesi earthquake
2025 Myanmar earthquake

style="{{shindo/color|mmi|11}}text-align:left" id="mmi-11" | 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 landslips occur on soft ground. Rails are greatly bent.

|2005 Kashmir earthquake
2008 Sichuan earthquake
2024 Noto earthquake

style="{{shindo/color|mmi|12}}text-align:left" id="mmi-12" | 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
2023 Turkey–Syria earthquakes

= Correlation with magnitude =

cellspacing="3" cellpadding="3" style="width:20%; border:1px aqua ; background:#eee; margin:0 auto 0 auto; float:right;"
style="text-align:left" |Magnitude

| Typical Maximum Modified Mercalli Intensity

style="text-align:left" |1.0–3.0

| I

style="text-align:left" |3.0–3.9

| II–III

style="text-align:left" |4.0–4.9

| IV–V

style="text-align:left" |5.0–5.9

| VI–VII

style="text-align:left" |6.0–6.9

| VII–IX

style="text-align:left" |7.0 and higher

| VIII or higher

colspan="2" style="text-align: center;" | [https://web.archive.org/web/20110623113247/http://earthquake.usgs.gov/learn/topics/mag_vs_int.php 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,{{Cite web |publisher=United States Geological Survey |title=M 7.0 – 26 km NNE of El Hoyo, Argentina – Impact |url=https://earthquake.usgs.gov/earthquakes/eventpage/usp000hsdc/impact |website=ANSS Comprehensive Earthquake Catalog}} while a magnitude 2.2 event in Barrow in Furness, England, in 1865, about 1 km deep, had a maximum felt intensity of VIII.{{Cite web |url=http://www.quakes.bgs.ac.uk/historical/query_eq/ |title=UK Historical Earthquake Database |publisher=British Geological Survey |access-date=2018-03-15}}

The small table is a rough guide to the degrees of the MMI scale.{{cite web|title=Modified Mercalli Intensity Scale|url=http://resilience.abag.ca.gov/shaking/mmi/|publisher=Association of Bay Area Governments|access-date=2017-09-02|archive-date=2023-03-26|archive-url=https://web.archive.org/web/20230326023832/http://resilience.abag.ca.gov/shaking/mmi/|url-status=dead}} 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 =

Dozens of intensity-prediction equations{{sfn|Allen|Wald|Worden|2012}} 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.{{cite web| url = http://www.gmpe.org.uk| title = Ground motion prediction equations (1964–2021) by John Douglas, University of Strathclyde, Glasgow, United Kingdom}} 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.{{sfn|Musson|2000}}

= Correlation with physical quantities =

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.{{cite web|title=ShakeMap Scientific Background|url=https://earthquake.usgs.gov/eqcenter/shakemap/background.php|publisher=United States Geological Survey|access-date=2017-09-02|archive-url=https://web.archive.org/web/20090825092714/http://earthquake.usgs.gov/eqcenter/shakemap/background.php|archive-date=2009-08-25|url-status=dead}}

= Comparison to the moment magnitude scale =

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

References

=Notes=

{{notelist}}

=Citations=

{{Reflist|}}

=Sources=

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{{refend}}

Further reading

  • {{Cite journal |last=Jones |first=Richard |date=2012 |title=Investigating the Mercalli Intensity Scale Through 'Lived Experience' |url=https://www2.hawaii.edu/~rmjones7/RMJ-Mercalli%20Scale%20Sci%20Scope%20Article.pdf |journal=Science Scope |volume=36 |issue=4 |pages=54–60 |issn=0887-2376 |jstor=43183283 |id={{ERIC|EJ1000835}}}}
  • {{Cite journal |last1=Wald |first1=David J. |author-link=David J. Wald |last2=Loos |first2=Sabine |last3=Spence |first3=Robin |last4=Goded |first4=Tatiana |last5=Hortacsu |first5=Ayse |date=2023 |title=A Common Language for Reporting Earthquake Intensities |url=http://eos.org/features/a-common-language-for-reporting-earthquake-intensities |journal=Eos |volume=104 |language=en-US |doi=10.1029/2023eo230160|doi-access=free }}