Mercalli Scale

What is the Modified Mercalli Intensity Scale?

How is the Mercalli Scale Used

The Mercalli and its Insurance Value: an Observable Data Measurement Tool.

The Modified Mercalli Intensity Scale:

Modified Mercalli – All about Observable data.

The Mercali scale is an “arbitrary ranking based on observed effects” and is used for measuring shaking. It does not have a “mathematical basis.” The scale measures perceived shaking of Land, Trees, Structures and other elements in the environment. It also incorporates earthquake as “felt by people”. It is based solely on observable data. This is exceptionally subjective. This is in direct contast to other measurements of earthquake shaking such as the moment magnitude or richter scale methods which use seismographs.

The modified mercalli intensity scale goes by numerous names, including: the mercalli intensity scale, the modified mercalli scale, the modified mercalli, or simply the mercalli. Sometimeit is capitalized, sometimes its not. We probably use all of the various forms of names in this post. You will see it, occassionaly abbreviated as simply MM.

The Modified Mercalli Scale vs the Richter Scale:

Two scales – lots of confusion.

The Modified Mercalli Intesity Scale measures “observable data” and is not based on a seismograph. It measures the effects of the Quake. Again its “observable.” The Richter Scale is based on a seismograph and attempts to measure the actual Quake. The Richter scale is abbrevaited ML. The Modified Mercalli scale is abbreviated MM. The MM scale is from I to XII and the ML is from 1 to 10.

“Observed data vs a Mechanical measurement“

The Richter Scale is no longer considered the best tool to measure the magnitude of an earthquake, that distinction goes to the Moment Magnitude or Mw. That does not stop the media obsessing over the Richter scale.

MM, ML, Ms, Mw, and Mb:

Actually the USGS has more than two scales.

There are lots of abbrevations starting with the M letter in the Earth Movement world. Two of them discussed in this article are MM – Mercalli Scale and ML – Richter Scale. To confuse you further there are at least three additional methods to measure magnitude. These include “surface-wave magnitude (Ms), body-wave magnitude (Mb), and moment magnitude (Mw).” With Mw being the current preferred method of measuring earthquakes. There are others.

A Very Brief History of the Mercalli Scale:

A Priest vulcanologist who was sadly burned to death started it all.

The Milan vulcanologist Giuseppe Mercalli created his first scale in 1883 and advanced it in 1902. Both Cancani and Sieberg tinkered and proposed changes in 1912 and 1923. Additionally Harry Wood and Frank Neumann translated the scale into English and it unofficially started to be remaned after them: Wood Neumann. Charles Richter [creator of the Richter scale] proposed to rechange the name to the Modified Mercalli Scale of 1956. He may have been motivated to keep it separated from his own scale. That partial name seems to have mostly stuck. Today it is officially called the Modified Mercalli Intensity Scale.

The Modified Mercalli Intensity Scale:

The Mercalli scale is represented in roman numerals from I to XII with I being the lowest and XII being the most extreme. Therefore the common mercalli scale has 12 levels from “Not Felt” to “Extreme”. Some of the higher levels have been added more recently. The scale is intended to be based on observations at or near the epicenter. The Modified Mercalli Intensity Scale is abbreviated MM for short.

The Mercalli Scale Roman numeral can be listed in a variety of ways. With a period, proceeded by the “MM” abbreviation, with or without a period. Examples could include III or III. or MMIII or MMIII. On occassion you may also see them expressed as pure numerals such as 12.

Earthquake Canada lists the Mercalli Scale with these descriptions:

I. or MMI. – “Not Felt…sometimes birds, animals, reported uneasy or disturbed”

II. MMII. – “Felt indoors by few…sometimes hanging objects may swing….sometimes birds, animals, reported uneasy or disturbed”

III. or MMIII. – “Felt indoors by several…Vibration like that due to passing of light, or lightly loaded trucks…Hanging objects may swing slightly.”

IV. or MMIV. – “Felt indoors by many, outdoors by few. Awakened few…Sensation like heavy body striking building, or falling of heavy objects to inside.”

V. or MMV. – “Felt indoors by practically all, outdoors by many or most…Awakened many, or most. Frightened few…Broke dishes, glassware, to some extent. Cracked windows… Overturned small or unstable objects…”

VI. or MMVI. – “Felt by all, indoors and outdoors. Frightened many, excitement general…many ran outdoors. Awakened all….Liquid set in strong motion. Small bells rang -church….Damage slight in poorly built buildings….. Fall of knick-knacks, books, pictures.”

VII. or MMVII. – “Frightened all – general alarm, all ran outdoors. Some, or many, found it difficult to stand….Waves on ponds, lakes, and running water….Damage negligible in buildings of good design and construction, slight to moderate in well-build ordinary buildings, considerable in poorly build or badly designed buildings…”

VIII. or MMVIII. – “Fright general – alarm approaches panic. Disturbed persons driving motor cars…. Considerable [damage] in ordinary substantial buildings, partial collapse: racked, tumbled down, wooden houses in some cases; threw out panel walls in frame structures, broke off decayed piling. Fall of walls.”

IX. or MMIX. – “Panic general. Cracked ground conspicuously. Damage considerable….underground pipes sometimes broken.”

X. or MMX. – “Cracked ground, especially when loose and wet, up to widths of several inches…Landslides considerable from river banks and steep coasts…. Damage serious to dams, dikes, embankments. Severe to well-build wooden structures and bridges, some destroyed. Developed dangerous cracks in excellent brick walls. Destroyed most masonry and frame structures, also their foundations…Open cracks and broad wavy folds in cement pavements…”

XI. or MMXI. – “Disturbances in ground many and widespread…Broad fissures, earth slumps, and land slips in soft, wet ground. Ejected water in large amounts….Few, if any (masonry), structures remained standing….Destroyed large well-built bridges…pipe lines buried in earthy completely out of service.”

XII. or MMXII. – “Damage total – practically all works of construction damaged greatly or destroyed….Waves seen on ground surfaces (actually seen, probably, in some cases). Distorted lines of sight and level….”

*Abbreviations are condensed from Earthquake Canada, which is itself a condensed version from “Sieberg’s Mercalli-Cancani scale, modified and condensed.” In other words you are simply reading highlights here.

It is interesting to note that apparently people inside and outside may have different observations. Additionally those in automobiles even more so.

As you can clearly see the MM of I. to III. are only slightly observable while the IV. to VI. are noticeable and begin to become “disturb[ing]” or scary. For VII. and beyond “General Alarm”,”Panic,” and possibly even “Damage Total” descriptions begin to become real.

Uses of Mercalli:

Observable Data in Use.

The Mercalli scale is a tool used to demonstrate the damage or strength of a present or past seismic event. Using this estimating method it allows us to compare different historical seismic events against each other. It is an invaluable tool at the field level especially when left with no devices to measure the ferocity of shaking. And in comparison with tools that feel shaking the Mercallis is more results driven: What did the shaking do?

Mercalli may be most helpful when attempting to consider the historical impact of earthquakes during times before we had our machines. How does one estimate an earthquake such as the Fort Tejon Earthquake of 1857? There were no scales in widespread use then.

The New Madrid earthquake event of 1811-1812 is a useful example. A series of quakes that occurred over a period of several months brought widespread disaster to of all places – Missouri. Boats were literally thrown out of the Mississippi river. The MM scale assists us in measuring this past historical event and comparing it with more recent seismic events such as the Loma Prieta quake of 1989.

Additionally the MM scale is practical from a consumer level. It is relatively non technical. With no technology whatsoever any person can assess the damage done by an earthquake and provide their own guidance and opinion on the level on the Modified Mercalli scale. [Our MM scale is massively abbreviated and if interested you can use the full description yourself.]

How do I Know if I am Looking at the Mercalli or Richter Scale?

Great Question. From past experience I can tell you that sometimes you will not know with certainty. Always start with the key, if the graph has one. But in general if its a Roman Numeral its a Mercalli. If its Roman and up to XII – its certainly the Mercalli Scale. If its numeric and only up to ten its probalby either the Richter Scale of the Motion Magnitude.

There is a way to be super clear when discussing with other Quake wonks. Use the preceding letter initials. Such as MMXI or ML4 or Mw2. That being said it is surprising how many times you see referanced an Earthquake scale with no information about what type of reading it is. That seems super important as one type of reading is observed data and another is a mechanical measurement.

Many people seem to be under the mistaken impression that there is ONLY one earthquake scale. There are many. In reporting for this article this author noticed numerous misstaken headlines and information conerning magnitude scales.

The Modified Mercalli Scale for Earthquake Insurance:

Earthquake property insurers uses lots of data including Modified Mercalli.

The MM scale is another tool that can assist underwriters in forcasting and underwriting earthquake insurance premiums. Premiums are derived from a number of terrestial factors including past earthquake history, fault lines, types of soil, and slope of land. There are building considerations used in underwriting and deriving premiums as well among other considerations.

The insurance industry loves data. Current and Past data it does not matter they love it all and the Modified Mercalli Intensity Scale is one of the many data tools that some earthquake insurers use.

The Mercalli Scale of Some Famous Historical Earthquakes:

A small set of examples of some large historical examples of the Merc Intensity Scale [MM]:

The 1700 Cascadia Region Quake, Mercalli Intesity IX.

The Portugal 1755 Quake, Mercalli Intesity X.

The 1868 Peru Quake, Mercalli Intesity XI.

The 2010 Chile Quake, Mercalli Intesity IX.

The 2004 Indonesia Quake, Mercalli Intesity IX.

The Merc:

The Mercalli is a surprisingly useful tool when you learn that it is solely based on observable data and not some machine in the corner. It is a system that has been in continuous use for over 100 years. Its value as a rear view mirror tool is exceptional. The Modified Mercalli Intensity Scale is one of the most unique measuring tools available in the earthquake world.

Additional Sources of Information:

A partial list of resources used by QuakeCov.

USGS.gov

Wikipedia

California Earthquake Authority

Thoughtco

Thank you for reading this post. Please seek the licensed advice of an insurance agent in your jurisdiction whenever you are considering ammending, cancelling, or purchasing any type of insurance product.