Rock and Roll: The New Madrid Fault System Part III: The Modified Mercalli Intensity Scale By Stephanie Osborn
Rock and Roll: The New Madrid Fault System Part III: The Modified Mercalli Intensity Scale
Excerpted from Rock and Roll: The New Madrid Fault System, ©2017
By Stephanie Osborn
Images in this article are public domain, unless otherwise specified.
This whole collection of writings started off with an email exchange, months ago. Our illustrious hostess was part of the discussion, and expressed interest in my converting the info to one or more blog articles. Then, when the LibertyCon programmers heard about it, they asked me to give a presentation on same, which I did.
The presentation was a full house, and at the end, there was a request for me to convert it to blogs and/or an ebook. I asked how many would like to see an ebook of the material; virtually every hand in the lecture hall went up.
A little over a month later, with additional research under my belt and factored into the manuscript, the book has gone live. And as promised, I am providing Sarah a series of blog articles on the subject. This series of blog articles is only a small fraction of the material contained in the ebook; it may be considered in the nature of a series of informative abstracts of the information contained therein. For additional information, may I recommend that you check out Rock and Roll: The New Madrid Fault System.
Part III: The Modified Mercalli Intensity Scale
The intensity of a quake is defined by its effects on Earth’s surface. The Richter scale, long the definitive measure of an earthquake’s strength, is a measure of quake energy, and even that has been recently superseded by the moment magnitude scale (as in recent years, the Richter scale was found to be inaccurate for the most powerful quakes), but before the advent of seismographic data, geologists developed various means of estimating quake intensity based upon the damage produced. The one that eventually survived is the Mercalli Intensity scale. It was originally developed by Italian vulcanologist Giuseppe Mercalli in 1806, and later modified to accommodate new data.
According to the U.S. Geological Survey, “The intensity scale consists of a series of certain key responses such as people awakening, movement of furniture, damage to chimneys, and finally — total destruction. Although numerous intensity scales have been developed over the last several hundred years to evaluate the effects of earthquakes, the one currently used in the United States is the Modified Mercalli (MM) Intensity Scale. It was developed in 1931 by the American seismologists Harry Wood and Frank Neumann. This scale, composed of increasing levels of intensity that range from imperceptible shaking to catastrophic destruction, is designated by Roman numerals. It does not have a mathematical basis; instead it is an arbitrary ranking based on observed effects.
[Modern quake damage to chimney; Mercalli VII]
“The Modified Mercalli Intensity value assigned to a specific site after an earthquake has a more meaningful measure of severity to the nonscientist than the magnitude because intensity refers to the effects actually experienced at that place.
[Modern home with crawlspace, shifted off foundations with partial collapse;
“The lower numbers of the intensity scale generally deal with the manner in which the earthquake is felt by people. The higher numbers of the scale are based on observed structural damage. Structural engineers usually contribute information for assigning intensity values of VIII or above.”
The Modified Mercalli Intensity Scale
|I||Not felt||Not felt except by a very few under especially favorable conditions.|
|II||Weak||Felt only by a few persons at rest, especially on upper floors of buildings.|
|III||Weak||Felt quite noticeably by persons indoors, especially on upper floors of buildings. Many people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibrations similar to the passing of a truck. Duration estimated.|
|IV||Light||Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rocked noticeably.|
|V||Moderate||Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects overturned. Pendulum clocks may stop.|
|VI||Strong||Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight.|
|VII||Very strong||Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken.|
|VIII||Severe||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.|
|IX||Violent||Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations.|
|X||Extreme||Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rails bent.|
[San Francisco, 1906; Richter mag. 8, Mercalli XI.
The USGS adds, “Another measure of the relative strength of an earthquake is the size of the area over which the shaking is noticed. This measure has been particularly useful in estimating the relative severity of historic shocks that were not recorded by seismographs or did not occur in populated areas. The extent of the associated felt areas indicates that some comparatively large earthquakes have occurred in the past in places not considered by the general public to be regions of major earthquake activity. For example, the three [principal] shocks in 1811 and 1812 near New Madrid, MO, were each felt over the entire eastern United States.”
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