eme Few, if any, (masonry) structures remain standing. Bridges destroyed. Broad fissures in ground. Underground pipe lines completely out of service. Earth slumps and land slips in soft ground. Rails bent greatly.
This is what an XI: Extreme earthquake is like. I have only lived through one this bad in 1971 which is in the following word button if you want to know more about this and it was one of the most terrifying experiences of my life. Power lines go down, masonry all crumbles everywhere unless it is rebar reinforced (and many buildings didn't have that in 1971 yet) but after this quake building codes got a lot stiffer statewide to prevent more deaths in future quakes throughout California. Even railroad tracks bend and roads have breaks or have different levels in them so if you drive on one and don't notice it's not really a road anymore that you can drive on you are dead or injured. This happened to several people then including a highway patrolman on an motorcycle when a bridge collapsed under him on a freeway then.
- The 1971 San Fernando earthquake (also known as the Sylmar earthquake) occurred in the early morning of February 9 in the foothills of the San Gabriel Mountains in ...
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XI (Extreme) [5]
Mercalli intensity scale
From Wikipedia, the free encyclopedia
For the Italian volcanologist, see Giuseppe Mercalli.
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usually reported for an earthquake (sometimes misreported as the Richter magnitude), which is a measure of the energy
released. The intensity of an earthquake is not entirely determined by
its magnitude. It is not based on first physical principles, but is,
instead, empirically based on observed effects.[1]The Mercalli scale quantifies the effects of an earthquake on the Earth's surface, humans, objects of nature, and man-made structures on a scale from I (not felt) to XII (total destruction).[2][3] Values depend upon the distance from the earthquake, with the highest intensities being around the epicentral area. Data gathered from people who have experienced the quake are used to determine an intensity value for their location. The Italian volcanologist Giuseppe Mercalli revised the widely used simple ten-degree Rossi–Forel scale between 1884 and 1906, creating the Mercalli Intensity scale which is still used nowadays.
In 1902, the ten-degree Mercalli scale was expanded to twelve degrees by Italian physicist Adolfo Cancani. It was later completely re-written by the German geophysicist August Heinrich Sieberg and became known as the Mercalli–Cancani–Sieberg (MCS) scale.
The Mercalli–Cancani–Sieberg scale was later modified and published in English by Harry O. Wood and Frank Neumann in 1931 as the Mercalli–Wood–Neumann (MWN) scale. It was later improved by Charles Richter, the father of the Richter magnitude scale.
The scale is known today as the Modified Mercalli scale (MM) or Modified Mercalli Intensity scale (MMI).
Contents
Modified Mercalli Intensity scale
The lower degrees of the Modified Mercalli 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.The large table gives Modified Mercalli scale intensities that are typically observed at locations near the epicenter of the earthquake.[2]
| 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. |
| XI. Extreme | Few, if any, (masonry) structures remain standing. Bridges destroyed. Broad fissures in ground. Underground pipe lines completely out of service. Earth slumps and land slips in soft ground. Rails bent greatly. |
| XII. Extreme | Damage total. Waves seen on ground surfaces. Lines of sight and level distorted. Objects thrown upward into the air. |
Correlation with magnitude
| Magnitude | Typical Maximum Modified Mercalli Intensity |
|---|---|
| Under 2.0 | I |
| 2.0 – 2.9 | II – III |
| 3.0 – 3.9 | III – IV |
| 4.0 – 4.9 | IV – V |
| 5.0 – 5.9 | V – VI |
| 6.0 – 6.9 | VI – VII |
| 7.0 – 7.9 | VII – VIII |
| 8.0 or higher | VIII or higher |
The small table is a rough guide to the degrees of the Modified Mercalli Intensity scale.[2][3] The colors and descriptive names shown here differ from those used on certain shake maps in other articles.
Correlation with physical quantities
The Mercalli 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 damages are best correlated with peak acceleration for lower-intensity events, and with peak velocity for higher-intensity events.[5]Comparison to the moment magnitude scale
The effects of any one earthquake can vary greatly from place to place, so there may be many Mercalli intensity values measured for the same earthquake. These values can be best displayed using a contoured map of equal intensity, known as an isoseismal map. However, each earthquake has only one magnitude.See also
- Japan Meteorological Agency seismic intensity scale
- Rohn Emergency Scale
- Seismic scale
- Spectral acceleration
- Strong ground motion
References
- Sources
- Wood, H. O.; Neumann, F. (1931), "Modified Mercalli intensity scale of 1931" (PDF), Bulletin of the Seismological Society of America (Seismological Society of America) 21: 277–283
External links
- National Earthquake Information Center (U.S.)
- Mercalli Scale Simulator, Northern Illinois University
- The Modified Mercalli Intensity Scale – United States Geological Survey
- U.S. Earthquake Intensity Database - NOAA
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