Mercalli intensity scale

Mercalli intensity scale

The Mercalli intensity scale is a seismic scale used for measuring the intensity of an earthquake. It measures the effects of an earthquake, and is distinct from the moment magnitude Mw usually reported for an earthquake (sometimes described as the obsolete Richter magnitude), which is a measure of the energy released. The intensity of an earthquake is not totally determined by its magnitude.

The 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).[1][2] Values depend upon the distance to 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 Mercalli (Intensity) scale originated with the widely-used simple ten-degree Rossi-Forel scale, which was revised by Italian vulcanologist Giuseppe Mercalli in 1884 and 1906.

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 or Modified Mercalli Intensity scale, and abbreviated MM or 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 small table is a rough guide to the degrees of the Modified Mercalli Intensity scale.[1][2] The colors and descriptive names shown here differ from those used on certain shake maps in other articles.

The large table gives Modified Mercalli scale intensities that are typically observed at locations near the epicenter of the earthquake.[1]

The correlation between magnitude and intensity is far from total, depending upon several factors including depth of the earthquake, terrain, and population density. For example, on 19 May 2011 an earthquake of magnitude 0.7 in Southern California, USA 4 km deep was classified as of intensity III by the United States Geological Survey (USGS), while a 4.5 magnitude quake in Salta, Argentina 164 km deep was of intensity I.[3]

Moment Magnitude Typical Maximum
Modified Mercalli Intensity
1.0 – 3.0 I
3.0 – 3.9 IIIII
4.0 – 4.9 IVV
5.0 – 5.9 VIVII
6.0 – 6.9 VIIIX
7.0+ VIII or higher


I. Instrumental Generally not felt by people unless in favorable conditions.
II. Weak Felt only by a few people at best, especially on the upper floors of buildings. Delicately suspended objects may swing.
III. Slight Felt quite noticeably by people indoors, especially on the upper floors of buildings. Many do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibration similar to the passing of a truck. Duration estimated.
IV. Moderate Felt indoors by many people, outdoors by few people during the day. At night, some awaken. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rock noticeably. Dishes and windows rattle alarmingly.
V. Rather Strong Felt outside by most, may not be felt by some outside in non-favorable conditions. Dishes and windows may break and large bells will ring. Vibrations like large train passing close to house.
VI. Strong Felt by all; many frightened and run outdoors, walk unsteadily. Windows, dishes, glassware broken; books fall off shelves; some heavy furniture moved or overturned; a few instances of fallen plaster. Damage slight.
VII. Very Strong Difficult to stand; furniture broken; damage negligible in building 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. Noticed by people driving motor cars.
VIII. Destructive Damage slight in specially designed structures; considerable in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture moved.
IX. Violent General panic; 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. Intense Some well built wooden structures destroyed; most masonry and frame structures destroyed with foundation. Rails bent.
XI. Extreme Few, if any masonry structures remain standing. Bridges destroyed. Rails bent greatly.
XII. Cataclysmic Total destruction – Everything is destroyed. Lines of sight and level distorted. Objects thrown into the air. The ground moves in waves or ripples. Large amounts of rock move position. Landscape altered, or leveled by several meters. In some cases, even the routes of rivers are changed.

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 shakings and building damages are best correlated with peak acceleration for lower-intensity events, and with peak velocity for higher-intensity events.[4]

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. Each earthquake, on the other hand, has only one magnitude.

Historical analysis

Earthquake Death Toll Moment Magnitude Mercalli intensity
Tangshan earthquake (1976) 250,000 7.8–8.2 XI [5]
Loma Prieta earthquake (1989) 63 6.9 IX [6]
Kobe earthquake (1995) 6,434 6.8 X-XI [5]
Haiti earthquake (2010) 316,000 7.0 X
Van earthquake (2011) 432+ 7.2 X
Christchurch earthquake (February 2011) 181 [7] 6.3 IX [8]
Tōhoku earthquake (2011) 18,500. Totals include tsunami. 9.0 IX [9]

See also

References

External links


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