Modified Mercalli Intensity Scale

The effect of an earthquake on the Earth's surface is called the intensity. 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 12 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.

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. After the occurrence of widely-felt earthquakes, the Geological Survey mails questionnaires to postmasters in the disturbed area requesting the information so that intensity values can be assigned. The results of this postal canvass and information furnished by other sources are used to assign an intensity value, and to compile isoseismal maps that show the extent of various levels of intensity within the felt area. The maximum observed intensity generally occurs near the epicenter. (source)

Mercalli Wikipedia Page

* Modified Mercalli equivalent Richter Magnitude


Mercalli Seismic Intensity
level description table

Modified
Mercalli

I

Perceived Shaking: NOT FELT
Comments
Not felt except by a very few under especially favorable conditions.
Peak Ground Acceleration
Less than 0.001 g (g = 9.81 m/s²)
Equivalent Richter Magnitude*
1.0 to 2.0

Modified
Mercalli

II

Perceived Shaking: WEAK
Comments
Felt only by a few people at rest, especially on upper floors of buildings.
Peak Ground Acceleration
0.002 g (g = 9.81 m/s²)
Equivalent Richter Magnitude*
2.0 to 3.0

Modified
Mercalli

III

Perceived Shaking: WEAK
Comments
Felt quite noticeably by people 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.
Peak Ground Acceleration
0.005 g (g = 9.81 m/s²)
Equivalent Richter Magnitude*
3.0 to 4.0

Modified
Mercalli

IV

Perceived Shaking: LIGHT
Comments
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.
Peak Ground Acceleration
0.010 g (g = 9.81 m/s²)
Equivalent Richter Magnitude*
4.0

Modified
Mercalli

V

Perceived Shaking: MODERATE
Comments
Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects overturned. Pendulum clocks may stop.
Peak Ground Acceleration
0.025 g (g = 9.81 m/s²)
Equivalent Richter Magnitude*
4.0 to .0

Modified
Mercalli

VI

Perceived Shaking: STRONG
Comments
Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight.
Peak Ground Acceleration
0.05 g (g = 9.81 m/s²)
Equivalent Richter Magnitude*
5.0 to 6.0

Modified
Mercalli

VII

Perceived Shaking: VERY STRONG
Comments
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.
Peak Ground Acceleration
0.10 g (g = 9.81 m/s²)
Equivalent Richter Magnitude*
6.0

Modified
Mercalli

VIII

Perceived Shaking: SEVERE
Comments
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.
Peak Ground Acceleration
0.25 g (g = 9.81 m/s²)
Equivalent Richter Magnitude*
6.0 to 7.0

Modified
Mercalli

IX

Perceived Shaking: VIOLENT
Comments
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.
Peak Ground Acceleration
0.5 g (g = 9.81 m/s²)
Equivalent Richter Magnitude*
7.0

Modified
Mercalli

X

Perceived Shaking: EXTREME
Comments
Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rails bent.
Peak Ground Acceleration
1.0 g (g = 9.81 m/s²)
Equivalent Richter Magnitude*
7.0 to 8.0