Intensity & Modified Mercalli Scale

Intensity is also used to gauge earthquakes. Intensity is based on the effects that can be seen or felt due to an earthquake at a particular location. The Modified Mercalli Scale is the most commonly used scale in the United States. The scale is divided into 12 categories, describing the effects of a quake.

Each category is designated by a Roman numeral. The descriptions are arranged in increasing levels of intensity, from imperceptible shaking at Level I, to catastrophic destruction at Level XII. While there will only be 1 magnitude for any given earthquake, there are many different levels of intensity depending on the observer's specific location.
Highest Projected Modified Mercalli Intensities Map
Measurement of Severity
Many find the Mercalli Scale to be a more meaningful measurement of severity as it refers to the effects actually seen and/or felt in a particular area, though it may not be as useful for scientific study or research. Since no specialized equipment is needed, anyone can make observations and estimate the level of intensity using the Mercalli Scale. A sample of the intensity level descriptions is printed below. Visit the U.S. Geological Survey website to see the complete scale.

Modified Mercalli Scale

Level Effect
I Not felt except by a very few under especially favorable conditions.
II Felt only by a few persons at rest, especially on upper floors of buildings.
III 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. The vibrations are similar to the passing of a truck. Duration estimated.
IV Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes, windows, and doors disturbed; the walls make a cracking sound. The sensation is like a heavy truck striking the building. Standing motor cars rocked noticeably.
V Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects overturned. Pendulum clocks may stop.
VI *A 6.0 to 6.9 magnitude quake on the New Madrid fault could produce some of the following effects in St. Charles County.

Felt by all and many frightened. Some heavy furniture moved; a few instances of fallen plaster. The damage is slight.
VII * A 7.0 to 7.9 magnitude earthquake on the New Madrid fault could produce some of the following effects in St. Charles County.

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 * An 8.0 to 8.9 magnitude earthquake on the New Madrid fault could produce some of the following effects in St. Charles County.

Damage slight in specially designed structures; considerable damage in ordinary substantial buildings with partial collapse. The damage is great in poorly built structures. The fall of chimneys, factory stacks, columns, monuments, and walls. Heavy furniture overturned.
IX Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb. The damage is great in substantial buildings, with a partial collapse. Buildings shifted off foundations.
X Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rails bent.
XI Few, if any (masonry) structures remain standing. Bridges destroyed. Rails bent greatly.