Page 41 - CCPS Monograph - Natural Hazards

The Modified Mercalli (MM) Intensity Scale assigns a value to an earthquake intensity that ranges from
imperceptible shaking to catastrophic destruction. 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. 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 following
is an abbreviated description of the levels of Modified Mercalli intensity. (USGS 2019b)

Table G.4 Modified Mercalli Intensity Scale (USGS 2019b)

Intensity Shaking Description/Damage
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.

Assessment of and planning for natural hazards 38

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