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 within the felt area. The maximum observed intensity generally occurs
near the epicenter.
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 12
levels of Modified Mercalli intensity.
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. Vibrations 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, doors disturbed; walls make cracking sound. Sensation like heavy truck
striking 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. Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen
plaster. Damage slight.
VII. 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. 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. 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. 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.
XII. Damage total. Lines of sight and level are distorted. Objects thrown into the air. |
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