Mercalli Scale - Earthquake Intensity
How are Earthquakes measured to determine their Intensity?
The effect an earthquake has on the Earth's surface is called the intensity. Intensity is classified based on responses such as people awakening, movement of furniture, damage to buildings and chimneys, and finally level of damage from mild to total destruction. Several intensity scales have been developed during the last several hundred years to determine and measure 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 features 12 increasing levels of intensity, ranging from minor shaking to catastrophic destruction and shows its increments in Roman numerals. Unlike the Richter Scale which is used to determine the magnitude of an earthquake, it does not have a mathematical formula basis. It ranks earthquakes based on observed effects.
The Modified Mercalli Intensity value assigned to a specific location after an earthquake has a more meaningful measure of severity to the general public compared to the magnitude because intensity refers to the effects people actually experienced at that place during the event.
The Mercalli Scale Increments:
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 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.
For information on how to determine and measure Earthquake Magnitude, please click: Richter Scale