The scale lists criteria that permit the seismologist to represent the severity of ground shaking in a community or part of a community by a number. Department of Commerce have for many decades been based on the Modified Mercalli Intensity Scale of 1931 (Wood and Neumann, 1931), which we usually refer to simply as the 'Modified Mercalli' or 'MM' scale. Geological Survey and (prior to 1973) by agencies in the U. The intensity of an earthquake at a location is a number that characterizes the severity of ground shaking at that location by considering the effects ofthe shaking on people, on manmade structures, and on the landscape. The following is an excerpt from Intensity Distribution and Isoseismal Maps for the Northridge, California, Earthquake of January 17,1994. (Public domain.)Ībridged from The Severity of an Earthquake, USGS General Interest Publication 1989-288-913 History and Details of MMI Structural engineers usually contribute information for assigning intensity values of VIII or above.Ībbreviated description of the levels of Modified Mercalli intensity. The higher numbers of the scale are based on observed structural damage.

The lower numbers of the intensity scale generally deal with the manner in which the earthquake is felt by people. 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. It does not have a mathematical basis instead it is an arbitrary ranking based on observed effects. This scale, composed of increasing levels of intensity that range from imperceptible shaking to catastrophic destruction, is designated by Roman numerals. It was developed in 1931 by the American seismologists Harry Wood and Frank Neumann. 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. 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. The effect of an earthquake on the Earth's surface is called the intensity.

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