In a quiet forest, you can sometimes hear a single leaf fall to the ground. Sound intensity is defined as the sound power per unit area, whereas amplitude is the distance between the resting position and the crest of a wave. While sound intensity is proportional to amplitude, they are different physical quantities. 277-283.Students may be confused between amplitude and intensity. Modified Mercalli Intensity Scale of 1931: Seismological Society of America Bulletin, v. Geological Survey Professional Paper 1527, 418 p. Seismicity of the United States, 1568-1989 (Revised): U. Intensity Distribution and Isoseismal Maps for the Northridge, California, Earthquake of January 17,1994, USGS Open-File Report 95-92. Criteria based on such phenomena are downweighted now in assigning of USGS intensities (Stover and Coffman, 1993). Since 1931 ithas become clear that many phenomena that Wood and Neumann (1931) originally used as criteria to define the highest Modified Mercalli intensities (X and above) are related less to the level of ground shaking than to the presence of ground conditions susceptible to spectacular failure or to the ease with which seismic faulting of different style and depth can propagate to the ground surface. the unabridged scale is reproduced in Stover and Coffman (1993). The Modified Mercalli scale is given as originally abridged by Wood and Neumann (1931). Assigning of MM intensity values therefore involves use of the original criteria of Wood and Neumann (1931) with amendments and modifications that have been developed in the decades since 1931. Moreover, construction methods have changed appreciably since the scale was introduced. Experience with the MM scale in the decades since 1931 has shown that some criteria are more reliable than others as indicators of the level of ground shaking. 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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