1732 Intensity

In general, seismic intensity is a metric of the effect, or the strength, of an earthquake hazard at a specific location. While the term can be generically applied to engineering measures such as peak ground acceleration (PGA), it is usually reserved for qualitative measures of location-specific earthquake effects, based on observed human behavior and structural damage. Numerous intensity scales developed in preinstrumental times — the most common in use today are the modified Mercalli (MMI) (Wood and Neumann 1931), Rossi-Forel (R-F), Medvedev-Sponheur-Karnik (MSK-64 1981; Grunthal 1998) and its successor the European Macroseismic Scale (EMS-98 1998), and Japan Meteorological Agency (JMA) (Kanai 1983) scales.

Modified Mercalli Intensity (MMI) is a subjective scale defining the level of shaking at specific sites on a scale of I to XII. (MMI is expressed in Roman numerals, to connote its approximate nature.) For example, moderate shaking that causes few instances of fallen plaster or cracks in chimneys constitutes MMI VI. It is difficult to find a reliable relationship between magnitude, which is a description of the earthquake's total energy level, and intensity, which is a subjective description of the level of shaking of the earthquake at specific sites, because shaking severity can vary with building type, design and construction practices, soil type, and distance from the event (Table 17.2).

Note that MMI X is the maximum considered physically possible due to ''mere'' shaking, and that MMI XI and XII are considered due more to permanent ground deformations and other geologic effects than to shaking.

TABLE 17.2 Modified Mercalli Intensity Scale of 1931 (after Wood and Neumann 1931)

I Not felt except by a very few under especially favorable circumstances

II Felt only by a few persons at rest, especially on upper floors of buildings. Delicately suspended objects may swing

III Felt quite noticeably indoors, especially on upper floors of buildings, but many people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibration like passing track. Duration estimated

IV During the day felt indoors by many, outdoors by few. At night some awakened. Dishes, windows, and doors disturbed; walls make creaking sound. Sensation like heavy truck striking building. Standing motorcars rock noticeably

V Felt by nearly everyone; many awakened. Some dishes, windows, etc., broken; a few instances of cracked plaster; unstable objects overturned. Disturbance of trees, poles, and other tall objects sometimes noticed. Pendulum clocks may stop

VI Felt by all; many frightened and run outdoors. Some heavy furniture moved; a few instances of fallen plaster or damaged chimneys. Damage slight

VII Everybody runs outdoors. Damage negligible in buildings of good design and construction slight to moderate in well-built ordinary structures; considerable in poorly built or badly designed structures. Some chimneys broken. Noticed by persons driving motor cars

VIII Damage slight in specially designed structures; considerable in ordinary substantial buildings, with partial collapse; great in poorly built structures. Panel walls thrown out of frame structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned. Sand and mud ejected in small amounts. Changes in well water. Persons driving motor cars disturbed

IX Damage considerable in specially designed structures; well designed frame structures thrown out of plumb; great in substantial buildings, with partial collapse. Buildings shifted off foundations. Ground cracked conspicuously. Underground pipes broken

X Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations; ground badly cracked. Rails bent. Landslides considerable from river banks and steep slopes. Shifted sand and mud. Water splashed over banks

XI Few, if any (masonry), structures remain standing. Bridges destroyed. Broad fissures in ground.

Underground pipelines completely out of service. Earth slumps and land slips in soft ground. Rails bent greatly

XII Damage total. Waves seen on ground surfaces. Lines of sight and level distorted. Objects thrown upward into the air

TABLE 17.3 Comparison of Modified Mercalli (MMI) and Other Intensity Scales
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