PART 2
Earthquakes Intensity: Modified Mercalli Scale
The first popular scale used to measure the strength of an earthquake
was developed in Europe in the 1880's by Italian seismologist De Rossi
and Swiss seismologist Forel. The Rossi-Ferel scale, with values from Roman
numerals I to X, was widely used for about two decades as a means of investigating
earthquakes and comparing the effects of various shocks throughout the
world. The scale's main defect was that it lumped a great deal of major
damage under classification X.
In 1902, the Italian seismologist Mercalli set up a new scale, with an expanded range from I to XII. This new scale allowed for a more refined analysis of major damage. The Mercalli Scale was updated in 1931 by two American seismologists, Harry O. Wood and Frank Neumann, to take into account modern features such as tall buildings, automobiles, underground pipes and utility lines. This " Modified Mercalli" scale, frequently abbreviated to "MM," is still used today. In the version printed here, the language has been slightly changed, but the basic ideas are the same as those used by professional seismologists to rate earthquakes. Keep in mind that conditions such as construction materials and nature of the bedrock or surficial materials will determine effects to some degree. The Modified Mercalli scale is based on observed damage, and the levels (I to XII) are called intensities.
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No noticeable effects. |
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Generally not felt by people, but dizziness or nausea may result. Animals, especially birds, may be uneasy. Trees, structures, liquids, bodies of water may sway gently, doors may swing very slowly. |
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May be felt indoors, especially on upper floors of multistory buildings and by sensitive or nervous persons. Animals, especially birds, are disturbed. Trees, structures, liquids and bodies of water may sway. Hanging objects may swing. |
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May be felt indoors by several people, usually as a rapid vibration that may not be recognized as an earthquake at first (similar to vibrations due to passing of a light truck or a heavy truck some distance away). Duration may be estimated. Movements may be appreciable on upper levels of tall structures. Standing vehicles may rock slightly. |
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Felt indoors by many, outdoors by few. May awaken light sleepers. Vibration like that due to passing of heavy trucks. Sensation like a heavy body striking the building or falling of heavy objects inside. Dishes, glassware, and windows rattle, and doors may creak, especially if the intensity is in the upper end of the range. Hanging objects often swing. Liquids in open vessels are disturbed slightly. Stationary vehicles rock noticeably. |
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Felt indoors by practically everyone, outdoors by most people. Direction to origin can often be estimated by those outdoors. Awakens most sleepers. Buildings tremble throughout. Dishes and glassware may break. Some windows may crack. Vases and small or unstable objects overturn and may fall. Generally, hanging objects and doors swing considerably. Pictures knock against walls or swing out of place. Doors and shutters open or close abruptly. Pendulum clocks stop or run erratically. Furniture may shift slightly. Small amounts of liquid spill from filled, open containers. Trees and bushes shake slightly. |
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Felt by everyone. Awakens all sleepers. Produces general excitement. Persons move unsteadily. Trees and bushes shake slightly to moderately. Liquids are set in strong motion. Small bells in churches and schools may ring. Poorly built buildings may be damaged. Plaster cracks and may fall in small amounts. Many dishes and glasses and a few windows break. Small objects, books and pictures fall. furniture moves and may be overturned. |
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General alarm. People find it difficult to stand. Drivers notice shaking in vehicles. Trees and bushes shake moderately to strongly. Waves form on ponds, lakes and streams. Water is muddied. Gravel and sand stream banks cave in. Large church bells ring. Suspended objects quiver. Damage is negligible in buildings of good design and construction; slightly to moderate in well-built ordinary buildings; considerably in poorly built or badly designed buildings, adobe houses, old walls, spires, etc. Plaster and some stucco fall. Many windows and some furniture break. Bricks and stones are dislodged, loosened brickwork and tiles shake down. Weak chimneys break at the roof line. Cornices fall from towers and high buildings. Heavy furniture overturns. Concrete irrigation ditches are considerably damaged. |
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General fright, alarm approaches panic. Trees shake strongly, branches and trunks break off. Sand and mud erupt in small amounts. Flow of springs and wells is temporarily (sometimes permanently) changed. Dry wells renew flow. Temperature of springs and well water varies from normal. Damage slight in brick structures built especially to withstand earthquakes; considerable in ordinary substantial buildings, with some partial collapse; heavy in some wooden houses, with some collapsing. Panel walls break away in frame structures. Decayed pilings break off. Walls fall. Solid stone walls crack and break seriously. Wet ground and steep slopes crack to some extent. Chimneys, columns, monuments and factory stacks and towers twist and fall. Heavy furniture moves conspicuously or overturns. |
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General panic. Ground cracks conspicuously. Damage is considerable in masonry structures built especially to withstand earthquakes; great in other masonry buildings, some collapse. Some wood-frame houses built to withstand earthquakes are thrown out of plumb, others are shifted wholly off foundations. Reservoirs are seriously damaged. Underground pipes sometimes break. |
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Ground, especially when loose and wet, cracks up to widths of several inches; fissures up to a meter in width run parallel to canal and stream banks. Land sliding is considerable from river banks and steep coasts. Sand and mud shift horizontally on beaches and flat land. Water level changes in wells. Water is thrown onto banks of canals, lakes and rivers. Dams, dikes, embankments are seriously damaged. Well-built wooden structures and bridges are severely damaged, and some collapse. Dangerous cracks develop in strong brick walls. Most masonry and frame structures, and their foundations are destroyed. Railroad rails bend slightly. Buried pipe lines tear apart or are crushed endwise. Open cracks and broad wavy folds open in cement pavements and asphalt road surfaces. |
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Disturbances in ground are many and widespread, varying with the ground material. Broad fissures, slumps and landslides develop in soft, wet ground. water charged with sand and mud is ejected in large amounts. Sea waves of significant magnitude may develop. Damage is severe to wood -frame structures and great to dams, dikes and embankments. Few masonry structures remain standing. supporting piers or pillars of large, well-built bridges are wrecked. Wooden bridges that "give" are less affected. Railroad rails bend greatly, and some are thrust endwise. Buried pipes are completely disrupted. |
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Damage is total, practically all works of construction are damaged greatly or destroyed. Disturbances in the ground are great and varied, numerous shearing cracks develop. Landslides, rock falls and slumps in river banks are numerous and extensive. Large rock masses are wrenched loose and torn off. Fault slips develop in firm rock, and horizontal and vertical offset displacements are notable. Water channels, both surface and underground, are disturbed and modified greatly. Lakes are dammed, new waterfalls are produced, rivers are deflected. Surface waves are seen on ground surfaces. Objects are thrown upward into the air. |