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There are two primary methods used to measure earthquakes. The earlier method is known as the Modified Mercalli Intensity Scale. This scale was invented by Giuseppe Mercalli in 1902. It has a graduated scale from 1 to 12 and is based on peoples' visual and physical observations at the time of the event. The levels of the Modified Mercalli Intensity Scale are as follows:
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Intensity
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Descriptive Name
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Effects
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Richter Scale
Comparison
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I
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Instrumental
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Not felt except by a very few under especially favorable circumstances. Recordable only with a seismograph.
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1 to 2
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II
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Feeble
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Felt only by a few persons at rest, especially on upper floors of buildings. Delicately suspended objects may swing.
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2 to 3
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III
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Slight
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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 a passing truck. Duration estimated.
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3 to 4
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IV
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Moderate
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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 a heavy truck striking building. Standing motor cars rock noticeably.
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4
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V
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Rather Strong
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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.
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4 to 5
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VI
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Strong
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Felt by all; many frightened and run outdoors. Some heavy furniture moved; a few instances of fallen plaster or damaged chimneys. Damage is light.
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5 to 6
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VII
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Very Strong
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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.
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6
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VIII
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Destructive
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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.
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6 to 7
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IX
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Ruinous
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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.
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7
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X
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Disastrous
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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.
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7 to 8
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XI
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Very Disastrous
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Few, if an (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.
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8
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XII
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Catastrophic
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Damage total. Waves seen on ground surfaces. Lines of sight and level distorted. Objects thrown upward into the air.
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> 8
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As you can see by reading the described effects in the table above, this method of "measuring" earthquake intensity is inaccurate primarily because it is so subjective. It relies on visual observations from untrained people who may exaggerate greatly. Even if it were totally objective, it would still not be satisfactory because it is based primarily upon observations of things going on in a populated area. For example, out in the desert there are no trees, furniture, or buildings to use to estimate the strength of the shock. There may not even be any people there to act as observers. So this method is not useful in remote areas.
68k jpeg
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The great Alaska earthquake of March 27, 1964 was felt over an area of 500,000 square miles. The shock snapped off the tops of trees near the epicenter and killed 114 people, some as far away as California.
Click photo for larger version.
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45k jpeg
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The 1906 San Francisco earthquake and fire killed 700 people, destroyed most of the city, and left 250,000 people homeless. These unfortunate horses were killed by a collapsing building.
Click photo for larger version.
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Definitions and Introduction | The Modified Mercalli Intensity Scale | The Richter Magnitude Scale | Colorado's Seismic Risk | Secondary Hazard Events | Survival Tips
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