A scale of seismic intensity is a way of measuring or rating the effects of an earthquake at different sites. The Modified Mercalli Intensity Scale is commonly used in the United States to seek information on the severity of effects of an earthquake . Intensity ratings are expressed as Roman numerals between I at the low end and XII at the high end.
The Intensity Scale differs from the Richter Magnitude Scale in that the effects of any one earthquake vary greatly from place to place, so there may be many Intensity values (e.g.: IV, VII) measured from one earthquake. Each earthquake, on the other hand, should have just one Magnitude, although the several methods of estimating it will yield slightly different values (e.g.: 6.1, 6.3).
The rating the Intensity of the effects an earthquake does not
require any instrumental measurements. Thus newspaper accounts, diaries,
and other historical records can be used to make intensity ratings of past
earthquakes, for which there are no instrumental recordings. Such research
helps promote the understanding of the earthquake history of a region,
and estimate future hazards.
| Scale | ||
|---|---|---|
| Mercalli | Richter | Description |
| I | 0-4.3 | Vibrations are recorded by intruments. People do not feel any Earth movement. |
| II | People at rest upstairs notice shaking. A few people might notice movement if they are at rest and/or on the upper floors of tall buildings. | |
| III | Shaking felt indoors; hanging objects swing. Many people indoors feel movement. Hanging objects swing back and forth. People outdoors might not realize that an earthquake is occurring. | |
| IV | 4.3-4.8 | Dishes rattle; standing cars rock; trees shake. Most people indoors feel movement. Hanging objects swing. Dishes, windows, and doors rattle. The earthquake feels like a heavy truck hitting the walls. A few people outdoors may feel movement. Parked cars rock. |
| V | Doors swing; liquid spills from glasses; sleepers awake. Almost everyone feels movement. Sleeping people are awakened. Doors swing open or close. Dishes are broken. Pictures on the wall move. Small objects move or are turned over. Trees might shake. Liquids might spill out of open containers. | |
| VI | 4.8-6.2 | People walk unsteadily; windows break; pictures fall off walls. Everyone feels movement. People have trouble walking. Objects fall from shelves. Pictures fall off walls. Furniture moves. Plaster in walls might crack. Trees and bushes shake. Damage is slight in poorly built buildings. No structural damage. |
| VII | Dificult to stand; plaster,bricks, and tiles fall; large bells ring. People have difficulty standing. Drivers feel their cars shaking. Some furniture breaks. Loose bricks fall from buildings. Damage is slight to moderate in well-built buildings; considerable in poorly built buildings. | |
| VIII | 6.2-7.3 | Car steering affected; chimneys fall; branches break; cracksin wet ground. Drivers have trouble steering. Houses that are not bolted down might shift on their foundations. Tall structures such as towers and chimneys might twist and fall. Well-built buildings suffer slight damage. Poorly built structures suffer severe damage. Tree branches break. Hillsides might crack if the ground is wet. Water levels in wells might change. |
| IX | General panic; damage to foundations; sand and mud bubble from ground. Well-built buildings suffer considerable damage. Houses that are not bolted down move off their foundations. Some underground pipes are broken. The ground cracks. Reservoirs suffer serious damage. | |
| X | Most buildings destroyed*; large landslides; water thrown out of rivers. Most buildings and their foundations are destroyed. Some bridges are destroyed. Dams are seriously damaged. Large landslides occur. Water is thrown on the banks of canals, rivers, lakes. The ground cracks in large areas. Railroad tracks are bent slightly. | |
| XI | 7.3-8.9 | Railway tracks bend; roads break up; large cracks appear in ground; rocks fall. Most buildings collapse. Some bridges are destroyed. Large cracks appear in the ground. Underground pipelines are destroyed. Railroad tracks are badly bent. |
| XII | Total destruction; "waves" seen on ground surface; river courses altered; vision distorted. Almost everything is destroyed. Objects are thrown into the air. The ground moves in waves or ripples. Large amounts of rock may move. | |
| Severity | Scale | |
|---|---|---|
| Mercalli | Richter | |
| Mild | I-III | 0-4.3 |
| Moderate | IV-V | 4.3-4.8 |
| Intermediate | VI-VII | 4.8-6.2 |
| Severe | VIII-X | 6.2-7.3 |
| Catastrophic | XI-XII | 7.3-8.9 |
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