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Visual Glossary

Birth of an earthquake: fault, focus, epicenter
Aftermath of the 1989 Loma Prieta earthquake. A search and rescue team, Marina District. Photo by J.K. Nakata, U.S.G.S.

Earthquake intensity
Modified Mercalli Intensity Scale

spacer image There are two basic ways to measure the strength of an earthquake; magnitude and intensity. Magnitude measures the total amount of energy released in an earthquake. Intensity measures of the effect of an earthquake on buildings and reactions of people. No instruments are used to determine intensity, just interviews with people in the affected area and observations of damaged structures. It is a useful measure in built-up urban areas, but not so useful in remote areas without any buildings to damage or people to react!
spacer image Intensity levels range from not felt (I) to total destruction (XII). Generally the larger the earthquake, the larger the area affected and the higher the maximum intensity.
spacer image People living in earthquake country like to use the modified Mercalli intensity scale (shown below) to gauge the effects of an earthquake in their neighborhood.

Modified Mercalli Intensity Scale

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What happens to people and structures

I Not felt except by a very few persons 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, but many people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibration like passing truck.
IV During the day, felt in doors by many, outdoors by few. At night, some awakened, Dishes, windows, doors disturbed; walls make creaking sound. Sensation like heavy truck striking building. Standing motor cars rocked noticeably.
V Felt by nearly everyone; many awakened. Some dishes, windows, etc., broken; a few instances of cracked plaster; unstable objects overturned. Disturbances 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 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 levels. Disturbs persons driving motor cars.
IX Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb; damage 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.
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This page was last updated on 9/6/00