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Slide from Supercourse Library - http://www.pitt.edu/~super1/lecture/lec32361/009.htm
Intensity is a measure of how severe the shaking was at a particular location and is determined by classifying the degree of shaking severity as measured by an intensity scale.
Modified Mercalli (MM) Scale Categories:
I. Felt only by a very few people under especially favorable circumstances
II. Felt only by a few people at rest, especially on the upper floors of buildings. Suspended objects may swing.
III. Felt quite noticeably indoors. Standing motor vehicles may rock slightly. Vibration like the passing of a truck.
IV. Felt indoors by many, outdoors by a few. At night, some awakened. Crockery, glassware, windows, doors rattle.
V. Felt by nearly everyone; damage to contents and structures uncommon but possible.
VI. Felt by all; many frightened and run outdoors; damage slight.
VII. Everybody runs outdoors; damage negligible to buildings seismically well-designed and constructed; slight to moderate to ordinary structures; considerable damage to poorly built or badly designed structures.
VIII. Damage slight in well-designed, considerable in ordinary, and great in poorly built structures; chimneys, monuments, walls, etc., fall.
IX. Damage considerable to well-designed structures, and great (including partial or complete collapse) in other buildings; buildings shifted off foundations; underground pipelines disrupted.
X. Some well-built wooden structures destroyed; most masonry and ordinary structures destroyed; railroad tracks bent; landslides common; water spills over banks of streams, lakes, etc.
XI. Few, if any, masonry structures remain standing; bridges are destroyed; broad fissures open in the ground; underground pipelines are completely out of service; earth subsides.
XII. Damage is total; waves are seen propagating along surface of the ground; nearly impossible to stand; objects thrown up into the air.