Earthquake Intensity

Intensity is a word used in reference to the effect of an earthquake on buildings, objects and people. For example, at a particular location, an earthquake might be strong enough to damage buildings severely, moving them off of their foundations and throwing their walls out of square and plumb. This description gives you some idea that the earthquake had a very powerful effect at this location.

At another location the effects of the same earthquake could be very different. At this location, buildings are not damaged, people are startled and objects are knocked from shelves and tables. The intensity at this location is much lower, probably because the epicenter is farther away.

Over the past few hundred years several scales have been developed to rate the intensity of earthquakes. The one in most common use today in the United States is the Modified Mercalli Intensity Scale. This scale was developed in 1931 by Harry Wood and Frank Neumann. It has twelve steps of increasing intensity which are designated by Roman numerals. A brief version of this scale, published by the National Earthquake Information Center, is given below.

I. Not felt except by a very few 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 of buildings, but many people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibration like passing of truck. Duration estimated.

IV. During the day felt indoors by many, outdoors by few. At night, some awakened. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rocked noticeably.

V. Felt by nearly everyone; many awakened. Knocked pictures against wall, or swung them out of place. Moved small objects and furniture, the latter to slight extent. 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.

VI. Felt by all; many frightened and run outdoors. Persons made to move unsteadily. Broke dishes, glassware, in considerable quantity, also some windows. Fall of knickknacks, books, pictures. Overturned furniture in many instances. 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 motorcars.

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. Moved conspicuously very heavy furniture. Sand and mud ejected in small amounts. Changes in well water.) Persons driving motor cars disturbed.

IX. 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.

X. Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations; ground badly cracked. Rails bent. Landslides considerable on riverbanks 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.