Mercalli Earthquake Intensity Scale

2015 Nepal earthquake damage
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The Modified Mercalli Intensity Scale of 1931 is the basis for the U.S. evaluation of seismic intensity. Intensity is different than the magnitude in that it is based on observations of the effects and damage of an earthquake, not on scientific measurements. This means that an earthquake may have different intensities from place to place, but it will only have one magnitude. In simplified terms, magnitude measures how big an earthquake is while intensity measures how bad it is.

The Mercalli Scale

The Mercalli scale has 12 divisions, using Roman numerals from I to XII.

  • 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 motorcars may rock slightly. Vibration like passing truck. Duration estimated.
  • IV. During the day felt indoors by many, outdoors by few. At night some awakened. Dishes, windows, and doors disturbed; walls make a creaking sound. Sensation like heavy truck striking building. Standing motorcars rock noticeably.
  • V. 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.
  • 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 motor 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. 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. The 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 landslips 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.

From Harry O. Wood and Frank Neumann, in Bulletin of the Seismological Society of America, vol. 21, no. 4, December 1931.

Although the correlation between magnitude and intensity is weak, the USGS has made a pretty good estimate of the intensity that might be felt near the epicenter of an earthquake of a specific magnitude. It is important to reiterate that these relationships are by no means precise:

Magnitude Typical Mercalli Intensity
Felt Near Epicenter
1.0 - 3.0 I
3.0 - 3.9 II - III
4.0 - 4.9 IV - V
5.0 - 5.9 VI - VII
6.0 - 6.9 VII - IX
7.0 and greater VIII and greater
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Your Citation
Alden, Andrew. "Mercalli Earthquake Intensity Scale." ThoughtCo, Aug. 25, 2020, Alden, Andrew. (2020, August 25). Mercalli Earthquake Intensity Scale. Retrieved from Alden, Andrew. "Mercalli Earthquake Intensity Scale." ThoughtCo. (accessed June 11, 2023).