Earthquake Magnitude and Intensity
Magnitude and Intensity measure different characteristics of earthquakes. Magnitude measures the energy released at the source of the earthquake and is determined from measurements on seismographs. Intensity measures the strength of shaking produced by the earthquake at a certain location. Intensity is determined from effects on people, human structures, and the natural environment. The effect of an earthquake on the Earth's surface is called the intensity. The Modified Mercalli (MM) Intensity Scale, developed in 1931 by the American seismologists Harry Wood and Frank Neumann, is composed of 12 increasing levels of intensity that range from imperceptible shaking to catastrophic destruction (designated by Roman numerals)
The following table gives intensities that are typically observed at locations near the epicenter of earthquakes of different magnitudes. More information is available through through the National Earthquake Information Center.
| Magnitude | Intensity | Description |
|---|---|---|
| 1.0 - 3.0 | I | I. Not felt except by a very few under especially favorable conditions. |
| 3.0 - 3.9 | II - III | II. Felt only by a few persons at rest, especially
on upper floors of buildings.
III. Felt quite noticeably by persons indoors, especially on upper floors of buildings. Many people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibrations similar to the passing of a truck. Duration estimated. |
| 4.0 - 4.9 | IV - V | IV. Felt indoors by many, outdoors by few during
the day. 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. Some dishes, windows broken. Unstable objects overturned. Pendulum clocks may stop. |
| 5.0 - 5.9 | VI - VII | VI. Felt by all, many frightened. Some heavy
furniture moved; a few instances of fallen plaster. Damage slight.
VII. Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken. |
| 6.0 - 6.9 | VII - IX | VIII. Damage slight in specially designed structures;
considerable damage in ordinary substantial buildings with partial
collapse. Damage great in poorly built structures. Fall of chimneys,
factory stacks, columns, monuments, walls. Heavy furniture overturned.
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. |
| 7.0 and higher |
VIII or higher |
X. Some well-built wooden structures destroyed;
most masonry and frame structures destroyed with foundations. Rails
bent.
XI. Few, if any (masonry) structures remain standing. Bridges destroyed. Rails bent greatly. XII. Damage total. Lines of sight and level are distorted. Objects thrown into the air. |