An earthquake intensity scale - The Modified Mercalli Scale

The intensity and magnitude of an earthquake are not the same thing, although related by cause and effect. The magnitude of an event that triggers and earthquake results in shaking of particular intensities.

The Modified Mercalli Scale associates values in Roman numerals, on a scale from I-XII, with specific effects on people and structures that occur with increasing intensities of shaking.

Unlike magnitudes, which are assigned a single value for each earthquake event, intensity is a characteristic that takes on a range of values around each earthquake epicenter.

Higher magnitude earthquakes tend to have higher intensities at their epicenters, but intensities always drop off to less than detectable levels at some distance. Unlike magnitude which describes the initial release of energy, intensity depends on what an earthquake wave happens to encounter as it moves away from an epicenter.

Actual intensities vary not only with distance, but depend on the:
1. Qualities of the intervening ground,
2. Direction in which earthquake waves pass through,
3. Qualities of buildings and other structures encountered
Intensity is an important measure because it describes shaking or damage at a particular place - the place you might be, or a place you hope to remain standing after an earthquake event.

Several scales exist, but two commonly used ones in the United States are the Modified Mercalli scale and the Rossi-Forel scale. ShakeMaps produced by the U.S. Geological Survey show intensities associated with recent earthquakes.

Modified Mercalli Scale
Source: U.S.Geological Survey

I Not felt except by a very few under especially favorable conditions.

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.

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.

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.

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.

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.