earthquake intensity scales

One way of describing the size of an earthquake is by the amount of shaking at some particular location. To actually measure ground motion requires a seismometer, but ground motion can be estimated from effects like damage to buildings, or from events that people who experience a 'quake typically remember, such as the ringing of church bells, or even whether they felt the earthquake at all (seated persons typically notice earthquakes that people moving around do not). A scale can be constructed whose steps are defined by the kinds of damage and events that typically occur together. With such a scale, scientists arriving at the scene of an earthquake can assign a number to its intensity at that location by gathering witnesses' impressions.

Rossi-Forel Scale

In 1883 M. S. de Rossi and F. A. Forel published a 10-step intensity scale which was widely used in the 19th and early 20th centuries. Measurements on this scale are prefixed with the initials “R.F.”, followed by a roman numeral. Its steps roughly correspond to those of the Mercalli scale, except “R.F. X” lumps together steps 10 through 12 on the Mercalli scale.

Mercalli Scale

In 1902, Giuseppe Mercalli greatly improved the Rossi-Forel Scale, later increasing the number of steps to 12, and the intensity scale has since been known as the Mercalli Scale. Further improvements were made by A. Sieberg in 1923, Harry O. Wood and Frank Neumann in 1931, and Charles Richter in 1956. All of these are “Modified Mercalli scales.” The improvements made the descriptions less regional and more precise (instead of “buildings fall,” the newer scales say what kinds of buildings fall), and have refined the groupings to be sure they include events that actually occur together.

The initials “M.M.” (for Modified Mercalli) are often used before the step number. Intensities on the Mercalli scale are usually shown in Roman numerals, a convention worth preserving because it helps to distinguish intensity ratings from magnitude ratings.

The Modified Mercalli Scale is a good way of saying how bad it was at some particular location, but it isn't a good way of saying how strong the earthquake was in general. For example, intensity depends a great deal on the nature of the ground. An earthquake's intensity will be much greater in a town built on fill than in one built on granite. In 1985, an earthquake 300 km away caused catastrophic damage in Mexico City, but mainly in 15- to 25-story buildings. The buildings had a natural resonance at a period of around two seconds, and the geological conditions beneath the city picked up such waves from the quake and amplified them. If an earthquake's “size” is to be described without reference to location, some other technique must be used.

Modified Mercalli Scale, 1956 version
Intensity value Characteristics Richter
(note 1)
I Only detectable by seismographs. (note 2) <3.5
II Felt by persons at rest on upper floors or favorably placed. 3.5
III Felt indoors. Hanging objects swing. Vibration like passing of light trucks. Duration estimated. May not be recognized as an earthquake. 4.2
IV Hanging objects swing. Vibration like passing of heavy trucks. Windows, dishes, doors rattle. Parked cars rock. Glasses clink. Crockery clashes. In the upper range of IV, wooden walls and frames creak. 4.5
V Felt outdoors; direction estimated. Sleepers awakened. Liquids disturbed, some spilled. Small unstable objects displaced or upset. Doors swing, close, open. Shutters, pictures, move. Pendulum clocks stop; start, change rate. 4.8
VI Felt by all. Many frightened and run outdoors. Persons walk unsteadily. Windows, dishes, glassware broken. Knickknacks, books, etc. fall off shelves. Pictures fall off walls. Furniture moves or overturned. Weak plaster and masonry D cracked. Small bells ring (church, school). Trees, bushes sway visibly or are heard to rustle. 5.4
VII Difficult to stand. Noticed by drivers. Hanging objects quiver; furniture breaks; damage to masonry D, including cracks. Weak chimneys broken off at roof line. Fall of plaster, loose bricks, stones, tiles, cornices, unbraced parapets and architectural ornaments. Some cracks in masonry C. Waves on ponds, water turbid with mud. Small slides and caving in along sand and gravel banks. Large bells ring. Concrete irrigation ditches damaged. 6.1
VIII Steering of cars affected. Damage to masonry C and partial collapse; some damage to masonry B, none to masonry A. Fall of stucco and some masonry walls. Twisting, fall of chimneys, factory stacks, monuments, towers, elevated tanks. Frame houses moved on foundations if not bolted down; loose panel walls thrown out. Decayed piling broken off. Branches broken from trees. Changes in flow or temperature of springs and wells. Cracks in wet ground and on steep slopes. 6.5
IX General panic. Masonry D destroyed; masonry C heavily damaged, sometimes with complete collapse; masonry B seriously damaged. General damage to foundations. Frame structures, if not bolted down, shift off foundations. Frames racked. Serious damage to reservoirs. Underground pipes break. Conspicuous cracks in ground. In alluviated areas sand and mud ejected, earthquake fountains, sand craters. 6.9
X Most masonry and frame structures destroyed with their foundations. Some well-built wooden structures and bridges destroyed. Serious damage to dams, dikes, and embankments. Large landslides. Water thrown on banks of canals, rivers, lakes, etc. Sand and mud shifted horizontally on beaches and flat land. Rails bent slightly. 7.3
XI Rails bent greatly. Underground pipelines completely out of service. 8.1
XII Damage nearly total. Large rock masses displaced. Lines of sight and level distorted. Objects thrown into the air. >8.1

Masonry A: shows good workmanship, mortar and design; reinforced, especially laterally, and bound together using steel, concrete, etc.; designed to reduce lateral forces.

Masonry B: Good workmanship and mortar; reinforced, but not designed in detail to resist lateral forces.

Masonry C: Ordinary workmanship and mortar; no extreme weaknesses like failing to tie in at corners, but neither reinforced nor designed against horizontal forces.

Masonry D: Weak materials, such as adobe; poor mortar; low standards of workmanship; weak horizontally.

From: Elementary Seismology, by Charles F. Richter. Copyright © 1958 by W. H. Freeman and Company. Reprinted with permission.

Notes

1. Any Richter scale equivalent to a Mercalli intensity is at best approximate, since the two scales measure different things. The number given is an estimate of the Mercalli intensity experienced at or near the epicenter of the earthquake.

2. Intensity I is assigned to some quakes that actually were detected by people, if the people were very far from the epicenter and could feel the quake only because of exceptionally favorable circumstances. In 1964 a few people in Seattle, for example, felt the Alaska earthquake because they were up in the Space Needle, and some people in Houston skyscrapers felt the 1985 Mexican earthquake.

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