The “size” of earthquakes is commonly expressed in two ways - magnitude and intensity.
Magnitude is a measure of the total energy released during an earthquake. It is determined from a seismogram, which plots the ground motion produced by seismic waves. As devised by C.F. Richter in 1935, the magnitude scale allows us to compare earthquakes in relative terms.
Though the term is used somewhat indiscriminately, magnitude is a highly technical calculation. Magnitude is defined as the logarithm (to the base 10) of the maximum wave-trace amplitude expressed in microns (1 micron = .0001 centimeter), as would be recorded by a standard short-period seismograph with specific constants, at an epicentral distance of 100 kilometers (62 miles).
The important thing to remember about magnitude is that the scale is logarithmic, which means that each step in magnitude represents a tenfold increase in amplitude of wave motion. Therefore, an earthquake of magnitude 6.0 has ten times the wave amplitude of an earthquake of magnitude 5.0, a hundred times the wave amplitude of a magnitude 4.0 earthquake, and one thousand times the wave amplitude of a magnitude 3.0 earthquake.
Because magnitude does not describe the extent of the damage, its usefulness is limited to an approximation of whether the earthquake is large, small, or medium sized. The destructiveness of an earthquake is a complex matter, related to the geology, population density, and cultural features of a specific area at a specific distance from the epicenter.
Seismologists and geologists also describe earthquakes by their intensity. Measured on a numerical scale, intensity is the degree of damage or the observable effects caused by an earthquake at a particular location. An earthquake of a particular magnitude will produce different intensities at different places, according to geology, population density, cultural features, and distance from the epicenter.
The most widely used intensity scale, the Modified Mercalli Scale, is divided into 12 degrees, each identified by a Roman numeral. For example, an earthquake intense enough to be felt by a person standing nearby has an intensity of MM III.
The relationship between Modified Mercalli intensity and Richter magnitude is as follows:
MM I (Richter 0.00 - 2.17) not felt or rarely felt under favorable circumstances. Sometimes, under certain conditions: trees, structures, liquids, and bodies of water sway; dizziness or nausea may be experienced; doors swing very slowly; birds and animals are uneasy or disturbed.
MM II (Richter 2.18 - 2.78) felt indoors by a few persons, especially on upper floors, or by sensitive or nervous persons. Sometimes: hanging objects swing; trees, structures, liquids and bodies of water sway; dizziness or nausea may be experienced; doors swing very slowly.
MM III (Richter 2.79 - 3.38) felt indoors by a number of people. Motion is usually a rapid vibration, and sometimes vibrations are not at first recognized as an earthquake; movement is significant on upper levels of tall structures; standing vehicles rock slightly; hanging objects swing.
MM IV (Richter 3.39 - 4.00) felt indoors by many and outdoors by a few. A few people awaken, especially light sleepers; vibrations feel like those of a heavily loaded truck passing by; dishes, windows, and doors rattle; walls and frames of structures creak; liquids in open vessels are slightly disturbed; standing vehicles rock noticeably.
MM V (Richter 4.01 - 4.62) felt indoors and outdoors by many or most people. Outdoors, the direction of the earthquake could be estimated. Buildings tremble; dishes and glassware break; small or unstable objects overturn and may fall; doors and shutters open or close abruptly; small objects move and furnishings move slightly; liquids in well- filled open containers spill slightly.
MM VI (Richter 4.63 - 5.20) felt by all people indoors and outdoors. People move unsteadily; some plaster cracks, and fine cracks appear in chimneys; dishes, glassware, and windows break; knick-knacks, books, and pictures fall; some furniture overturns; moderately heavy furniture moves.
MM VII (Richter 5.21 - 5.81) all people are frightened and run outdoors, general alarm. Many people find it difficult to stand; water is stirred and muddied; some sand and gravel stream banks cave in; chimneys crack considerably, and walls crack somewhat; plaster and stucco fall in considerable to large amounts; loosened bricks and tiles fall.
MM VIII (Richter 5.82 - 6.42) people are generally frightened, with alarm approaching panic. Persons driving vehicles are disturbed; trees shake strongly and branches break off; sand and mud are ejected from earth in small amounts; temporary and permanent changes occur in springs and wells; ground becomes wet to some extent, even on steep slopes; chimneys, columns, monuments, and towers fall.
MM IX (Richter 6.43 - 7.04) general panic. Ground cracks conspicuously; masonry structures are thrown out of plumb; large parts of well-built masonry buildings collapse; some buildings shift off foundations, and frames crack; reservoirs are seriously damaged; some underground pipes break.
MM X (Richter 7.05 - 7.64) ground cracks at widths up to several inches. Parallel to canals and streams, fissures form up to a yard wide. Numerous landslides occur on river banks and steep coasts; dams, dikes, and embankments are seriously damaged; most masonry and frame structures are destroyed; buried pipelines are torn apart or crushed; cracks and broad, wavy folds open in concrete pavements and asphalt road surfaces.
MM XI (Richter 7.65 - 8.23) disturbances in the ground are many and widespread, varying with ground material. Broad fissures, landslides, and liquefaction occur; water, sand and mud is ejected from the earth in large amounts; dams, dikes, and embankments are greatly damaged; few masonry structures remain standing; large, well-built bridges are destroyed; railroad rails are greatly bent and thrust endwise.
MM XII (Richter 8.24 - up ) damage is total, and nearly all works of construction are greatly damaged or destroyed. Landslides, and numerous shearing cracks appear; large rock masses are wrenched loose and torn off; lakes are dammed, waterfalls form, and rivers are deflected; waves are seen on ground surfaces; lines of sight and level are distorted; objects are thrown upward into the air.
This article adapted from Vol. 1, No. 3, p. 14.