Earthquake magnitudes and intensities

The measurement of earthquake intensity, based on observed effects and their magnitude, calculated with instrument readings, is useful not only for scientists who study and predict earthquakes but also for land-use planning and other aspects of public policy.

Intensity Scales

Magnitude is a numerical rating of the size or strength of an earthquake, based on instrument readings. Intensity is a different kind of numerical rating having to do with the actual effects of an earthquake on people, buildings, and the landscape. Magnitude rating values allow comparison between earthquakes on a worldwide basis, whereas intensity ratings are more useful for comparing relative effects in areas surrounding the epicenter.

Because only human judgment is required for an intensity rating, intensity scales were developed first. Many different scales of earthquake effects have been devised in different countries since Renaissance times. The earliest known scale was developed in Italy and had only four rating values. The earliest widely accepted intensity scale, in use after 1883, was the Rossi-Forel scale, which had ten value ratings. In the United States, a revised European scale, the 1931 modified Mercalli scale, has become the standard. This scale was modified by Giuseppe Mercalli in 1884, and after several further revisions was expanded to a twelve-value scale. The values of intensity range from I, which are barely felt, to XII, which are the most violent. Each earthquake has a zone of maximum intensity, surrounded by zones of successively lesser intensity.

Intensity Ratings

Intensity I on the modified Mercalli scale are felt by very few people. This level of earthquakes may still trigger nausea or dizziness if they occur in the marginal zone of a large earthquake. In an area experiencing intensity II effects, ground vibration may be felt by some people at rest, especially on upper floors, where building motion may exaggerate ground motion. Regions experiencing level III intensity are characterized by a brief period of vibration like that of a passing loaded truck. Many do not recognize it as an earthquake. Zones of intensity IV are felt indoors by many, but only a few outdoors. Buildings may shudder slightly; windows and doors of older homes may rattle, and glassware in cupboards may clink. In an intensity V earthquake, which is widely felt, people may be awakened and frightened. Windows and glassware may break, and cracks may appear in plaster.

Intensity VI earthquakes are felt by everyone, can lead to frightening of the general public, and result in panic. Plaster and brick chimneys may be damaged, and furniture relocated. Objects fall from shelves, and trees visibly shake. An intensity VII earthquake brings strong shaking that may last for many seconds, causing considerable damage to older brick buildings and slight to moderate damage to well-built wood- or steel-frame structures. These tremors are detectable by persons driving vehicles. In an intensity VIII earthquake, damage to buildings is considerable. Specially designed earthquake-resistant structures may hold up, but many older brick buildings may totally collapse. Branches and trunks of trees may break. Intensity IX earthquakes cause damage to most buildings. This can range from collapse to being thrown off its foundations, or out of plumb. Conspicuous ground cracks appear and buried water and gas pipes break. Intensity X earthquakes cause most buildings to collapse at least partially, but some can be totally destroyed. Railroad tracks are bent, and buried pipes buckle or break. Landsliding along riverbanks and steep slopes is triggered and obvious ground cracks are widespread. Strong shaking may last for tens of seconds. After intensity XI quakes, few structures remain standing. Broad fissures appear in the ground. The earthquake may cause a large tsunami if it occurs near a coastal area. The strong shaking may last a minute or more. Finally, in an intensity XII earthquake, objects are thrown into the air. Waves are “frozen” on the ground surface. Fewer than a half-dozen earthquakes have been rated at this level of intensity.

Assigning Intensity Ratings

Assignment of the lower values on the modified Mercalli scale is possible only if people are present to experience the effects. In the middle and upper values, effects on structures are a primary basis for the assignment of ratings, although earthquakes of greatest intensity may produce long-term geologic effects on the landscape, including ground fissures, fault scarps (cliff-like features visible at the earth's surface), landslides, and sandblows (small volcano-like mounds of sediment that erupt from water-saturated ground as a result of severe shaking). Thus, earthquakes that occur in uninhabited areas of the earth cannot be rated unless the shaking was sufficiently strong to produce geologic effects; similarly, rating is impossible for quakes occurring below large areas of the ocean with few or no populated islands, although in some cases, intensity can be estimated if a tsunami is generated.

Because effects on buildings are an important means of differentiating between ratings in the middle and upper part of the scale, the nature of construction becomes an important differentiating criterion. In earthquake-prone California, for example, earthquake-resistant design practices mandated by law have made the average new building less susceptible to damage or destruction than older buildings. Unless this factor is considered, equal-sized earthquakes would be rated lower over time because of less damage as older buildings are replaced.

Isoseismal Lines

Despite such problems, the modified Mercalli scale is still quite useful. There are many more people around the world who can serve as a potential observer of earthquake effects than there are earthquake-recording instruments. Earthquake intensity ratings begin to be collected soon after earthquakes are large enough to be felt by more than a few people. A government agency such as the US Geological Survey sends questionnaires to people in the area where the earthquake was felt. The forms contain questions related to the specific location and activity of the observer at the time of the earthquake. It also queries on details of what happened before, during, and after the quake. The responses are then rated, using the modified Mercalli scale, and the ratings are plotted on a map. Lines separating the various values, or “isoseismal lines,” can then be drawn to show areas having equal intensity. Frequently an irregularly shaped bull's-eye pattern emerges with the highest rating zone in the middle. This zone of maximum intensity usually contains the earthquake's epicenter.

The size and shape of the pattern of isoseismal lines can be invaluable in land-use planning or zoning of areas that experience frequent earthquake activity. The pattern may give clues about the distribution of land that would make a poor foundation for buildings because of greater susceptibility to seismic shaking. Certain types of sediment overlying bedrock can amplify seismic vibrations.

The highest intensity value determined for a specific earthquake is only a rough indicator of the quake's real strength. Quite large shocks can occur at many hundreds of kilometers of depth, which are called deep-focus earthquakes. This term is also used because their most damaging vibrations are largely absorbed by rock before arriving at the surface, and thus, their maximum intensity ratings are low.

Magnitude: The Richter Scale

Unlike earthquake intensity determinations, which vary with distance from the epicenter and depend on factors such as the depth of focus and soil depth, the magnitude rating of an earthquake is reported as a single number that is normally calculated from an instrumental recording of ground vibrations. There is also an alternative method for magnitude determination known as moment magnitude. Every earthquake large enough to be detected and recorded by seismographs can be assigned a magnitude value. The Richter scale of earthquake magnitude was the first to be used widely. It is named for its originator, Charles F. Richter, professor of seismology at the California Institute of Technology.

When an earthquake occurs, it generates ground vibrations that radiate out to surrounding areas, much as ripples do when a small pebble is dropped into a quiet pond. A seismograph is an instrument designed to detect and record, over time, even very small ground vibrations. It does this by amplifying the motion of the ground on which the seismograph sits when seismic waves pass through it. For some seismographs, the amplified recording (seismogram) is simply a piece of paper wrapped around a cylindrical, clock-driven drum, on which the ground vibrations are recorded as a zigzag line drawn by a recording pen. Larger ground vibrations result in larger zigzags or displacement on the paper. The amount of displacement from the normal or average position is called the amplitude, and it can be measured accurately with a simple fine-scale ruler.

In 1935, Richter first published his method for determining the relative size of the many earthquakes for which he had recordings in southern California. Most of these were small and moderate-sized shallow-focus earthquakes. Eventually, after various modifications, the Richter scale came to be used around the world to measure and compare earthquakes.

Magnitude Ratings

Richter's original definition of the magnitude of an earthquake is as follows: “The magnitude of any shock is taken as the logarithm of the maximum trace amplitude, expressed in microns, with which the standard short-period [Wood-Anderson] seismometer … would register that shock at an epicentral distance of 100 kilometers.” Simply stated, the Richter scale magnitude of an earthquake is determined by measuring the greatest horizontal displacement from the average of the recording-pen tracing on the seismogram of a standard instrument at a standard distance from the earthquake epicenter (earthquake ground motions weaken as they travel out from the source). The width of the largest amplitude is measured in microns (thousandths of a millimeter). The logarithm of this number (to the base 10) is calculated and becomes the rating number “on the Richter scale.” If an earthquake produced a maximum recording-pen displacement of 1 centimeter, that would be equivalent to 10,000 microns (which is equal to 10⁴ microns). The logarithm of 10⁴ is 4 (the exponent); thus, the earthquake would be rated at 4 on the Richter scale. If the maximum displacement had been 10⁵ microns, it would be rated 5.

Because of the logarithmic nature of the Richter scale, each higher value in whole numbers represents a tenfold increase in the amount of ground motion recorded on the seismogram. Compared with an earthquake rated at 4, one rated at 5 involves 10 times more ground-shaking displacement; one rated at 6 would involve 100 times more displacement than 4, and 7 would be 1,000 times more. In other words, the largest earthquakes produce more than 1,000 times more ground motion than those that just begin to damage average American homes.

Another way to compare earthquakes is to determine the amount of energy they release. The energy released by earthquakes is about 30 times greater for each higher Richter scale value. For example, an earthquake rated at 6 would be about 900 times (30 H 30) more energetic than one rated at 4.

Other Magnitude Scales

Methods for magnitude determination have advanced substantially since the original magnitude scale was developed by Richter. His original scale was valid only for shallow-focus earthquakes occurring in the local region of southern California, because it depended on a Wood-Anderson type of seismograph. That instrument is “tuned” to pick up the higher-frequency vibrations typical of the small and medium-sized earthquakes of southern California. Later work by Richter and others extended his original scale (which could not be applied much above magnitude 6) for use with other instruments tuned to pick up earthquakes that occurred at greater distances (more than 1,000 kilometers from a seismograph station) and had a deeper focus. These magnitude extensions were designed to coincide, as much as possible, with the values of the original scale. The extensional scales, however, use bases that are different from those of the original. One extensional scale, for example, requires a seismograph tuned to pick up only low-frequency vibrations. As a result, magnitude values differ somewhat and are not strictly “on the Richter scale.” Technical literature limits the use of the term “Richter scale” to magnitude values determined essentially according to the original specifications.

For the very largest earthquakes, even the extensional scales become inadequate for ranking accurately the relative strength of earthquakes, because the sensitive instruments are said to become “saturated”—essentially thrown off scale. To solve this problem, a “moment magnitude” scale was developed; it is based not on instrument readings but on data obtained in the field, along the earthquake-generating fault. The average amount of fault offset, the length and width of slippage along the fault, and rock rigidity data are used to calculate the moment magnitude. Because of its greater accuracy, the moment magnitude scale is becoming more commonly used, particularly for medium-sized and larger earthquakes. Although the values of the moment magnitude scale essentially merge with those of the Richter scale for medium-sized earthquakes, at higher values there can be substantial differences. Thus, the largest earthquake rated by moment magnitude is 9.5.

Prediction of Earthquake Recurrence

A key element to the prediction of earthquake recurrence, particularly for the less frequent but larger and more destructive earthquakes, is a detailed record of the relative sizes of earthquakes occurring along a particular fault (or fault segment) through time. The most accurate and consistent indicator of size is the magnitude value. Because instruments needed for magnitude determination have been in existence for less than one hundred years, magnitudes of earlier events must be estimated. This can be done using several approaches.

Maximum intensity values correlate with different but known magnitude values in areas where the depth of focus is thought to be consistent through time and where the nature of bedrock absorption of seismic waves is known. For determining maximum intensity values for unrecorded earthquakes of the past, archives and historical documents sometimes yield useful data. For prehistoric earthquakes, newly developed techniques are proving successful in defining the occurrence of large earthquakes and, under the right circumstances, even of their relative sizes. Such old events may be judged by the nature and extent of geologic traces preserved in radiocarbon-datable buried sediments.

Another avenue of study of earthquake recurrence is based on the amount of stored-up energy that is released by earthquakes worldwide in a year. A curve of the energy release can be compared to the occurrences of every possible recurrent earthquake-triggering mechanism—for example, tidal forces. To determine the energy released, the moment magnitude must be determined for as many earthquakes as possible, but especially for the largest ones because they release much more energy overall than do the more numerous smaller ones. One study, for example, revealed a strong correlation between times of higher-than-average earthquake activity (around 1910 and again around 1960) and the extent of “wobbling” of the earth's axis of rotation.

Determination of Seismic Risk

In earthquake-prone areas, seismic risk must be considered in urban and regional planning. The effects of ground shaking at any location are dependent primarily on magnitude, distance from the source of the earthquake waves, and the nature of the bedrock and the type and thickness of materials above it. After historical patterns of earthquake recurrence are determined, particularly their characteristic (or most typical) size, it is possible to make estimates of probable intensity patterns in the vicinity of faults. Detailed maps have been prepared of the areas along the San Andreas fault and for many miles on either side of it, outlining the zones of greatest potential seismic risk. Such maps can be of great value when decisions are being made regarding sites for potential secondary earthquake hazards such as nuclear power plants, fuel storage depots, and dams.

MagnitudeIntensity (Mercalli)Description1.0-3.0INot felt except by a very few under especially favorable conditions.3.0-3.9II-IIIII 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.4.0-4.9IV-VIV 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.5.0-5.9VI-VIIVI 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.6.0-6.9VII-IXVIII. 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+X-XIX. Some well-built wooden structures were 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.

Detection of Underground Nuclear Explosions

Additionally, seismic detection and characterization of distant underground nuclear explosions is of considerable political importance. One method of attempting to discriminate between a nuclear explosion and a natural seismic event is by analysis of its magnitude as recorded by several types of seismographs, each “tuned” to pick up different frequencies of ground vibrations. Ratios between such magnitude values appear to be quite useful for this purpose.

Significance of Numbers on the Richter Scale

Nearly every time a news broadcast makes reference to a damaging earthquake somewhere in the world, a number on the Richter scale is mentioned to give the listener some idea of the relative size of the event. If the earthquake has happened in a remote part of the globe and has not caused significant damage, it becomes merely another of the many facts that are soon forgotten. If the earthquake has happened where one's relative or a friend lives, however, that number on the Richter scale becomes extremely important because it is one of the first available indicators of possible severity. It can be determined within minutes after earthquake waves have been detected at seismological observatories, whereas direct communication from and damage assessment at the site of the earthquake may be very slow in coming.

Value of Seismic Maps

Aspects of earthquake intensity are less likely to be mentioned in the media except, perhaps, when covering local aspects of seismic risk along a certain fault or the risk of earthquakes in various areas of the United States. Maps identifying zones of potential seismic hazard are likely to become more common as their need becomes more apparent. The lack of such knowledge and of the will to act on it could be costly in terms of lives and property.

One of the most instructive illustrations of seismic intensity patterns is a map comparison of the 1906 San Francisco earthquake and a similar-sized earthquake in southern Missouri. The seismic wave absorptive properties of the bedrock along the western margin of North America are much greater than that of the central and eastern states. As a result, except in California, little damage is expected to occur from earthquakes in the United States—even from major ones. A great earthquake in the New Madrid fault zone of southern Missouri, however, is likely to have a wide zone of maximum intensity, resulting in severe damage in cities hundreds of miles from the epicenter.

Principal Terms

amplitude: the displacement of the tracings of the recording pen (or light beam) on a seismogram from its normal position

deep-focus earthquakes: earthquakes whose focus is greater than 300 kilometers below the surface

epicenter: the point on the earth's surface directly above the focus of an earthquake

focus: the point within the earth that is the source of the seismic waves generated by an earthquake

high-frequency seismic waves: those earthquake waves that shake the rock through which they travel most rapidly

low-frequency seismic waves: those earthquake waves that shake the rock through which they travel most slowly (also called long-period waves)

seismogram: an image of earthquake wave vibrations recorded on paper, photographic film, or a video screen

seismograph: the mechanical or mechanical-electrical instrument that detects and records passing earthquake waves

shallow-focus earthquakes: earthquakes having a focus less than 60 kilometers below the surface

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