Intensity is based on the observed effects of ground shakingon people, buildings, and natural features. It varies from placeto place within the disturbed region depending on the location ofthe observer with respect to the earthquake epicenter. Magnitude is related to the amount of seismic energyreleased at the hypocenter of the earthquake. It is based on theamplitude of the earthquake waves recorded on instruments whichhave a common calibration. The magnitude of an earthquake isthus represented by a single, instrumentally determined value. Earthquakes are the result of forces deep within the Earth'sinterior that continuously affect the surface of the Earth. Theenergy from these forces is stored in a variety of ways withinthe rocks. When this energy is released suddenly, for example byshearing movements along faults in the crust of the Earth, anearthquake results. The area of the fault where the suddenrupture takes place is called the focus orhypocenter of theearthquake. The point on the Earth's surface directly above thefocus is called the epicenter of the earthquake. San Fernando, California, 1971. Highway interchange heavily damaged by the magnitude 6.5 earthquake[Click on image for a larger view]The Richter Magnitude Scale Seismic waves are the vibrations from earthquakes thattravel through the Earth; they are recorded on instruments calledseismographs. Seismographs record a zig-zag trace that shows thevarying amplitude of ground oscillations beneath the instrument. Sensitive seismographs, which greatly magnify these groundmotions, can detect strong earthquakes from sources anywhere inthe world. The time, location, and magnitude of an earthquakecan be determined from the data recorded by seismographstations. The Richter magnitude scale was developed in 1935 by CharlesF. Richter of the California Institute of Technology as amathematical device to compare the size of earthquakes. Themagnitude of an earthquake is determined from the logarithm ofthe amplitude of waves recorded by seismographs. Adjustments areincluded in the magnitude formula to compensate for the variationin the distance between the various seismographs and theepicenter of the earthquakes. On the Richter Scale, magnitude isexpressed in whole numbers and decimal fractions. For example, amagnitude of 5.3 might be computed for a moderate earthquake, anda strong earthquake might be rated as magnitude 6.3. Because ofthe logarithmic basis of the scale, each whole number increase inmagnitude represents a tenfold increase in measured amplitude; asan estimate of energy, each whole number step in the magnitudescale corresponds to the release of about 31 times more energythan the amount associated with the preceding whole numbervalue. Van Norman Dam, San Fernando, California, 1971. Earthquake-induced liquefaction of the earth-filled dam resulted in a landslide that caused partial collapse[Click on image for a larger view] At first, the Richter Scale could be applied only to therecords from instruments of identical manufacture. Now,instruments are carefully calibrated with respect to each other. Thus, magnitude can be computed from the record of any calibratedseismograph. Earthquakes with magnitude of about 2.0 or less are usuallycalled microearthquakes; they are not commonly felt by people andare generally recorded only on local seismographs. Events withmagnitudes of about 4.5 or greater--there are several thousandsuch shocks annually--are strong enough to be recorded bysensitive seismographs all over the world. Great earthquakes,such as the 1964 Good Friday earthquake in Alaska, havemagnitudes of 8.0 or higher. On the average, one earthquake ofsuch size occurs somewhere in the world each year. Although theRichter Scale has no upper limit, the largest known shocks havehad magnitudes in the 8.8 to 8.9 range. Recently, another scalecalled the moment magnitude scale has been devised for moreprecise study of great earthquakes. The Richter Scale is not used to express damage. Anearthquake in a densely populated area which results in manydeaths and considerable damage may have the same magnitude as ashock in a remote area that does nothing more than frighten thewildlife. Large-magnitude earthquakes that occur beneath theoceans may not even be felt by humans. (Top) San Francisco, California, 1906. Collapse of City Hall after the 8.3magnitude earthquake. Most of the property destruction was caused by the fire that raged after the earthquake.(Bottom) Anchorage, Alaska, 1964. Much of the damage after this magnitude 8.6 earthquake was due to huge landslides, such as this one under Government Hill elementary School.[Click on image for a larger view]The Modified Mercalli Intensity Scale The effect of an earthquake on the Earth's surface is calledthe intensity. The intensity scale consists of a series ofcertain key responses such as people awakening, movement offurniture, damage to chimneys, and finally--total destruction. Although numerous intensity scales have been developed over thelast several hundred years to evaluate the effects ofearthquakes, the one currently used in the United States is theModified Mercalli (MM) Intensity Scale. It was developed in 1931by the American seismologists Harry Wood and Frank Neumann. Thisscale, composed of 12 increasing levels of intensity that rangefrom imperceptible shaking to catastrophic destruction, isdesignated by Roman numerals. It does not have a mathematicalbasis; instead it is an arbitrary ranking based on observedeffects. The Modified Mercalli Intensity value assigned to a specificsite after an earthquake has a more meaningful measure ofseverity to the nonscientist than the magnitude because intensityrefers to the effects actually experienced at that place. Afterthe occurrence of widely-felt earthquakes, the Geological Surveymails questionnaires to postmasters in the disturbed arearequesting the information so that intensity values can beassigned. The results of this postal canvass and informationfurnished by other sources are used to assign an intensity value,and to compile isoseismal maps that show the extent of variouslevels of intensity within the felt area. The maximum observedintensity generally occurs near the epicenter. (Top) Mindanao, Phillippines, 1976. Apartment building destroyedby a magnitude 7.9 earthquake.(Bottom) Long Beach, California, 1933. Exterior walls collapsed ontoparked cars after this magnitude 6.3 earthquake (photo by Southern California Earthquake Pictures).[Click on image for a larger view] The lower numbers of the intensity scale generally deal withthe manner in which the earthquake is felt by people. The highernumbers of the scale are based on observed structural damage. Structural engineers usually contribute information for assigningintensity values of Vlll or above. Ninety percent of the world's earthquakes occur in specific areas that are the boundaries of the Earth's major crustal plates. Shown on the map are the epicenter locations of earthquakes of magnitude 4.5 or greater that occurred from 1978 through 1987.[Click on image for a larger view] The following is an abbreviated description of the 12 levelsof Modified Mercalli intensity.Not felt except by a very few under especially favorable conditions.Felt only by a few persons at rest, especially on upper floors of buildings. Delicately suspended objects may swing.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. Vibration similar to the passing of a truck. Duration estimated.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.Felt by nearly everyone; many awakened. some dishes, windows broken. Unstable objects overturned. Pendulum clocks may stop.Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight.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.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.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.Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rail bent.Few, if any (masonry) structures remain standing. Bridges destroyed. Rails bent greatly.Damage total. Lines of sight and level are distorted. Objects thrown into the air. Another measure of the relative strength of an earthquake isthe size of the area over which the shaking is noticed. Thismeasure has been particularly useful in estimating the relativeseverity of historic shocks that were not recorded byseismographs or did not occur in populated areas. The extent ofthe associated felt areas indicates that some comparatively largeearthquakes have occurred in the past in places not considered bythe general public to be regions of major earthquake activity. For example, the three shocks in 1811 and 1812 near New Madrid,Mo., were each felt over the entire eastern United States. Because there were so few people in the area west of New Madrid,it is not known how far it was felt in that direction. The 1886Charleston, S.C., earthquake was also felt over a region of about2 million square miles, which includes most of the eastern UnitedStates. *U.S. GOVERNMENT PRINTING OFFICE: 1989-288-913This publication is one of a series of general interestpublications prepared by the U.S. Geological Survey to provideinformation about the earth sciences, natural resources, and theenvironment. To obtain a catalog of additional titles in theseries "General Interest Publications of the U.S. GeologicalSurvey," write: U.S. Geological Survey Information Services Box 25286 Denver, CO 80225As the Nation's principal conservation agency, the Department ofthe Interior has the responsibility for most of our nationally owned publiclands and natural resources. This includes fostering the wisest use of ourland and water resources, protecting our fish and wildlife, preserving theenvironmental and cultural values of our national parks and historical places, and providingfor the enjoyment of life through outdoor recreation. The Departmentassesses our energy and mineral resources and works to assure that theirdevelopment is in the best interests of all our people. The Department also has amajor responsibility for American Indian reservation communities and for people who livein Island Territories under U.S.administration. Accessibility FOIA Privacy Policies and Notices
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