Why Are There So Many Earthquake Magnitude Scales?Įarthquake size, as measured by the Richter Scale, is a well-known, but not well understood, concept. Although the Richter Scale has no upper limit, the largest known shocks have had magnitudes in the 8.8 to 8.9 range. Because of the logarithmic basis of the scale, each whole number increase in magnitude represents a tenfold increase in measured amplitude as an estimate of energy, each whole number step in the magnitude scale corresponds to the release of about 31 times more energy than the amount associated with the preceding whole number value. For example, a magnitude of 5.3 might be computed for a moderate earthquake, and a strong earthquake might be rated as magnitude 6.3. On the Richter Scale, magnitude is expressed in whole numbers and decimal fractions. Adjustments are included in the magnitude formula to compensate for the variation in the distance between the various seismographs and the epicenter of the earthquakes. The magnitude of an earthquake is determined from the logarithm of the amplitude of waves recorded by seismographs. Richter of the California Institute of Technology as a mathematical device to compare the size of earthquakes. The Richter magnitude scale was developed in 1935 by Charles F. The time, location, and magnitude of an earthquake can be determined from the data recorded by seismograph stations. Sensitive seismographs, which greatly magnify these ground motions, can detect strong earthquakes from sources anywhere in the world. Seismographs record a zigzag trace that shows the varying amplitude of ground oscillations beneath the instrument. Seismic waves are the vibrations from earthquakes that travel through Earth they are recorded on instruments called seismographs. Magnitude is thus represented by a single, instrumentally determined value. It is based on the amplitude of the earthquake waves recorded on instruments, which have a common calibration. Magnitude is related to the amount of seismic energy released at the hypocenter of the earthquake. It varies from place to place within the disturbed region depending on the location of the observer with respect to the earthquake epicenter. Intensity is based on the observed effects of ground shaking on people, buildings, and natural features. The two terms are quite different, however, and they are often confused. The severity of an earthquake can be expressed in terms of both intensity and magnitude. The point on Earth's surface directly above the focus is called the epicenter of the earthquake. The area of the fault where the sudden rupture takes place is called the focus or hypocenter of the earthquake. When this energy is released suddenly-by shearing movements along faults in the crust of Earth, for example-an earthquake results. The energy from these forces is stored in a variety of ways within the rocks.
Geological SurveyĮarthquakes are the result of forces deep within Earth's interior that continuously affect its surface. Source: National Earthquake Information Center, U.S.