Seismologists have developed simple ways to identify earthquake size from the records of seismographs. The most common measure of size is Magnitude, Ml. Astronomers have long graded the size of the stars according to the Stellar magnitude scale, which is based on the relative brightness of stars as seen through a telescope.
In 1935, Charles Richter developed at the California Institute of Technology an analogue measure for earthquakes. Richter proposed that earthquakes be graded according to the seismic wave amplitudes detected by a seismograph.
Because the size of earthquakes varies enormously, the amplitudes of the ground motions differ by factors of thousands from earthquake to earthquake. Therefore it is most conveniant to compress the range of wave amplitudes measured on seismograms by using mathematical theories.
Richter defined the magnitude of a local earthquake as the logarithm to base ten of the maximum seismic-wave amplitude (in thousands of a millimeter) recorded on a standard seismograph at a distant of 100 kilometers from the earthquake epicentre. Using the logarithmic scale, everytime the magnitude goes up by 1 unit, the amplitude of the earthquake waves increases 10 times.
From its inception the magnitude scale has been expanded from recognition from its modest beginnings. The conveniance of describing the size of an earthquake by just one number, the magnitude, has required that the method be extended to apply to a number of types of seismographs throughout the world. Consequently, there are a variety of magnitude scales , which are based on different formulas for epicentral distance and ways of choosing an appropriate wave amplitude.
Earthquake magnitudes are used in three main ways. First, they are recognised by the general public, as by scientists, engineers, and technicians, as a measure of the relative size of an earthquake; people correlate a magnitude, at least roughly, with the severity of an earthquake. Second, magnitudes are of significance in the ongoing efforts to draw up a comprehensive nuclear test ban treaty; research has indicated that comparison of different kinds of magnitude is one of the best ways to distinguish between a nuclear explosion and an earthquake due to natural causes. Third, magnitudes of previous earthquakes are used in an approximate way to predict what the greatest acceleration of the ground shaking may be in an earthquake at a site of an important structure. The information is then used by the engineer to design a structure that will withstand such strong motion.
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