Why Are There So Many Magnitude Scales?
Earthquake size, as measured by the Richter Scale is a well known, but not
well understood, concept. What is even less well understood is the
proliferation of magnitude scales and their relation to Richter's original
magnitude scale. The idea of a logarithmic earthquake magnitude scale was
first developed by Charles Richter in the 1930's for measuring the size of
earthquakes occurring in southern California using relatively high-frequency
data from nearby seismograph stations. This magnitude scale was referred to
as ML, with the L standing for local. This is what was to eventually become
known as the Richter magnitude.
As more seismograph stations were
installed around the world,
it became apparent that the method developed by Richter was strictly valid only for
certain frequency and distance ranges. In order to take advantage of the
growing number of globally distributed seismograph stations, new magnitude
scales that are an extension of Richter's original idea were developed. These
include body-wave magnitude, mb, and surface-wave magnitude, MS. Each is
valid for a particular frequency range and type of seismic signal. In its
range of validity each is equivalent to the Richter magnitude.
Because of the limitations of all three magnitude scales, ML, mb, and MS, a
new, more uniformly applicable extension of the magnitude scale, known as
moment magnitude, or MW, was developed. In particular, for very large
earthquakes moment magnitude gives the most reliable estimate of earthquake
size. New techniques that take advantage of modern telecommunications have
recently been implemented, allowing reporting agencies to obtain rapid
estimates of moment magnitude for significant earthquakes.
See also: Magnitude / Intensity Information and Links
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