Content Previous Next

[c6, p177]

The original magnitude scale of Richter (1935) was introduced for the purpose of providing an objective measure of the energy of each earthquake in the initial listing of earthquakes in the southern California region compiled by the Seismological Laboratory in Pasadena. Rather than attempting to measure the energy of the earthquake source directly, he chose to construct an empirical scale derived from a simple measure of the complex seismic waveform. Using only the maximum excursion of the seismogram as measured on a single type of instrument, the Wood-Anderson seismograph, he defined the local magnitude of an earthquake as

ML = log10 A - log10 A0(delta),

where the empirical function A0 depends only on the epicentral distance of the station, delta. The zero point was arbitrarily set by Richter to avoid negative magnitudes in the course of routine work. Use of common logarithms means that two earthquakes located at the same distance from a station and having peak amplitudes differing by a factor of 10 will differ by 1 magnitude unit. In practice, readings from all observing stations are averaged after adjustment with station-specific corrections to obtain the ML value. Although Richter (1935) predicted that the local-magnitude scale "cannot hold to any high accuracy," history has proved it to be a powerful quantitative tool for ordering the relative sizes of earthquakes.

Several points about ML should be emphasized. First, it is strictly defined only for the southern California region, although its applicability to coastal central and northern California has since been shown. Recent studies of the A0 curve suggest that it will require revision and regionalization. Second, because ML has no actual physical units associated with it, other empirical magnitude scales may be freely adjusted to coincide with it. The local-magnitude scale has, in fact, been used as the basis for establishing essentially all other magnitude scales. Finally, because ML is derived from measurements taken from a single, band-limited seismograph, ML values saturate once an earthquake becomes large enough. Thus, the "correct" Richter magnitude ML=6.9 for the great 1906 earthquake obtained by Jennings and Kanamori (1979) reflects the amplitude of seismic waves at periods near 1 s but not the total energy of this earthquake. Uniformly valid characterization of the "size" of an earthquake requires use of magnitude scales based on longer-period measures of the event.