Richter (1935) and Gutenberg and Richter (1942) developed the local magnitude scale using Wood-Anderson seismographs. Richter (1935) defined "local" magnitude as:
= - +
where: is the local magnitude estimate; is the logarithm of the maximum trace amplitude (in mm) recorded by a standard Wood-Anderson torsion seismograph; is the logarithm of a standard event of magnitude zero at the same epicentral distance in km; and, is the station adjustment. The estimate for an event is then the arithmetic average of the individual estimates from the Wood-Anderson seismographs that recorded the event.
In the past thirty years, the instrumentation with which we measure earthquakes has changed, and we can now process the data digitally. Nonetheless, we would like to continue to assign events with local magnitudes which are consistent with those that have been calculated in Northern and Southern California for the past 75 years.
Berkeley Seismological Laboratory
215 McCone Hall, UC Berkeley, Berkeley, CA 94720-4760
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