were made in Italy as early as the late eighteenth century. In the 1880s, M.S. Rossi of Italy and F. Forel of Switzerland defined standards for grading qualitative observations by integer values that increase with the amount of shaking and disruption. Versions of their “intensity scale,” as modified by G. Mercalli and others, are still used to map intensity after strong events (27), but they do not measure the intrinsic size of an earthquake, nor can they be applied to events that humans have not felt and observed (i.e., almost all earthquakes). The availability of instrumental recordings and the desire to standardize the seismological bulletins motivated seismologists to estimate the intrinsic size of earthquakes by measuring the amplitude of the seismic waves at a station and correcting them for propagation effects, such as the spreading out of wave energy and its attenuation by internal friction. Several such scales were developed, including one by Wadati in 1931, but the most popular and successful schemes were based on the standard magnitude scale that Charles Richter of Caltech published in 1935.
Richter recognized that seismographic amplitude provides a first-order measure of the radiated energy but that these data are highly variable depending on the type of seismograph, distance to the earthquake, and local site conditions. To normalize for these factors, he considered only southern California earthquakes recorded on Caltech’s standardized network of Wood-Anderson torsion seismometers (28). He defined the local magnitude scale for such events by the formula
ML = log A – log A0, (2.3)
where A is the maximum amplitude of the seismic trace on the standard seismogram; A0 is the amplitude at that same distance for a reference earthquake with ML = 0; and all logarithms are base 10. He fixed the reference level A0 by specifying a magnitude-zero earthquake as an event with an amplitude of 1 micron on a standard Wood-Anderson seismogram at a distance of 100 kilometers (29). An earthquake of magnitude 3.0 thus had an amplitude of 1 millimeter at 100 kilometers, which was about the smallest level measurable on this type of pen-written seismogram (30). Corrections for recordings made at other distances were determined empirically and incorporated into a simple graphic procedure.
During the next decade, Richter and Gutenberg refined and extended the methodology to include earthquakes recorded by various instrument types and at teleseismic distances. Gutenberg published a series of papers in 1945 detailing the construction of magnitude scales based on the maximum amplitude of long-period surface waves (MS), which could be applied to shallow earthquakes at any distance, and teleseismic body waves (mb), which could be applied to earthquakes too deep to excite ordinary