QUANTIFICATION OF EARTHQUAKES AND MAGNITUDE SCALES

Physical measures of the complex mechanical event producing the earthquake take many forms, including the dimensions of the faulted region, the amount of slip, and the strength of the radiated elastic waves. To relate the characteristics of one event to another, the observed quantities must generally be summarized through the use of either an empirical relation, such as magnitude, or a quantity derived from a physical model, such as seismic moment. Both procedures have their place, and the choice of one metric over another depends principally on the purposes of the comparison and the availability of common data.

Because no single procedure for determining magnitude can be applied to the entire historical record, the catalog must be quantified by using various magnitude scales. Each scale is briefly described below to define its origin and to clarify its relation to the other scales. I emphasize that each scale has a particular range of validity and that different magnitude scales will, in general, yield slightly different values for the same event. Such differences in magnitude seem to provide a never-ending source of interest and controversy for the news media, who commonly lump all scales together under the heading of "Richter scale." To the seismologist, such differences are neither surprising nor controversial and can, in fact, provide information on the underlying physical processes of the earthquake source.

I also emphasize that intensity scales characterize the effects of the earthquake at a particular location and are not magnitude scales. Strictly speaking, intensity values (or, for that matter, instrumentally measured values of ground-motion parameters) describe the vibratory motions that are the actual earthquake as observed at a particular location, whereas magnitude values describe the faulting event that generates the earthquake.