Earthquake frequency-magnitude relationships can be well described as a polymodal composite of normal distributions, a pattern consistent with varying causal mechanisms in different tectonic settings and/or magnitude ranges but a pattern also in conflict with the assumption of self-similarity or the fractal nature of earthquakes. From the Giardini Catalog, 4660 deep (> 350 km) worldwide earthquakes presumably homogeneous with respect to causal mechanism, are well described as a single normally distributed population. Analysis of 10,341 earthquakes ranging in magnitude from 2.0 to 8.2 from the 1989 to 1991 National Earthquake Information Center - Quick Epicenter Determination catalogue yields three populations each of whose magnitude frequency is expressed in terms of probability. Some nonsimilar frequency-magnitude distributions for low magnitudes are also explicable as the result of one or more normally distributed populations. In our model, b is a measure of the dispersion of the population.

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