THE POLYMODAL FREQUENCY-MAGNITUDE RELATIONSHIP OF EARTHQUAKES

Earthquake frequency-magnitude relationships can be well described as a polymodal composite of normal distributions, a pattern consistent wi th varying causal mechanisms in different tectonic settings and/or mag nitude ranges but a pattern also in conflict with the assumption of se lf-similarity or the fractal nature of earthquakes. From the Giardini Catalog, 4660 deep (> 350 km) worldwide earthquakes presumably homogen eous with respect to causal mechanism, are well described as a single normally distributed population. Analysis of 10,341 earthquakes rangin g in magnitude from 2.0 to 8.2 from the 1989 to 1991 National Earthqua ke Information Center-Quick Epicenter Determination catalogue yields t hree populations each of whose magnitude frequency is expressed in ter ms of probability. Some nonsimilar frequency-magnitude distributions f or low magnitudes are also explicable as the result of one or more nor mally distributed populations. In our model, b is a measure of the dis persion of the population.