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The hypothesis1 of subduction of oceanic plates under active continental margins has become a fundamental idea in plate tectonics. Yet the evidence for the existence of oceanic plates beneath continental borders is still largely circumstantial. In seismology, the presence of a new layer or structural unit is taken as confirmed only after seismic phases corresponding to reflections or refractions from that layer have been identified on seismograms. I report here the presence of a mantle phase, interpreted as a seismic wave refracted from a dipping interface on records of aftershocks from the intermediate earthquake of 24 October 1980 in the Mixtec Highlands of south-central Mexico which may be direct evidence on the structural position of the subducted plate under the Mexican active continental margin.

DIFFICULTIES have arisen in attempting to reconcile the tectonic evolution of continental borders with the theory of ocean floor spreading. In the case of the Chilean coast, sea floor spreading has apparently been confirmed by magnetic evidence and disproved by the presence of undisturbed sediments in the trench. Scholl, von Huene, and Ridlon1 have suggested that there are only four alternative solutions to this difficulty, none of which is supported by experimental evidence.

THIS communication outlines a probability model which seems to provide an adequate basis for making predictions concerning the occurrence of largest earthquake magnitudes over time: (a) The number of earthquakes in a year is a Poisson random variable with mean α; (b) X, the earthquake magnitude, is a random variable distributed with cumulative distribution function