Geological Implications of Deep-Focus Earthquakes

Five years ago at the Pasadena meeting of the Seismological Society the writer ventured to state in a preliminary report of the evidence for a focal depth of 400 km for the Japanese earthquake of March 29, 1928, that the occurrence of earthquakes at a depth of 400 km may well have large significance for isostasy and related problems, as well as in regard to structure, constitution, and condition of the Earth. At that time deep foci enjoyed only a modicum of scientific respectability, but since then they seem to have found an accepted place in the family of scientific data, and interest has arisen in regard to their significance and implications. The present paper is an attempt to develop, to a little extent, some of these implications.