Early in the history of seismology it was occasionally suggested that, in addition to earthquakes with foci comparatively near the surface of the earth, shocks might also originate at depths of the order of several hundred kilometers. However, down to a comparatively recent date, all such conclusions were either purely speculative or were based on inadequate or misinterpreted data.
The first scientifically sound determinations of great focal depth were made by H. H. Turner (1922), the first results being published in 1922, followed by much more detailed and accurate work in connection with his editing of the International Seismological Summary. These conclusions were based on the abnormally early arrival of the first waves at distant observatories. However, he also reported several instances in which these first waves apparently arrived late at great distances, indicating focal depth less than normal, or “high focus.” In his first paper, shocks were . . .