On the Nature of the 20°—Discontinuity in the Earth’s Mantle

Abstract

The structure of the earth’s mantle, mainly its upper part, having been investigated by the analysis of seismic waves of five Kamchatka-Kurile Islands earthquakes of shallow focus and some others, the problems on the existence and property of the 20°-discontinuity and the low-velocity layer in the mantle are discussed in some detail. The time-distance graphs obtained in Japan are in fair agreement with Jeffreys-Bullen’s Table (1939), and also the 20°-discontinuity is considered to exist certainly at nearly the same depth as given by Jeffreys. The amplitude relation observed at the epicentral distance near 20° between the direct wave propagating through the medium entirely above the 20°-discontinuity and the refracted wave penetrating down into that discontinuity is just the reverse of that commonly expected, that is, the former being small and the latter large, both for the P- and S-waves. Concerning this phenomenon some arguments are given. Moreover, the existence of a new discontinuity was ascertained at some depth below the 20°-discontinuity, as deduced from the appearance of a prominent later phase. As to the existence of a low-velocity layer in the mantle, it is somewhat questionable at least in the form after Gutenberg from the analysis of seismic waves observed at the stations in Japan of the earthquakes of Kamchatka-Kurile Islands. But its existence is still probable in some forms, which is uncertain at present, at some depths below the Mohorovi?i? discontinuity. This problem will be postponed to a succeeding paper.