On the layer of relatively low wave velocity at a depth of about 80 kilometers

More than twenty years ago, the author (Gutenberg, 1926) pointed out that the longitudinal waves of shallow earthquakes decrease considerably in amplitude at epicentral distances between a few (say 2) degrees and about 15 degrees. At greater distances the amplitudes become again rather large, at 16° epicentral distance about as large as at 2°. With further increase in epicentral distance the amplitudes decrease slowly, The whole phenomenon was interpreted as due to a decrease in wave velocity at a depth between 70 and 80 km. Further investigations (Gutenberg and Richter, 1931; 1935, pp. 335-343; 1939a; Gutenberg, 1945a; 1945c, pp. 126-127) indicate that the decrease in velocity must be rather small, that the shadow zone is less significant in earthquakes which start at a depth of 150 to 250 km. and is absent in earthquakes originating at a depth of more than about 300 km