On the Layer of Relatively Low Wave Velocity at A

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 kin. 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. It was pointed out by Gutenberg (1928, 1945a) that all the findings can be explained on the assumption that at a critical depth of about 80 hn. the melting point of the material is reached. There has been no clear indication thus far whether the decrease in velocity is gradual over a certain depth interval or sudden at a certain depth, or both. The present paper is a first attempt to find more accurately this decrease in wave velocity with increasing depth.