Variations in physical properties within the Earth's crustal layers

In two recent papers (Gutenberg, 1943a, 1944b) the author has investigated with as high an accuracy as practicable travel times and average wave velocities in the crustal layers in southern California. However, it is not possible to use the observed time data for conclusions as to the changes of the velocity within a given layer; the expected effects are of the same order of magnitude as the effects of local differences in structure (Gutenberg, 1943c). Zoeppritz (1912) was the first to point out that changes with distance in the amplitudes of the observed waves may give information of this kind. The method outlined by him is employed in the present paper to study the changes in the velocity of elastic waves, the bulk modulus and the coefficient of rigidity with depth within the layers of the earth's crust. The results are compared with data found from laboratory experiments. The depth at which the melting point is reached is discussed.