Computer Science
Scientific paper
Apr 1953
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1953gecoa...3..169k&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 3, Issue 4, pp.169-185
Computer Science
1
Scientific paper
The velocity of propagation of longitudinal and transversal ultrasonic waves has been determined in a glass-like substance at various temperatures both above and below the softening point, the viscosity and density of the substance being determined at the same temperatures. It was found that the propagation of the transversal waves continues up to a temperature 30-40° above the softening point, demonstrating that transversal waves can be propagated in media which are unquestionable liquids. From the experimental results the modulus of torsion, the modulus of compression and the mean time of relaxation of shear stresses in the medium could be calculated in the range of temperatures considered. A decrease of longitudinal velocity was observed as the material passes through the softening range, a decrease whose magnitude could be explained quantitatively only by assuming that the relaxation of shear stress occurring in the softening range of the material involves a simultaneous relaxation of about 30% of the modulus of compressibility. These results provide support for the theory that there is no discontinuous change in density and composition at the earth core. The reduction in longitudinal velocity occurring at the seismic discontinuity at a depth of 2900 km can in fact quantitatively be accounted for by the assumption that the relaxation time of the shear stresses in the medium at that depth equals the period of the seismic disturbance and that, as above, the disappearance of the modulus of torsion results in a reduction of about 30% in the modulus of compression. The modulus of compression as calculated in the outer 2900 km of the earth from the velocity of seismic waves is thus, in reality, a dynamic quantity, i.e. if a static pressure were applied for a period exceeding some 300 years the modulus of compressibility then observed would be about 30% smaller. This conclusion has to be borne in mind particularly when calculating changes in the density of the interior of the earth as a result of gravitational effects. A compressibility about 30% higher than hitherto assumed should now be inserted in such calculations. A characteristic and necessary result deriving from the assumption of a continuous variation of density and composition in the earth is a marked dispersion of the velocities of propagation at the boundary of the earth's "core" and a variation in the size of the earth's "core" with the frequency of the earthquake wave, a disturbance of higher frequency corresponding to a somewhat smaller "core". It is interesting, therefore, that these effects (dispersion and dependence of "core" diameter on frequency) have in fact been established experimentally by K. Altenburg by subjecting seismographic records to Fourier analysis.
Kühn Werner
Viemauer Siegfried
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