抄録
Arrival times of P and S waves at Japanese stations from 33 intermediate and deep earthquakes in and near Japan are plotted in the form of S-P vs. P plots. Most of the plotted points for each earthquake (except one which occurred in Kyushu) fit a straight line with a slope of about 0.77. This means that the velocity ratio of P to S waves is about 1.77 and Poisson's ratio is about 0.265 for the portion of the upper mantle concerned. It should be emphasized that the ratio does not differ significantly between the two distinctive portions of the upper mantle, i. e., the inclined deep seismic layer of high-Q and high-V and the overlying wedge-shaped region of low-Q and low-V. For certain deep earthquakes, plotted points in ranges of larger S-P deviate systematically from the straight lines fitted to points in ranges of smaller S-P. From this deviation it is suggested that a portion of the upper mantle under the deep seismic layer has a comparatively large Poisson's ratio.
