Abstract
We present a method suitable for ray tracing in a three-dimensional structure composed of several zones. This is a method extended from two-dimensional ray tracing and is characterized by a small number of calculation steps for a ray. Applying this method to four events occurring around Hokkaido and off the east coast of Kyushu, we investigate the anomalous structure in the upper mantle beneath Japan. P velocities of 8.05 and 7.50km/s are estimated as a model in the uppermost parts of the high and low velocity zones, respectively; the velocity difference of 6.8 percent is comparable with those in the studies made to date. At the lower boundary of the high velocity lithosphere, the velocity contrast is suggested to be lower than 2 percent, while it is over 2 to 3 percent at the upper boundary.
