抄録
The velocity distributions of P and S waves in a seismic dipping zone in Hokkaido and Tohoku regions are calculated by travel-time analysis based on a simple ray tracing method. The earthquakes analyzed are located in the middle part of Tohoku and west off Aomori Prefecture.
The velocity in the crust is specified for all the earthquakes in each of three regions; Hokkaido, Pacific coast of Tohoku and middle part of Tohoku, whereas the velocity is assumed to increase as a linear function of depth from the top of mantle toward the hypocenter.
Comparison of the velocity structures between the foci and Hiroo with those between the foci and Kushiro indicates that there are three percent velocity contrast for P waves and two percent contrast for S waves. The rays from the foci to Hiroo pass through a seismic active zone. As to the rays from the foci to Kushiro, however, two thirds of their lengths are below the seismic active zone in the seismic dipping zone.
The velocity structure for P and S waves is proved to be both vertically and laterally heterogeneous in the seismic dipping zone.
