Abstract
P-waves from the CANNIKIN explosion were recorded at sensitive stations densely distributed in Japan. The distribution of the travel-time residuals suggests a distinct heterogeneity of the upper mantle associated with the underthrusting of the Pacific and Philippine plates, The negative anomalies in Hokkaido are most prominent in the south and are reduced to normal in the north, making a remarkable contrast with the positive anomalies near the subduction region of the Philippine plate. These contradictory distributions may be caused by the predominances of the inner and outer low-Q zones beneath Hokkaido and beneath Southwest Japan, respectively. A narrow belt of negative anomaly is apparently developed from the central part of Japan to the west along the latitude of 35.5°N, which might be explained by the particular deformation of the plate near the junction of North Honshu and Izu-Bonin arcs.
