Abstract
The nuclear explosion Cannikin in the Central Aleutian islands detonated in 1971 was observed at about 150 seismometrical stations in Japan. These data were used with the purpose of investigation of lateral inhomogeneity in the upper mantle associated with the Japan Island arc by means of a three-dimensional-ray-tracing method recently developed. The upper mantle was divided into three zones in terms of velocity contrast based on the work by Utsu; 1) the low velocity zone, uppermost 275km in the mantle, on the continental side of the inclined deep and intermediate earthquake zone, 2) the high velocity inclined seismic zone of 100-km thick reaching to a 350-km depth, 3) the low velocity zone, uppermost 350km in the mantle, on the Pacific side of the inclined seismic zone. The best-fit velocity contrasts in each zone to a standard earth model were -4, +3, and -1% for the above mentioned zones 1), 2), and 3), respectively. This model is in good agreement with the observed P-wave traveltime anomalies in north-east Japan, however, inconsistent in detail with the observation in south-west Japan, although it explains a general trend of the observed traveltime anomalies in whole Japan. Suitability of the present model is discussed comparing with tho observation.
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