40. Travel Times to Japanese Stations from Longshot and their Geophysical Implications
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Travel times to Japanese stations from the Longshot underground explosion of October 29, 1965, were studied. The arrival times of P waves are 1.5 to 3 seconds earlier than the Jeffreys-Bullen (J-B) time. A systematic variation of the J-B residual was found in relation to the seismic plane which dips downward from the Pacific Ocean side towards the continent; a station on the Pacific Ocean side registered a relatively early arrival (-3 sec), while stations on the Japan Sea side registered relatively late arrivals (-1.5 sec). This systematic variation can be interpreted in terms of a structural anomaly associated with the deep seismic plane. The simplest of all the models which explain this systematic variation is the one in which the P-wave velocity in the top 250 km of the mantle is reduced by 0.4 km/sec on the continental side as compared with that on the ocean side of the deep seismic plane. The amplitude ratio of PcP to P also provides a favorable evidence for this model. The average Q is estimated as 80 in the top 250km of the mantle on the continental side of the deep seismic plane. The low velocity and Q can be explained in terms of a temperature excess of about 500℃ coupled with a partial melting of about 2%. Such anomalous structure would have an important bearing on various island-arc tectonics such as the magma generation and convection. The arrival times at Japanese stations are compared with those at stations elsewhere in the world. The earliest arrival among the Japanese stations is comparable to that at stations on relatively old crust, and the latest to that at stations on relatively young crust. The residual at Japanese stations, on the average, is not anomalous as compared with the world average.|1965年10月29日に行なわれた地下爆発Longshotの日本への走時をしらべた.P波の走時はJeffreys-Bullenの走時にくらべて筑波で約3sec早く,近畿,中国地方の観測点では約1.5sec早い.PcPについても同様の傾向がみられた.このような系統的な走時異常は深発地震面に関係する構造異常によつて説明できる.もつとも筒単なモデルは,深発地震面を境にして深さ約250kmより浅いマントル内でのP波の平均的な速さが陸側では海側においてより約0.4km/sec遅いとするものである.筑波と白木で観測されたPcPとPの振幅比は著しく異なる.この事実は陸側のマソトルでのQが海側での値にくらべて小さく,約80であるとすれば説明できる.このような低速度と小さいQを説明するためには陸側のマソトルでの温度が海側にくらべて平均的に500℃高く,同時に約2%程度の部分溶融があるとすればよい.日本の走時を世界の他の観測所での値と比較すると,日本におけるもつとも早い走時はシールド地域のものに対応し，もつとも遅い走時は中生代以後の比較的若い造山帯での値に対応する.
- 1968-10-31
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