63. Observation of Seismic Waves Generated by Explosions with an Ocean Bottom Seismograph

海で爆破して陸の観測点で観測し,海から島への地殻構造を決定する方法は多くの利点を持つが,本邦では漁業問題のために海岸近くの浅い海で爆破出来ない場合が多く,地殻上部の速度情報に欠けることがある.この欠点を補うために海底地震計を製作し海に観測点を設置することにした.地震計は4.5Hzの上下動地震計を用い,記録方式は直接アナログ磁気録音方式を採用し,7インチリールを用いると,0.5mm/sの送り速度で0.3～40Hzの周波数帯域で600時間記録可能であり,深度500mまでの耐圧容器に入れるようにした.この海底地震計は昭和44年7月,北海道積丹沖爆破の際に水深116mの地点に設置され,600kg,11個;300kg,2個の海中爆破の観測を行なった.明瞭な初動と第2の位相が観測され,初動は5.7,6.5km/sの見掛速度を与え,第2の位相は1.47km/sの速度を与え,直接水中音波によるものであることがわかった.従つて,本邦における海底地震計による最初の爆破の観測の結果は,海岸近くでの観測を可能にし地殻上部の速度情報を求めるのに有効であると同時に,爆破点,距離の決定にも有効であることを示した.|Observation of seismic waves in temporary stations on land from explosions at sea is a powerful method to obtain data on the crustal structure from the ocean to the island or to the continent. However, since it is difficult to fire explosives in shallow water around Japan because of fishing problems, the data on the upper part of crust could not be obtained fairly often. In order to improve this deficiency, observation with an ocean bottom seismograph was carried out in July, 1969 in the case of the Off Shakotan Explosion. The direct analogue recording technique was applied for the magnetic tape recording, so that the total recording time was extended to 600 hours with a 7 inch magnetic tape and with a tape speed of 0.5 mm/s for the frequency band 0.3-40 Hz. The geophone used was of vertical type with a natural frequency of 4.5 Hz manufactured by Geo Space Company. The maximum depth for this ocean bottom seismograph is 500m and the actual observation was carried out at the depth of 116m. The charge size for eleven shots was 600 kg and that for two shots was 300 kg. Clear first arrivals and direct water waves were recorded. The apparent velocities 5.7 and 6.5km/s are derived from first arrivals by applying an approximate correction of water depth