Report on DELP 1989 Cruise in the TTT Junction Areas : Part 3: Geomagnetic Anomalies over the Triple Junction Area off the Boso Peninsula, Japan

The three components and total intensity of the geomagnetic field were measured using STCM and a proton precession magnetometer during the DELP 1989 triple junction cruise. The magnetic anomaly lineation located at 34°N, 142°30'E, which may be the M7 (129Ma) Mesozoic anomaly, crosses over the Japan Trench, with the trend N45°E. Lineations in the north western Pacific basin east of this region is N70°E. Therefore, the N45°E trend over the triple junction area means anticlockwise rotation of the triple junction area by 25° relative to the northwestern Pacific basin.1989年にDELP航海により,海溝三重点海域での地磁気調査がなされた.測定にはフラックスゲート船上三成分磁力計とプロトン磁力計を用いて,地磁気三成分異常と全磁力異常分布を明らかにすることが目的であった

Report on DELP 1985 Cruises in the Japan Sea : Part IV : Geomagnetic Anomalies over the Seamounts in the Yamato Basin

Seamount magnetic anomaly studies have been conducted on four seamounts (Yamato, B, Meiyo, Meiyo Daisan) in the Yamato Basin. These seamounts lie in the Yamato Seamount Chain trending approximately NE-SW parallel to the axis of the Basin. The total intensities and the three components of the geomagnetic field over four seamounts were measured. Bathymetric depths were measured by 12 kHz PDR. The minimum and maximum values of peak amplitudes of the magnetic anomalies observed over the Meiyo Seamount and B Seamount were 172 to 611 nT respectively. We have attemped to apply the method of Talwani to explain magnetic anomaly patterns over the four seamounts. The results indicate that B Seamount has normal magnetization and the remaining three seamounts have polarity reversals among different sections of each seamount. For all seamounts the declinations of magnetization lie around 0° or 180°. Thus, it appears that the Yamato Seamount Chain includes several polarity reversals and all seamounts have never been rotated relative to the average magnetic coordinate since their generation.大和海盆には大和,B,明洋,明洋第3の4海山がある.4つの海山は海盆の中軸部にあり,南西-北東の方向に海山列をつくっている.地磁気全磁力と地磁気三成分を各海山の上で観測した.全磁力異常振幅の最大値はB海山で観測された611nTであった.また最小値は明洋海山の172nTであった.観測地を説明するために,各海山にTalwani法を適用しモデル計算をした.B海山は正帯磁に磁化し,大和,明洋,明洋第3の3海山は正帯磁と逆帯磁の部分が混在していることがわかった.B海山の偏角は340°,伏角は40°であった.大和海山は,2つの峰が偏角O°伏角40°と偏角O°伏角60°の正帯磁で残りの1つの峰が偏角180°伏角-40°の逆帯磁と考えると,観測値をよく説明できた.明洋海山は中央にある峰が逆帯磁で南東部と西部の峰が正帯磁に磁化していることがわかった.逆帯磁の偏角と伏角は180°,-40°,正帯磁の偏角と伏角は各々O°,50°と10°,40°であった.明洋第3海山は山体上部が逆帯磁で下部が正帯磁とすると観測値をうまく説明できた.上部の偏角は70°伏角は-30°で下部は偏角O°,伏角60°であった.正帯磁と逆帯磁の混在から大和海山列の生成期間に少なくとも1つ以上の磁場反転を含むことがわかった.また4海山の偏角がO°(あるいは180°)からあまりずれていないことから,大和海山列は生成後に回転しなかったものと考えられる

1989年度DELP海溝三重会合点海域調査研究航海報告 : 第3部:海溝三重会合点海域における地磁気異常

The three components and total intensity of the geomagnetic field were measured using STCM and a proton precession magnetometer during the DELP 1989 triple junction cruise. The magnetic anomaly lineation located at 34°N, 142°30\u27E, which may be the M7 (129Ma) Mesozoic anomaly, crosses over the Japan Trench, with the trend N45°E. Lineations in the north western Pacific basin east of this region is N70°E. Therefore, the N45°E trend over the triple junction area means anticlockwise rotation of the triple junction area by 25° relative to the northwestern Pacific basin.1989年にDELP航海により,海溝三重点海域での地磁気調査がなされた.測定にはフラックスゲート船上三成分磁力計とプロトン磁力計を用いて,地磁気三成分異常と全磁力異常分布を明らかにすることが目的であった

Report on DELP 1986 Cruise in the Northwestern Pacific : Part 4: Measurement of Three Components and Total Force of the Geomagnetic Field

During the DELP-1986 cruise, three components of the geomagnetic field and the total force intensity were measured with an STCM (Shipboard Three Component Magnetometer) and a proton precession magnetometer. STCM provides three component geomagnetic field intensities on board a research vessel, of which results can be used to identify the orientation of the magnetic lineament from only one single ship track. Our data give some additional information on the magnetic anomaly lineation patterns of the northwestern Pacific and the direction of lineation of the area surveyed by the present cruise is reexamined by the three component magnetic field data. The direction of geomagnetic anomaly lineation of this area is confirmed to run with a N70°E strike. Two vessels were used for the present study, i.e. WakashioMaru for three component magnetometer run and Daisan KaikoMaru for single proton magnetometer run.1986年度夏期に実施されたDELP並びに地震予知共同観測で得られた,地磁気三成分(水平二成分,鉛直一成分)及び全磁力の異常分布について報告する.三成分の測定は船舶用フラックスゲート磁力計により,全磁力は,プロトン型磁力計により行われ,両者間では補正を行なうことが可能である.また三成分磁力計はジャイロによる方位の測定と船体磁気の補正により,地理方位三成分の値として得られる.三成分の利点は,航跡が単数であっても,海底下に分布する地磁気異常源の走向を推定できることにある.今日までに得られている,標題海域の地磁気異常分布について今回の様な測定により,新たな吟味を加える事が可能である.この海域に於ける地磁気異常の走向はN70°Eであることが確認された

Report on DELP 1988 Cruises in the Okinawa Trough : Part 5. Measurement of the Three Components and Total Intensity of the Geomagnetic Field in the Okinawa Trough

Three components and total intensity of the geomagnetic field were measured by Shipboard Three Component Magnetometer (STCM) and a proton precession magnetometer, respectively, during the DELP 1988 cruises of the R/V Dai-5 Kaiko-maru in the Okinawa Trough. Geomagnetic survey was mainly conducted in the region of the southwestern part of the trough, and detected magnetic anomalies with amplitudes exceeding 300 nT. Generally, these anomalies are associated with topographic highs in the trough. The anomalies are distributed discontinuously except the axial rift area of the trough. This anomaly pattern suggests that the back-arc spreading have occurred only within the rift area, the Yaeyama Graben, in the southwestern part of the trough.DELP 1988航海において,地磁気三成分および全磁力の観測をおもに沖縄トラフの南西部海域で行った.その績果,トラフ底に分布する地形的高まりに対応した300nTをこえる地磁気異常が認められた.地磁気三成分異常について解析した結果,トラフ南西部では,八重山海底地溝域を除いて顕著な磁気異常の連続性は認められなかった.八重山海底地溝域においては,地溝の伸び(ほぼ東西)に沿った地磁気異常の連続性が認められ,地磁気異常の原因物体の2次元的な分布が推定された.八重山海底地溝には,火山岩より成ると思われる海丘が点在している.これらに対応した全磁力異常の分布および三成分異常の解析結果より,堆積層下にも貫入岩体の存在が示唆された.この様なことから,少なくとも地溝東半の約50kmの範囲においては.火山活動をともなう背弧海盆形成の活動が行われたことが推定された

Measurement of geomagnetic field at sea during JARE-30, 1988-1989

Three components and total intensity of the geomagnetic field were measured by STCM (Shipboard Three Components Magnetometer) and a proton magentometer at the same time at sea during the 30th Japanese Antarctic Research Expedition. The measurements of total intensity of the geomagnetic field by a proton magnetometer showed a variation of noises in harmony with the variation of ship's velocity. This variation of noises may be caused by instability of the sensor due to large velocity of the ship. The measurements of three components of the geomagnetic field by STCM revealed short wavelength noiscs which were caused by small yawing of the ship. Reliable absolute values of three components of the geomagnetic field were obtained by adapting the total intensity of the geomagnetic field measured by a proton magnetometer to the data of three components of the geomagnetic field measured by STCM. In the present state of STCM, it is necessary to measure three components and total intensity of geomagnetic field by STCM and a proton magnetometer simultaneously, in case of obtaining the absolute values of three components of the geomagnetic field. These results suggest that improvement of measurement of the geomagnetic field at sea will be required

PRELIMINARY REPORT OF THREE COMPONENTS OF GEOMAGNETIC FIELD MEASURED ON BOARD THE ICEBREAKER SHIRASE DURING JARE-30, 1988-1989

Measurement of three components of geomagnetic field was carried out on board the icebreaker SHIRASE during the 30th Japanese Antarctic Research Expedition (JARE-30). Vector anomalies of geomagnetic field were obtained and the directions of magnetic lineations were determined from the vector anomalies. They are in good agreement with the results previously reported along the ship\u27s tracks (e. g. ROYER et al. : Tectonophysics, 155,235,1988; ROYER and SANDWELL : J. Geophys. Res., 94,13755,1989), except for the Antarctic continental margin and the Enderby Basin. In the Antarctic continental margin, N-S trending magnetic structure that coincides with the Australian-Antarctic depression is detected between Australia and Antarctica and the local magnetic anomaly that seems to be caused by the Napier Complex appears between 50°E and 60°E along 63°S. Further, in the Enderby Basin around 60°S, N-S and NNE-SSW trending magnetic lineations which have never been reported before are detected. These results may suggest new constraints on the evolution of the Indian Ocean

Report on DELP 1987 Cruises in the Ogasawara Area : Part V. Measurement of Three Components and Total Intensity of the Gomagnetic Field in the Ogasawara Trough

The three components and total intensity of the geomagnetic field were measured using STCM and a proton precession magnetometer during the DELP 1987 Ogasawara cruises. The total intensity anomalies are generally negative and small in amplitude in the trough, their lineation pattern can hardly be defined. However, the anomaly patterns of the three vector components of the geomagnetic field show widely distributed lineation features trending north-south over the trough. Two total intensity negative peaks in the northern and southern ends of the trough are located at around the same positions as the free air gravity negative anomaly peaks.1987年秋のDELP航海により小笠原トラフ周辺海域において地磁気連続測定を行った.測定に於いては船舶用フラックスゲート地磁気三成分測定系ならびに曳航型プロトン磁力計により観測を行ったが,後者については航海中船上測定系に異常が出たことなどもあって完全な三成分対全磁力比較検討の材料とは成らなかったものの,全磁力についてはこの海域に於て地質調査所などから多ぐのデータが提供されているので,解析に支障は無いと思われる.船舶上での三成分磁力測定はかねてから神戸大学理学部により信頼度の高いデータが得られる様に成ってきたが,本観測地域の例にもみられるように低緯度でかつ海底の基盤もまた低緯度で発生したと予測される地域の地磁気観測には三成分磁力計は必要不可欠なものである.この報告にも,従来より明瞭な地磁気縞模様の認められていなかったトラフ内にもその存在が確認されたことは,トラフがリフティングとオープニングによって形成されたことを示唆している.今後この縞模様が何時のものであるかを同定せねばならないが,そのためにはこの海域の基盤岩の年代などに関する資料を入手したいと考える