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

DELP 1985年度日本海研究航海報告 : IV. 大和海盆における海山の磁気異常

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

Trial of Multidisciplinary Observation at an Expandable Sub-Marine Cabled Station "Off-Hatsushima Island Observatory" in Sagami Bay, Japan

Sagami Bay is an active tectonic area in Japan. In 1993, a real-time deep sea floor observatory was deployed at 1,175 m depth about 7 km off Hatsushima Island, Sagami Bay to monitor seismic activities and other geophysical phenomena. Video cameras monitored biological activities associated with tectonic activities. The observation system was renovated completely in 2000. An ocean bottom electromagnetic meter (OBEM), an ocean bottom differential pressure gauge (DPG) system, and an ocean bottom gravity meter (OBG) were installed January 2005; operations began in February of that year. An earthquake (M5.4) in April 2006, generated a submarine landslide that reached the Hatsushima Observatory, moving some sensors. The video camera took movies of mudflows; OBEM and other sensors detected distinctive changes occurring with the mudflow. Although the DPG and OBG were recovered in January 2008, the OBEM continues to obtain data

Preliminary Report of the KT 86-10 Cruise for the Milura and Hachijo Basin

The Mikura Basin and Hachijo Basin lie just beyond the volcanic front of the Izu-Ogasawara arc-trench system and the existence of submarine hydrothermal activity is expected. This area was surveyed during the cruise of KT 86-10 by the R/V Tansei-Maru, Ocean Research Institute, University of Tokyo from the 12th to the 21st of July, 1986. A small topographic high west of the Inanbajima which occupies the central part of the basin was found by a 12 kHz echo sounder as well as a seismic profiler. The small topographic high consists mostly of the boulder of volcanic rocks which were recognized later as two pyroxene andesites by the submersible "Shinkai 2000" of JAMSTEC. The other parts of the basins are covered by thick volcanogenic and biogenic materials having bioturbation structures on the surface by the bottom dwelling organisms. The geologic developments of the basins viewed from the present observations are as follows: Basement of the region was cut by the normal faults relating to the initiation of rifting the northern Izu-Ogasawara backarc area and followed to subside. Thick volcanic materials covered the basement. Compressional stress field after the end of the rifting may take place to form folding and fissure eruption of andesitic lava to make small knoll. CTD (Conductivity, Temperature, Depth) measurements across the knoll were carried out and temperature anomalies were found near the knoll. The temperature and salinity relationship observed along the knoll gives negative possibility to support the existence of presentday hydrothermal activity. Multichannel seismic profiler data show the existence of a low velocity part, which may possibly be a magma chamber, about 1.5 km beneath the Mikura Basin. All the other data support the existence of hydrothermal activity in the basins. However, it will be necessary to have more elaborate surveys of this area in future studies.御蔵海盆と八丈海盆は伊豆.小笠原島弧-海溝系の火山フロントのすぐ背後に位置している.これらの海盆には海底熱水活動の存在が期待されている.東京大学海洋研究所の淡青丸によるKT86-10次航海が1986年7月12日から21日の間行われ,この地域の地質学,地球物理学,及び地球化学的な調査がなされた.御蔵海盆の中央にある藺灘波島の西部からPDRと音波探査によって小さな地形的高まりが発見された.この高まりは北北西-南南東の伸長方向を持ちその幅数100メートル,高さ数10メートルで2マイル程度連続する.これは,火山岩の巨礫の積み重なった小丘で,その深部には断層が発達している.この火山岩の大部分は両輝石安山岩であることが後の「しんかい2000」の潜水調査で確かめられた.周辺の海盆底は厚い火山源及び生物源物質によって埋立てられており,堆積物が底生生物によって著しく乱されている.これらの海盆の発達史は音波探査の記録から以下の通りまとめられる.まず伊豆・小笠原背弧のリフティングに関連した正断層群によって,この地域の基盤が切られ沈降した.ひきつづきこの基盤を厚い火山物質が覆った.次に,北北西-南南東方向のσhmaxに平行な安山岩質溶岩の広域割れ目噴火を起こし,活断層と伴に小丘が形成され,この小丘を横切ってCTDの観測が行われ小丘の近くで0.05℃程の温度異常が見つかった.小丘に沿って観測された温度と塩分濃度の関係は現在の熱水活動の存在に否定的である.しかし,マルチチャソネル音波探査の結果は御蔵海盆の深さ約1.5kmのところにマグマ溜りの存在を示している.島弧を横切る方向の地殻熱流量の値の分布が火山フロントで高く,背にでもやや高いこと,断層による陥没及び活断層の存在,深海カメラや潜水艇で熱水性堆積物らしいものが撮影されていること,潜水艇のマニピュレータで得られた安山岩にFe-Mnに富む堆積物が付着していたことなどは海盆に熱水活動の存在することを支持しているが,この海域のもっと丹精なる調査が将来必要である