Large-scale ripple marks on the shelf margin of the Northern Okinawa Trough     

Hideo Kagami  Shigeaki Kubo  Noriyuki Nasu 《Journal of Oceanography》1984,40(3):230-242

Trains of large-scale ripple marks (megaripples and sand waves) were found on the Amakusa and East China Sea shelves bordering the northern Okinawa Trough. Side-scan sonar surveys were carried out in 1974 and 1976 to investigate sea-floor features lying along a proposed submarine cable line. Megaripples were found on the outer margin of the Amakusa shelf between depths of 140 and 200 m. The megaripples were especially well developed at a depth of 167 m. They were typically straight-transverse crested with asymmetrical profiles, and measured 7 to 15 m in wavelength and 0.4 to 1.4 m in waveheight. Formation of the megaripples on the Amakusa shelf is probably controlled by relatively complex oceanographic conditions. A secondary circulation associated with the Gotô-nada clock-wise Current may be responsible for formation of the ripple marks. Local vorticities generated in the coastal boundary layer as a result of curvature of the Gotô-nada Current are known to cause the complex flow pattern at the Gotô and Amakusa shelf margins. The main semidiurnal (M₂) tidal current may also interact with these fluid processes.On the East China Sea shelf, megaripples and sand waves were found between depths of 140 and 220 m. Sand waves (200 m in wavelength) were observed in seismic reflection profiles. Large-scale lunate megaripples were observed at a depth of 154 m by the side-scan sonar. They had wavelengths of 10 to 30 m and waveheights of 1 to as high as 3 m. It appears from the types and nature of distribution of the megaripples that they are responding to the present-day flow regime, and it is partly ascertained from our observations over an interval of two years that the megaripples appear to be short-term response elements compared wit hteh sand waves. We conclude that the megaripples on the East China Sea shelf are current-formed during peak typhoon flow in August to November. From their distribution, the long term path of the main flow of the Tsushima Current is inferred at the edge of the East China Sea shelf. An area of low sediment mud content (less than 20 per cent) coincides with this path giving further support to our interpretation.  相似文献   

RFLP analysis of a mitochondrial gene,cytochrome oxidase I (COI) of three species of the genusCalyptogena around Japan     

Shigeaki Kojima  Takanori Kobayashi  Jun Hashimoto  Suguru Ohta 《Journal of Oceanography》1995,51(2):257-259

Phylogenetic relationship among three species of deep-sea vesicomyid bivalvesCalyptogena, i.e.,C. soyoae, C. solidissima, C. fausta and an undescribed species of the Iheya Ridge, Okinawa Trough was analyzed on the basis of the RFLP analysis of the fragment (about 1 kbp) of cytochrome oxidase I. Both the two populations ofC. soyoae (off Hatsushima and Okinoyama Bank, Sagami Bay) consisted of two haplotypes, which could be discriminated by only one restriction site (1.7% sequence divergence).Calyptogena of the Iheya Ridge could not be distinguished from one of the two haplotypes ofC. soyoae. Nucleotide substitution rates between species were calculated and dendrograms were constructed.  相似文献   

Microseismicity and permeability enhancement of hydrogeologic structures during massive fluid injections into granite at 3 km depth at the Soultz HDR site   总被引：1，自引：0，他引：1  

K. F. Evans  H. Moriya  H. Niitsuma  R. H. Jones  W. S. Phillips  A. Genter  J. Sausse  R. Jung  R. Baria 《Geophysical Journal International》2005,160(1):389-412

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Preface     

Nobuaki Niitsuma  Roland von Huene  Isao Motoyama  Saneatsu Saito  Kiyoshi Suyehiro  I. Selwyn Sacks 《Island Arc》2004,13(1):178-179

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Middle Miocene to Pleistocene magneto-biostratigraphy of ODP Sites 1150 and 1151, northwest Pacific: Sedimentation rate and updated regional geological timescale     

Isao Motoyama  Nobuaki Niitsuma  Toshiaki Maruyama  Hiroki Hayashi  Shin-ichi Kamikuri  Masamichi Shiono  Toshiya Kanamatsu  Kaori Aoki  Chikako Morishita  Kyoko Hagino  Hiroshi Nishi  Motoyoshi Oda 《Island Arc》2004,13(1):289-305

Abstract Shipboard and shore‐based investigation on siliceous and calcareous microfossil biostratigraphy, magneto‐stratigraphy and tephrostratigraphy identified numerous datum events from the sedimentary sequences of Sites 1150 and 1151 drilled on the forearc basin of northern Japan by the Ocean Drilling Program Leg 186. Some 83 datum events were selected to construct new age–depth models for the sites. Based on the reliable magneto‐stratigraphy from the Pleistocene to the Upper Miocene, which were correlated to the standard geomagnetic polarity timescale, and on excellent records of diatom and radiolarian biostratigraphy throughout the sequences, the shipboard age model was revised. Major revisions referred to stratigraphic position of the Miocene–Pliocene boundary that has been shifted more than 200 m downward in each sequence. The age–depth relations of the forearc sites represent drastic changes in the sedimentation rate—extremely high (40 cm/k.y. on average) in the Early Pliocene and low (less than 2 cm/k.y. on average) in the Middle Miocene—and several hiatuses exist throughout the sequence. The drastic changes can be related mostly to changes in diatom sedimentation and the tectonics of the Japanese Island Arc. Local ages for some foraminiferal, calcareous nannofossil and radiolarian bioevents are estimated from the age–depth models at each site. These newly calibrated bioevents and biozones as well as established diatom biostratigraphy are incorporated into the updated magneto‐biochronologic timescale, which will contribute to an improvement in biochronologic accuracy of Neogene sediments in northern Japan and adjacent areas.  相似文献   

Estimation of Deeper Structure at the Soultz Hot Dry Rock Field by Means of Reflection Method Using 3C AE as Wave Source     

N. Soma  H. Niitsuma  R. Baria 《Pure and Applied Geophysics》1997,150(3-4):661-676

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Rupture and delamination of arc crust rupture and delamination of island arc crust due to the arc–arc collision in the South Fossa Magna, central Japan     

Nobuaki Niitsuma 《Island Arc》1999,8(4):441-458

Three collisional cycles, the Tanzawa, Izu and Shichito, are known to have occurred in the South Fossa Magna, central Japan, since the late Miocene, based on geologic evidence. The cycles consist of six stages. At present the South Fossa Magna is in the later part of stage 5 of the Izu Cycle and stage 2 of the Shichito Cycle. Because the collisional processes are ongoing we can observe, measure and correlate them with the geologic records of the former cycles. The collisional processes are progressing intermittently because of the rupture and deformation of the collided and colliding island arc crusts. Rupture in the subducting crust can be explained by the geometry of the subducting plate along a boundary that is not straight. The delamination of the upper crust is detected from the geologic and crustal structure in the collided Tanzawa Block; it is an important factor in the deformation of the crust.  相似文献