S-wave anisotropy off Lofoten, Norway, indicative of fluids in the lower continental crust?   总被引：2，自引：0，他引：2  

Rolf Mjelde  Markvard A. Sellevoll  Hideki Shimamura  Takaya Iwasaki  Toshihiko Kanazawa 《Geophysical Journal International》1995,120(1):87-96

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Groundwater microtemperature in earthquake regions     

Hideki Shimamura  Morio Ino  Hisayoshi Hikawa  Takaya Iwasaki 《Pure and Applied Geophysics》1984,122(6):933-946

Microtemperature measurements of groundwater with a relative precision better than 1/1000°C have been made in several seismically active areas in Japan. The measured temperatures show clear coseismic signals as well as a correlation with atmospheric pressure. Simultaneous observations at various depths have shown that these temperature changes were not induced by simple groundwater level changes. Also, distinctive signals occurred before several earthquakes and seem to be caused by a different mechanism than the coseismic signals. The microtemperature at some observation sites shows excellent correlation with records of nearby sensitive borehole strainmeters. Simultaneous recording of microtemperature and strain has been initiated in some boreholes.  相似文献   

Crustal and upper mantle structure in the Ryukyu Island Arc deduced from deep seismic sounding   总被引：4，自引：0，他引：4  

T. Iwasaki  N. Hirata  T. Kanazawa  J. Melles  K. Suyehiro  T. Urabe  L. Möller  J. Makris  H. Shimamura 《Geophysical Journal International》1990,102(3):631-651

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Ocean Bottom Seismographs used in a crustal study of an area covered with flood-basalt off Lofoten,N. Norway     

R. Mjelde  M.A. Sellevoll  H. Shimamura  T. Iwasaki  T. Kanazawa 《地学学报》1993,5(1):76-84

Seismic refraction measurements were made in August 1988 to study the crustal structure off Lofoten, Northern Norway. Twenty-four 3-component Ocean Bottom Seismographs (OBS) were used, of which seven were deployed in the area covered by landward-flowing basalt deposited during the early Eocene break-up between Norway and Greenland. The main purpose of the OBS survey was to investigate whether this method can be used to map structures below the basalt, which is not easy to penetrate with conventional seismic reflection techniques. The records obtained showed that the OBS data contain considerable information about structures below the flood-basalt; preopening sediments up to 4.0 km thick is indicated below the 1.0–2.5 km-thick landward-flowing basalt. The success of the OBS survey indicates that such measurements can become an important tool in investigations on passive volcanic margins and, potentially, in other areas where highly reflective boundaries make the reflection technique difficult to apply.  相似文献   

Structure of a Volcanic Continental Margin Derived from Ocean-bottom Seismographic Data: The Northern Vøring Margin, off Norway     

S. Kodaira  R. Mjelde  H. Shiobara  T. Kanazawa  H. Shimamura  E. W. Berg  O. Riise 《Pure and Applied Geophysics》1998,152(1):1-21

—This paper presents a crustal model derived from an Ocean Bottom Seismograph (OBS) study along the northern Vøring margin off Norway. The profile was acquired to map the crustal structure in the northernmost part of the Vøring Basin, and to link crustal models of the Lofoten and central Vøring Basin obtained by previous OBS studies. The Vøring margin, as well as the Lofoten margin to the north, was created by continental breakup between Norway and Greenland in late Paleocene-early Eocene. The rifting and continental breakup process were accompanied by intense extrusive and intrusive magmatic activities. The OBS data provide the whole crustal structure along the northern Vøring margin, in the area where the deep crustal structure cannot be resolved by conventional multichannel reflection data due to sill intrusions in the sedimentary sequence. The shallow part of the crustal model is characterized by up to 10 km thick sediments, a sequence of flood basalts and sill intrusions. The P-wave velocities in the flood basalts and sill intrusions are estimated to 5.0 km/s and 4.7–5.8 km/s, respectively. The model indicates an abrupt thickening of the upper crystalline crust from approx.3 km in the NE, to about 10 km towards the SE, with velocities of 6.0–6.2 km/s. The lower crustal velocities are not well resolved due to lack of clear refraction arrivals from the lower crust. However, the observed amplitude versus offsets are best explained by a model with a change in lower crustal velocities from 6.8 to 7.2 km/s beneath the Bivrost lineament. The modelling infers the presence of a lower crustal reflector beneath the lineament, which represents the landward continuation of the Bivrost lineament. Reflection arrivals from the Moho reveal a Moho depth of 23 km in the middle of the profile and 18– 20 km in the northeastern part of the profile. A 370 km long crustal section from the central part of the Vøring Basin to the Lofoten margin, obtained by the results of this study and previous OBS studies, shows a simple thinned continental crust on the Lofoten margin, and a high velocity lower crust underlying an upper crust of varying thickness in the Vøring Basin. The transition between these structures is situated beneath the Bivrost lineament in the lower crust, and beneath the basement high about 40 km south of the lineament in the upper crust.  相似文献   

Enhancement/reduction of biological pump depends on ocean circulation in the sea-ice reduction regions of the Arctic Ocean   总被引：1，自引：0，他引：1  

Shigeto Nishino  Takashi Kikuchi  Michiyo Yamamoto-Kawai  Yusuke Kawaguchi  Toru Hirawake  Motoyo Itoh 《Journal of Oceanography》2011,67(3):305-314

The biological pump is a central process in the ocean carbon cycle, and is a key factor controlling atmospheric carbon dioxide (CO₂). However, whether the Arctic biological pump is enhanced or reduced by the recent loss of sea ice is still unclear. We examined if the effect was dependent on ocean circulation. Melting of sea ice can both enhance and reduce the biological pump in the Arctic Ocean, depending on ocean circulation. The biological pump is reduced within the Beaufort Gyre in the Canada Basin because freshwater accumulation within the gyre limits nutrient supply from deep layers and shelves hence inhibits the growth of large-bodied phytoplankton. Conversely, the biological pump is enhanced outside the Beaufort Gyre in the western Arctic Ocean because of nutrient supply from shelves and greater light penetration, enhancing photosynthesis, caused by the sea ice loss. The biological pump could also be enhanced by sea ice loss in the Eurasian Basin, where uplifted isohaline surfaces associated with the Transpolar Drift supply nutrients upwards from deep layers. New data on nitrate uptake rates are consistent with the pattern of enhancement and reduction of the Arctic biological pump. Our estimates indicate that the enhanced biological pump can be as large as that in other oceans when the sea ice disappears. Contrary to a recent conclusion based on data from the Canada Basin alone, our study suggests that the biological CO₂ drawdown is important for the Arctic Ocean carbon sink under ice-free conditions.  相似文献   

Dehydration kinetics of antigorite using in situ high-temperature infrared microspectroscopy     

Michiyo Sawai  Ikuo Katayama  Arisa Hamada  Makoto Maeda  Satoru Nakashima 《Physics and Chemistry of Minerals》2013,40(4):319-330

The dehydration kinetics of serpentine was investigated using in situ high-temperature infrared microspectroscopy. The analyzed antigorite samples at room temperature show relatively sharp bands at around 3,655–3,660 cm^?1 (band 1), 3,570–3,595 cm^?1 (band 2), and 3,450–3,510 cm^?1 (band 3). Band 1 corresponds to the Mg–OH bond, and bands 2 and 3 correspond to OH associated with the substitution of Al for Si. Isothermal kinetic heating experiments at temperatures ranging from 625 to 700 °C showed a systematic decrease of the OH band absorbance with heating duration. The one-dimensional diffusion was found to provide the best fit to the experimental data, and diffusion coefficients were determined with activation energies of 219 ± 37 kJ mol^?1 for the total water band area, 245 ± 46 kJ mol^?1 for band 1, 243 ± 57 kJ mol^?1 for band 2, and 256 ± 53 kJ mol^?1 for band 3. The results indicate that the dehydration process is controlled by one-dimensional diffusion through the tetrahedral geometry of serpentine. Fluid production rates during antigorite dehydration were calculated from kinetic data and range from 3 × 10^?4 to 3 × 10^?5  $ {\text{m}}_{\text{fluid}}^{ 3} \,{\text{m}}_{\text{rock}}^{ - 3} \,{\text{s}}^{ - 1} $ . The rates are high enough to provoke hydraulic rupture, since the relaxation rates of rocks are much lower than these values. The results suggest that the rapid dehydration of antigorite can trigger an intermediate-depth earthquake associated with a subducting slab.  相似文献   

Crustal structure of the Lofoten continental margin, off northern Norway, from ocean-bottom seismographic studies   总被引：2，自引：0，他引：2  

S. Kodaira  A. Goldschmidt-Rokita  J. M. Hartmann  H. B. Hirschleber  T. Iwasaki  T. Kanazawa  H. Krahn  S. Tomita  H. Shimamura 《Geophysical Journal International》1995,121(3):907-924

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Ca isotope variations in Allende     

M.H.A. Jungck  T. Shimamura  G.W. Lugmair 《Geochimica et cosmochimica acta》1984,48(12):2651-2658

We present Ca isotopic measurements on Allende Ca-Al-rich inclusions (CAIs) and an apatite enriched fraction from Orgueil. The results on CAIs show widespread excesses on the neutron-rich isotope ⁴⁸Ca. All other isotopes agree with the normal within our presently obtainable precision. Seven out of 11 CAIs analyzed exhibit isotopic anomalies ranging up to ~ +6? units (1? = 1 part in 10,000). This abundant occurrence of isotopic excesses places Ca alongside Ti and O, elements which show isotopic anomalies in all Allende CAIs measured so far. However, at present no clear correlation can be found between excesses in ⁴⁸Ca and ⁵⁰Ti, the isotopes which are thought to be coproduced by neutron-rich nucleosynthetic processes within stars. We believe that the relatively higher volatility of Ca compared to Ti compounds could have led to a variable dilution with isotopically “normal” Ca in vaporization and recondensation processes in stellar envelopes, the interstellar medium and/or the solar nebula. High precision measurements of ⁴⁶Ca limit possible anomalies in this 33 ppm abundance isotope to 10? units or less and, together with the observed ⁴⁸Ca and ⁵⁰Ti anomalies, constrain possible nucleosynthetic mechanisms capable of producing these neutron-rich nuclei.  相似文献   

Ni isotopic compositions in Allende and other meteorites     

T. Shimamura  G.W. Lugmair 《Earth and Planetary Science Letters》1983,63(2):177-188

A new technique for high-precision isotopic analyses of Ni was developed and applied to terrestrial samples, Allende inclusions and materials from other meteorites. Most of the Allende inclusions analysed here were previously reported to contain isotopically anomalous Ti. In contrast, the Ni isotopic abundances are indistinguishable from normal within presently obtainable precision with only one possible exception. The latter inclusion was shown by others to contain a significantly fractionated magnesium isotopic pattern of 9‰/amu. A normal Ni isotopic pattern has also been observed for the chromite/carbon fraction of an Allende acid residue which is known to contain heavy noble gases of highly anomalous isotopic composition. All other meteoritic samples analysed (Khohar matrix and chondrules, Murray matrix, a Tieschitz chondrule and an Orgueil magnetic fraction) also show normal isotopic compositions of Ni; no evidence for effects from now extinct⁶⁰Fe could be detected. In spite of ubiquitous isotopic anomalies in Ti from normal Allende inclusions, there is no signature of isotopic variations in Ni from the same samples. Possible constraints for the nucleosynthesis of iron peak elements and for astrophysical and cosmochemical conditions during formation of the solar system are discussed.  相似文献