Impact of flood events on macrobenthic community structure on an intertidal flat developing in the Ohta River Estuary     

Wataru Nishijima  Yoichi Nakano  Satoshi Nakai  Tetsuji Okuda  Tsuyoshi Imai  Mitsumasa Okada 《Marine pollution bulletin》2013

We investigated the effects of river floods on the macrobenthic community of the intertidal flat in the Ohta River Estuary, Japan, from 2005 to 2010. Sediment erosion by flood events ranged from about 2–3 cm to 12 cm, and the salinity dropped to 0‰ even during low-intensity flood events. Cluster analysis of the macrobenthic population showed that the community structure was controlled by the physical disturbance, decreased salinity, or both. The opportunistic polychaete Capitella sp. was the most dominant species in all clusters, and populations of the long-lived polychaete Ceratonereis erythraeensis increased in years with stable flow and almost disappeared in years with intense flooding. The bivalve Musculista senhousia was also an important opportunistic species that formed mats in summer of the stable years and influenced the structure of the macrobenthic community. Our results demonstrate the substantial effects of flood events on the macrobenthic community structure.  相似文献   

Hysteresis and Surface Shear Stresses During Snow-Particle Aeolian Transportation     

Niiya  Hirofumi  Nishimura  Kouichi 《Boundary-Layer Meteorology》2022,183(3):447-467

Boundary-Layer Meteorology - Surface shear stresses produced by wind and particle collision play a key role in aerodynamic entrainment and splash processes. The fluid shear stress at the surface...  相似文献   

Crustal transect from the North Atlantic Knipovich Ridge to the Svalbard Margin west of Hornsund   总被引：1，自引：0，他引：1  

Frode Ljones  Asako Kuwano  Rolf Mjelde  Asbjrn Breivik  Hideki Shimamura  Yoshio Murai  Yuichi Nishimura 《Tectonophysics》2004,378(1-2):17-41

The crustal structure along a 312 km transect, stretching from the axial mountains of the North Atlantic Knipovich Ridge to the continental shelf of Svalbard, has been obtained using seismic reflection data and wide angle OBS data. The resulting seismic V_p and V_s models are further constrained by a 2-D-gravity model. The principal objective of this study is to describe and resolve the physical and compositional properties of the crust in order to understand the processes and creation of oceanic crust in this extremely slow-spreading counterpart of the North Atlantic Ridge Systems. V_p is estimated to be 3.50–6.05 km/s for the upper oceanic crust (oceanic layer 2), with a marked increase away from the ridge. The measured V_p of 6.55–6.95 km/s for oceanic layer 3A and 7.10–7.25 km/s for layer 3B, both with a V_p/V_s ratio of 1.81, except for slightly higher values at the ridge axis, does not allow a clear distinction between gabbro and mantle-derived peridotite (10–40% serpentized). The thickness of the oceanic crust varies a lot along the transect from the minimum of 5.6 km to a maximum of 8.1 km. The mean thickness of 6.7 km for the oceanic crust is well above the average thickness for slow-spreading ridges (<10 mm/year half-spreading rate). The areas of increased thickness could be explained by large magma production-rates found in the zones of axial highs at the ridge axis, which also have generated the off-axial highs adjacent the ridge. We suggest that these axial and off-axial highs along the ridge control the lithological composition of the oceanic crust. This approach suggests normal gabbroic oceanic crust to be found in the areas bound by the active magma segments (the axial and off-axial highs) and mantle-derived peridotite outside these zone.  相似文献   

The Wide Band Spectrometer on the SOLAR-A     

M. Yoshimori  K. Okudaira  Y. Hirasima  T. Igarashi  M. Akasaka  Y. Takai  K. Morimoto  T. Watanabe  K. Ohki  J. Nishimura  T. Yamagami  Y. Ogawara  I. Kondo 《Solar physics》1991,136(1):69-88

The SOLAR-A spacecraft has spectroscopic capabilities in a wide energy band from soft X-rays to gamma-rays. The Wide Band Spectrometer (WBS), consisting of three kinds of spectrometers, soft X-ray spectrometer (SXS), hard X-ray spectrometer (HXS) and gamma-ray spectrometer (GRS), is installed on SOLAR-A to investigate plasma heating, high-energy particle acceleration, and interaction processes. SXS has two proportional counters and each counter provides 128-channel pulse height data in the 2–30 keV range every 2 s and 2-channel pulse count data every 0.25 s. HXS has a NaI scintillation detector and provides 32-channel pulse height data in the 20–400 keV range every 1 s and 2-channel pulse count data every 0.125 s. GRS has two identical BGO scintillation detectors and each detector provides 128-channel pulse height data in the 0.2–10 MeV range every 4 s and 4-channel pulse count data (0.2–0.7, 0.7–4, 4–7, and 7–10 MeV) every 0.25–0.5 s. In addition, each of the BGO scintillation detectors provides 16-channel pulse height data in the 8–100 MeV range every 4 s and 2-channel pulse count data (8–30 and 30–100 MeV) every 0.5 s. The SXS observations enable one to study the thermal evolution of flare plasma by obtaining time series of electron temperatures and emission measures of hot plasma; the HXS observations enable one to study the electron acceleration and heating mechanisms by obtaining time series of the electron spectrum; and the GRS observations enable one to study the high-energy electron and ion acceleration and interaction processes by obtaining time series of electron and ion spectra.After the launch the name of SOLAR-A has been changed to YOHKOH.  相似文献   

The Krakatau islands: The geotectonic setting     

Susumu Nishimura Dr.  H. Harjono Dr.  S. Suparka 《GeoJournal》1992,28(2):87-98

The Sunda Strait is located in a transitional zone between two different modes of subduction, the Java frontal and Sumatra oblique subductions. Western Java and Sumatra are, however, geologically continuous.The Krakatau complex lies at the intersection of two graben zones and a north-south active, shallow seismic belt, which coincides with a fracture zone along this seismic belt with fissure extrusion of alkali basaltic rocks commencing at Sukadana and continuing southward as far as the Panaitan island through Rajabasa, Sebuku and Krakatau.Paleomagnetic studies suggest that the island of Sumatra has been rotating clockwise relative to Java from at least 2.0 Ma to the present at a rate of 5–10h/Ma, and therefore the opening of the Sunda Strait might have started before 2 Ma (Nishimura et al. 1986).From geomorphological and seismological studies, it is estimated that the western part of Sumatra has been moving northward along the Semangko fault and the southern part of Sunda Strait has been pulled apart.Assuming that the perpendicular component (58 mm/yr; Fitch 1972) of the oblique subduction has not changed, we can estimate that the subduction started at 7–10 Ma. Huchon and LePichon (1984) also estimated that the subduction started at 13 Ma.Recent crustal earthquakes in the Sunda Strait area are clustered into three groups: (1) beneath the Krakatau complex where they are typically of tectonic origin, (2) inside a graben in the western part of the strait, and (3) in a more diffuse zone south of Sumatra. The individual and composite focal mechanisms of the events inside the strait show an extensional regime. A stress tensor, deduced from the individual focal mechanisms of the Krakatau group shows that the tensional axis has a N 130°E orientation (Harjono et al. 1988).These studies confirm that the Sunda Strait is under a tensional tectonic regime as a result of clockwise rotation along the continental margin and northward movement of the Sumatra sliver plate along the Semangko fault zone.  相似文献   

Snow ripples and their contribution to the mass transport in drifting snow     

Kenji Kosugi  Kouichi Nishimura  Norikazu Maeno 《Boundary-Layer Meteorology》1992,59(1-2):59-66

Morphological characteristics of snow ripples formed by drifting snow were investigated as functions of wind velocity in a cold wind tunnel at -15 °C. Wave-length, wave height and migration rate of snow ripples increased from 5 to 20 cm, 3 to 5 mm and 1 to 8 cm/min, respectively, with increasing wind velocity from 5 to 7 m/s. Measured size distributions of snow particles in snow ripples showed sorting of large particles in ridges, suggesting that the snow ripple migration is caused by creeping of large particles. The snow drift rate caused by creep, that is, by the ripple migration, was estimated to amount, at least, to 6% of the total snow drift rate.  相似文献   

Alvin-SeaBeam studies of the Sumisu Rift, Izu-Bonin arc     

B. Taylor  G. Brown  P. Fryer  J.B. Gill  A.G. Hochstaedter  H. Hotta  C.H. Langmuir  M. Leinen  A. Nishimura  T. Urabe 《Earth and Planetary Science Letters》1990,100(1-3)

Bimodal volcanism, normal faulting, rapid sedimentation, and hydrothermal circulation characterize the rifting of the Izu-Bonin arc at 31°N. Analysis of the zigzag pattern, in plan view, of the normal faults that bound Sumisu Rift indicates that the extension direction (080° ± 10°) is orthogonal to the regional trend of the volcanic front. Normal faults divide the rift into an inner rift on the arc side, which is the locus for maximum subsidence and sedimentation, and an outer rift further west. Transfer zones that link opposing master faults and/or rift flank uplifts further subdivide the rift into three segments along strike. Volcanism is concentrated along the ENE-trending transfer zone which separates the northern and central rift segments. The differential motion across the zone is accommodated by interdigitating north-trending normal faults rather than by ENE-trending oblique-slip faults. Volcanism in the outer rift has built 50–700 m high edifices without summit craters whereas in the inner rift it has formed two multi-vent en echelon ridges (the largest is 600 m high and 16 km long). The volcanism is dominantly basaltic, with compositions reflecting mantle sources little influenced by arc components. An elongate rhyolite dome and low-temperature hydrothermal deposits occur at the en echelon step in the larger ridge, which is located at the intersection of the transfer zone with the inner rift. The chimneys, veins, and crusts are composed of silica, barite and iron oxide, and are of similar composition to the ferruginous chert that mantles the Kuroko deposits. A 1.2-km transect of seven alvin heat flow measurements at 30°48.5′N showed that the inner-rift-bounding faults may serve as water recharge zones, but that they are not necessarily areas of focussed hydrothermal outflow, which instead occurs through the thick basin sediments. The rift basin and arc margin sediments are probably dominated by permeable rhyolitic pumice and ash erupted from submarine arc calderas such as Sumisu and South Sumisu volcanoes.  相似文献   

Near-infrared observations of the dust coma of Comet Kohoutek (1973f) with a tilting-filter Fabry-Perot photometer     

C. Barbieri  C.B. Cosmovici  K.W. Michel  T. Nishimura  A.E. Roche 《Icarus》1974,23(4):568-576

By means of narrow-band Fabry-Perot filters, which exclude the interference from molecular line fluorescence, the brightness of Comet Kohoutek (1973f) has been measured at 8560 and 8748 Å. Data reduction on the basis of averaged Mie-scattering cross sections indicates that the dust production rate was different before and after perihelion at the same heliocentric distances. This asymmetry suggests that vaporization and dust entrainment were governed by fractionation of a multicomponent mixture of parent molecules in a comparatively porous cometary nucleus.  相似文献   

Interannual variability and predictability of summertime significant wave heights in the western north pacific     

Wataru Sasaki  Hibiya Toshiyuki 《Journal of Oceanography》2007,63(2):203-213

We propose and validate a linear regression model which enables us to predict the summer (June–August) mean of the monthly 90th percentile of significant wave heights (H₉₀) in the western North Pacific (WNP). The most prevailing interannual variability of H⁹⁰ is identified by applying an Empirical Orthogonal Function analysis to H⁹⁰ obtained from the ERA-40 wave reanalysis as well as from the optimally interpolated TOPEX/Poseidon (OITP) wave data. It is found that the increase of H⁹⁰ is correlated with cyclonic circulation in the WNP which links with warm SST anomalies in the Niño-3.4 region. We adopt zonal wind anomaly averaged over the region 5°N–15°N, 130°E–160°E (U^10N) as a predictor of the first principal component (PC1) of H⁹⁰, since U^10N is closely correlated with the PC1 of H⁹⁰. It is revealed that regression models obtained from two different wave datasets are nearly identical. The predictability of the regression model is assessed in terms of the reduction of the root-mean-square (rms) errors between H⁹⁰ and the reconstructed data. The predictor is found to be successful in reducing the rms errors by up to 40% for the ERA-40 wave reanalysis and by up to 70% for the OITP wave data within the latitudinal band 10°N–25°N, though rms errors exceeding 0.3 m still remain, particularly in the East China Sea.  相似文献   

Applicability of numerical models to nonlinear dispersive waves     

Takumi Ohyama  Wataru Kioka  Akihide Tada 《Coastal Engineering》1995,24(3-4)

The applicability of three different wave-propagation models in nonlinear dispersive wave fields has been investigated. The numerical models tested here are based on three different wave theories: a fully nonlinear potential theory, a Stokes second-order theory, and a Boussinesq-type theory with an improved dispersion relation. Physical experiments and computations were conducted for wave evolutions during passage over a submerged shelf under various wave conditions. As expected, the fully nonlinear solutions agree better with the measurements than do the other solutions. Although the second-order solution has sufficient accuracy for smaller-amplitude wave cases, the truncation after the third harmonics causes significant discrepancies in wave form for larger waves. In addition, the second-order model markedly overestimates the first- and second-harmonic amplitudes in transmitted waves. The Boussinesq model provides excellent predictions of wave profile over the shelf even in larger wave cases. However, this model also overestimates the magnitudes of several higher harmonics in transmitted waves. These facts may indicate that energy transfer from bound components into free waves in these higher harmonics cannot be accurately evaluated by the Boussinesq-type equations.  相似文献