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排序方式: 共有292条查询结果,搜索用时 31 毫秒
241.
242.
Hideyuki Kawamura Toshimichi Ito Takuya Kobayashi Shigeyoshi Otosaka Naoki Hirose Orihiko Togawa 《Journal of Oceanography》2010,66(5):649-662
A numerical experiment is carried out to reproduce distribution of concentration of 90Sr and 137Cs, estimate their total amount and verify their source in the Japan Sea. Model results are in good agreement with observational
findings in the Japan Sea expeditions between 1997 and 2002 by the Japan Atomic Energy Agency. Vertical profiles of the concentration
of 90Sr and 137Cs show exponential decreases with depth from the sea surface to the sea bottom. From the model and observational results,
it is suggested that the concentration of 90Sr and 137Cs in the surface layer is approximately in the range of 1.0–1.5 Bq/m3 and 2.0–2.5 Bq/m3, respectively. On the other hand, it is found that the concentration in the intermediate and deep layer
is higher than that observed in the northwestern Pacific Ocean, suggesting active winter convection in the Japan Sea. The
total amount of 90Sr and 137Cs in the seawater is evaluated to be 1.34 × 1015 Bq and 2.02 × 1015 Bq, respectively, in the numerical experiment, which demonstrates an estimation by observational data obtained in the Japan
Sea expeditions. The total amount of 90Sr and 137Cs changed during the second half of 20th century corresponding to deposition at the sea surface with the maximums of 4.86
× 1015 Bq for 90Sr and 7.33 × 1015 Bq for 137Cs, respectively, in the mid-1960s. The numerical experiment suggests that the main source of 90Sr and 137Cs has been global fallout, although there have been some potential sources in the Japan Sea. 相似文献
243.
This article examines how a community of oyster farmers in Hokkaido, Japan recovered from the 2011 Tōhoku Earthquake and Tsunami, which damaged their aquaculture in Lake Akkeshi. It focuses on how, over the years, nested governance structures facilitated iterative learning, which helped build resilience against unexpected external crises. Drawing on adaptive management and resilience studies, this study examines the historical development of the local fishery cooperative association (FCA) as a series of responses to socio-ecological disasters — responses that this article characterizes as iterative learning. During the recovery after the 2011 tsunami, the FCA functioned as an information hub that directed the flow of knowledge and resources through government and industrial hierarchies, to the benefit of the farmers. This research, conducted between 2015 and 2017, is based on interviews with farmers, cooperative administrators, and government officials, as well as an analysis of archived records, fishery regulations and post-disaster subsidy programs. 相似文献
244.
An oxygen minimum zone (OMZ) currently exists at intermediate water depths on the northern Japanese margin in the northwestern Pacific. The OMZ results largely from a combination of high surface–water productivity and poor ventilation of intermediate waters. We investigated the late Quaternary history (last 27 kyr) of the intensity of this OMZ using changes in benthic foraminiferal carbon isotopes and assemblages in a sediment core taken on the continental slope off Shimokita Peninsula, northern Japan, at a water depth of 975 m. The core was located well within the region of the present-day OMZ and high surface–water productivity. The benthic foraminiferal δ13C values, which indicate millennial-scale fluctuations of nutrient contents at the sediment–water interface, were 0.48‰ lower during the last glacial maximum (LGM) than during the late Holocene. These results do not indicate the formation of glacial intermediate waters of subarctic Pacific origin, but rather the large contribution of high-nutrient water masses such as the Antarctic Intermediate Water, implying that the regional circulation pattern during the LGM was similar to that of modern times. Benthic foraminiferal assemblages underwent major changes in response to changes in dissolved oxygen concentrations in ocean floor sediments. The lowest oxygen and highest nutrient conditions, marked by dysoxic taxa and negative values of benthic foraminiferal δ13C, occurred during the Bølling/Allerød (B/A) and Pre-Boreal warming events. Dysoxic conditions in this region during these intervals were possibly caused by high surface–water productivity at times of reduced intermediate–water ventilation in the northwestern Pacific. The benthic assemblages show dysoxic events on approx. 100- to 200-year cycles during the B/A, reflecting centennial-scale productivity changes related to freshwater cycles and surface–water circulation in the North Pacific. 相似文献
245.
Motohiko Kashima Shin-Ichi Ito Kaoru Ichikawa Shiro Imawaki Shin-Ichiro Umatani Hiroshi Uchida Takashi Setou 《Journal of Oceanography》2009,65(1):73-80
A quasiperiodic variation of 100–110 days in the Kuroshio path off Cape Ashizuri, resulting from the passage of small meanders,
was detected by observation with moored current meters during 1993–1995. TOPEX/POSEIDON altimeter data covering 9 years showed
that the quasiperiodic variation period was not persistent and modulated twice, with a ∼110-day period from mid-1993 to late
1996, a ∼150-day period from late 1996 to mid-1999, and a ∼110-day period from mid-1999 to late 2001. The quasiperiodic variations
of the Kuroshio path migration were contemporaneous with the quasiperiodic arrivals of mesoscale eddies from the east along
27–32°N over the same ∼110- and ∼150-day period quasiperiodic variations. The periodic arrivals of the eddies configure the
periodic variations of the Kuroshio path and its inter-annual modulation. 相似文献
246.
The potential role of the tide-induced time-mean flow (the tidal residual current) in determining transport through the Tsugaru
Strait (located between the East/Japan Sea and the North Pacific) is investigated using a high-resolution numerical barotropic
model. The calculated K1, O1, M2, and S2 tidal fields agree well with available observational records derived from both tide gauge and current meter measurements
in the strait and the adjacent seas. The tidal residual current speed reaches 0.3 ms−1 in two narrow “neck” areas where topographic sills are located. This result suggests that tides should be taken into account
in estimating the long-term water mass and nutrient transport through narrow regions between the East/Japan Sea and the North
Pacific. An interesting aspect of the tidal residual current field is the prediction of several active eddy zones in which
sequences of eddy triplets develop in the vicinity of capes. Our vorticity analysis reveals that the interplay of topographic
effects arising from both the headland and the sill around capes plays a critical role in the formation of these triple eddy
patterns. 相似文献
247.
Keisuke Ito 《Pure and Applied Geophysics》1992,138(4):531-548
Recent advances in the theory of fracture and fragmentation are reviewed. Empirical laws in seismology are interpreted from a fractal perspective, and earthquakes are viewed as a self-organized critical phenomenon (SOC). Earthquakes occur as an energy dissipation process in the earth's crust to which the tectonic energy is continuously input. The crust self-organizes into the critical state and the temporal and spatial fractal structure emerges naturally. Power-law relations known in seismology are the expression of the critical state of the crust. An SOC model for earthquakes, which explains the Gutenberg-Richter relation, the Omori's formula of aftershocks and the fractal distribution of hypocenters, is presented. A new view of earthquake phenomena shares a common standpoint with other disciplines to study natural complex phenomena with a unified theory. 相似文献
248.
The relationship between the slip activity and occurrence of historical earthquakes along the Median Tectonic Line (MTL), together with that of the fault systems extending eastward has been examined. The MTL is divided into three segments, each containing diagnostic active faults. No historical earthquakes have been recorded along the central segment, although the segment has faster Quaternary slip rates compared with the other segments that have generated historical earthquakes. This discrepancy between earthquake generation and slip rate can be explained by a microplate model of southwest Japan. The microplate model also provides spatial and temporal coupling of slip on adjacent fault systems. In the context of this model, slip on adjacent faults reduces the normal stress on the MTL. Historical data and paleoseismic evidence indicate that slip on this segment occurs without significant strong ground motion. We interpret this as indicating anomalously slow seismic slip or aseismic slip. Slip on the central segment of the MTL creates transpressional regions at the eastern and western segments where historical earthquakes were recorded. Alternatively, the earthquakes at the eastern and western segments were triggered and concentrated shear stress at the edge of the segments resulted in postseismic slip along the central segment. The sequence of historical events suggests that the MTL characteristically does not produce great earthquakes. The microplate model also provides a tectonic framework for coupling of events among the MTL, the adjacent fault systems and the Nankai trough. 相似文献
249.
Dynamic compaction tests of bentonite-based materials (BBMs) with 100, 70 and 50% bentonite contents have been performed using five powdery bentonites with different physicochemical properties to establish the simplified evaluation method for dynamic compaction properties of BBMs. For a given bentonite content and a total compaction energy condition, the maximum dry density, ρdmax, and the optimum water content, wopt, which are well-known indexes of compaction properties, for BBMs were determined according to the type of bentonite used for BBMs. For evaluation of those values of BBMs derived in this study, the plastic limit of BBM, wpbbm, was defined as the plastic limit that was measured using the sample pulverized to a maximum grain size of less than 425 μm in the case of BBM with sand having a maximum grain size of more than 425 μm and was measured using the powdery bentonite itself in the case of BBM without sand. This study proposed equations for evaluating ρdmax and wopt of BBMs with more than 50% bentonite content under the total compaction energy conditions of 551–2755 kN-m/m3 using wpbbm. Finally, we related the equations derived in this study to the equation for evaluating hydraulic properties of compacted BBMs proposed in previous work and proposed the preparation method of BBMs with more than 50% bentonite content for constructing BBM buffer by in-situ compaction method. 相似文献
250.
Fe(III) complexed by organic ligands (Fe(III)L) is the primary form of dissolved Fe in marine and coastal environments. Superoxide, typically produced in biological and photochemical processes, is one of the reducing agents that contributes to transformation of Fe(III)L to bioavailable, free dissolved Fe(II) (Fe(II)′). In this work, the kinetics of superoxide-mediated Fe(II)′ formation from Fe(III)L in a simulated coastal water system were investigated and a comprehensive kinetic model was developed using citrate and fulvic acid as exemplar Fe-binding ligands. To simulate a coastal environment in laboratory experiments, Fe(III)L samples with various ligand/Fe ratios were incubated for 5 min to 1 week in seawater medium. At each ratio and incubation time, the rate of superoxide-mediated Fe(II)′ formation was determined in the presence of the strong Fe(II) binding ligand ferrozine by spectrophotometrically measuring the ferrous-ferrozine complex generated at a constant concentration of superoxide. The Fe(II)′ formation rate generally decreased with incubation time, as Fe(III)L gradually dissociated to form less reactive Fe(III) oxyhydroxide. However, when the ligand/Fe ratio was sufficiently high, the dissociation of Fe(III)L (and subsequent Fe precipitation) was suppressed and Fe(II)′ was formed at a higher rate. The rate of Fe(II)′ produced during the experiment was explained by the kinetic model. The model confirmed that both the ligand/Fe ratio and incubation time have a significant effect on the pathway via which Fe(II)′ is formed from Fe(III)-fulvic acid complexes. 相似文献