Distribution of boulders at Miyara Bay of Ishigaki Island,Japan: A flow characteristic indicator of tsunami and storm waves     

Kazuhisa Goto  Tetsuya Shinozaki  Koji Minoura  Kiyohiro Okada  Daisuke Sugawara  Fumihiko Imamura 《Island Arc》2010,19(3):412-426

This study investigates the distribution of boulders at Miyara Bay of Ishigaki Island, Japan. These boulders were deposited on a reef flat extending approximately 400–1300 m in width. Most boulders were rectangular to ellipsoidal, without sharp broken edges. They are reef and coral rock fragments estimated as <335 m³ (<633 t). Locally in the bay, the relationship between the boulder weight and position shows that boulders of a given weight have a clear limit on seaward distribution on the reef flat. For example, more than 1, 10, and 100 tons of boulders were deposited, respectively, more than 500, 300, and 100 m from the reef edge. The line is consistent with the possible landward transport limit by maximum storm waves at the Ryukyu Islands, suggesting that the line was formed by the reworking of some boulders by maximally strong storm waves, although we can not exclude the possibility that the line was formed by tsunamis. Furthermore, 68% of boulders at the bay are deposited beyond this line. Therefore, the presence of these boulders at their present positions is difficult to explain solely by storm waves, implying the possible tsunami origin of these boulders. The boulders are characteristically concentrated along the high‐tide line, suggesting the drastic reduction of the tsunami hydraulic force along the line. Previous studies using radiocarbon age dating, as well as our study, imply that at least 69 boulders at Miyara Bay were probably deposited at their present positions by the 1771 Meiwa tsunami, although some of these boulders might have been emplaced and displaced on the reef flat by prior tsunami or storm surges.  相似文献   

Effects of Earth＇s curvature and radial hetero-geneity in dislocation studies： Case studies of the 2008 Wenchuan earthquake and the 2004 Sumatra earthquake     

Guangyu Fu  Wenke Sun  Yoichi Fukuda  Shanghua Gao  Takashi Hasegawa 《地震学报(英文版)》2010,(4):301-308

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Assessing the role of forests in mitigating eutrophication downstream of pasture during spring snowmelt          下载免费PDF全文

Masaaki Chiwa  Sachiko Inoue  Naoaki Tashiro  Daisuke Ohgi  Yoshitoshi Uehara  Hideaki Shibata  Atsushi Kume 《水文研究》2015,29(4):615-623

Little research has examined whether forests reduce stream water eutrophication in agricultural areas during spring snowmelt periods. This study evaluated the role of forests in ameliorating deteriorated stream water quality in agricultural areas, including pasture, during snowmelt periods. Temporal variation in stream water quality at a mixed land‐use basin (565 ha: pasture 13%, forestry 87%), northern Japan, was monitored for 7 years. Synoptic stream water sampling was also conducted at 16 sites across a wide range of forest and agricultural areas in a basin (18.3 km²) in spring, summer and fall. Atmospheric nitrogen (N) and phosphorus (P) deposition were measured for 4 years. The results showed that concentration pulses of nitrate, organic N and total P in stream water were observed when discharge increased during spring snowmelt. Their concentrations were high when silicate concentrations were low, suggesting surface water exported from pasture largely contributed to stream water pollution during snowmelt. Atmospheric N and P deposition (4.1 kg N ha^?1 y^?1; 0.09 kg P ha^?1 y^?1, respectively) was too low to affect the background concentrations of N and P in streams from forested areas. Reduction of eutrophication caused by nutrients from pasture was mainly due to dilution by water containing low concentrations of N and P exported from forested areas, whereas in‐stream reduction was not a dominant process. Results indicate that forests have a limited capacity to reduce the concentration pulses of N and P in stream water during snowmelt in this study basin. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

The behavior and the role of the anti-cyclonic eddy in the Kuroshio large meander development     

Futoki Sakaida  Daisuke Egusa  Hiroshi Kawamura 《Journal of Oceanography》1998,54(1):101-114

The processes underlying the development of the Kuroshio large meanders that occurred in 1986 and 1989 are investigated using a satellite SST data set and hydrographic data. In both processes visible on the satellite SST images, a round-shaped, lower SST region with a diameter of about 200 km is found to the east of the Kuroshio small “trigger” meander (Solomon, 1978) until the region became extinguished near theEnshu Nada. The lower SST region can be interpreted as an anti cyclonic eddy, mainly because of the existence of a warm water mass in the subsurface layer of this region. The warm water mass is characterized by a constant temperature of 18–19°C, the maximum thickness of which is about 400 m. The satellite images show that the eddy is closely related to the Kuroshio path transforming into a shape like the letter “S”. This means that the eddy plays an important role in the development of the Kuroshio large meander since this, too, tends to follow an “S”-shaped path. Added to this, the subsurface layer structure of the eddy is similar to that of the warm water mass offShikoku. This similarity, together with the eddy behavior visible on the satellite SST images, implies that the examined eddy corresponds to the warm water mass offShikoku. In other words, the warm water mass offShikoku can be advected near to theEnshu Nada when the Kuroshio large meander occurs.  相似文献   

Estimating the thickness of a faulted mechanical layer based on fault spacing: An example from Miyako‐jima Island,southern Ryukyu Arc,Japan     

Sumire Maeda  Makoto Otsubo  Daisuke Asahina  Yuki Furuno 《Island Arc》2020,29(1)

The thickness of the mechanical layer that hosts a group of faults can be estimated from the spacing of saturated faults (i.e. the constant spacing between faults when the fault system is fully developed and has attained its final fault density). We measured fault spacing for a group of saturated active normal faults on Miyako‐jima Island (southern Ryukyu Arc, Japan) and estimated the thickness of the faulted mechanical layer. The measured fault spacing is 1.30 ±0.14 km, and the thickness of the mechanical layer is < 3 km, which is the upper limit of the seismogenic zone. This faulted mechanical layer corresponds to a sedimentary layer in which earthquakes cannot occur. Results indicate that the shallow (< 3 km depth) normal faults on Miyako‐jima Island do not have the potential to cause medium‐size earthquakes as individual faults. The origin of the shallow normal faults might be related to the presence of a larger‐scale, deeper fault. The results indicate that fault spacing provides important information on the potential magnitude of earthquakes associated with active faults.  相似文献   

Climate projections over different climatic regions of Afghanistan under shared socioeconomic scenarios     

Sediqi  Mohammad Naser  Hendrawan  Vempi Satriya Adi  Komori  Daisuke 《Theoretical and Applied Climatology》2022,149(1-2):511-524

Theoretical and Applied Climatology - The global climate models (GCMs) of Coupled Model Intercomparison Project phase 6 (CMIP6) were used for spatiotemporal projections of precipitation and...  相似文献   

Movement of the Kuroshio axis to the northeast shelf of Taiwan during typhoon events     

Akihiko Morimoto  Shoichiro Kojima  Sen Jan  Daisuke Takahashi 《Estuarine, Coastal and Shelf Science》2009

We investigated the movement of the Kuroshio axis on the northeast shelf of Taiwan associated with the passage of typhoons, using sea surface current data observed by the ocean radar system on Yonaguni and Ishigaki islands. First, we examined daily Kuroshio axis variation on the northeast shelf of Taiwan during typhoon events. The ocean radar data showed that the Kuroshio axis moved onto the shelf after passages of typhoons. The Kuroshio moved onto the shelf and stayed there after the passage of Typhoon Hai-Tang; while the Kuroshio maintained this pattern, southerly wind blew continuously for 4 days. The mean current speed northeast of Taiwan after the typhoon's passage increased by 18 cm s⁻¹. In addition, the sea level difference between two satellite altimetry tracks east of Taiwan increased by 14.4 cm. These results suggest that coastal upwelling east of Taiwan caused by the southerly wind generated an east–west sea level difference that, in turn, generated a northward geostrophic current. This current could have enhanced the Kuroshio east of Taiwan, and pushed it onto the shelf.  相似文献   

Surface current structure of the Tsushima Warm Current region in the Japan Sea derived by satellite-tracked surface drifters     

Tatsuro Watanabe  Daisuke Simizu  Kou Nishiuchi  Toru Hasegawa  Osamu Katoh 《Journal of Oceanography》2009,65(6):791-801

We examined the surface current structure in the Tsushima Warm Current (TWC) region of the Japan Sea based on a dataset from satellite-tracked surface drifters, including our new deployments during 2004–2008. The gridded mean current velocity and mean kinetic energy (KE) fields calculated from all available drifter data revealed the structure and seasonality of the three branches of the TWC. By comparing the eddy kinetic energy (EKE) field to the KE field, we found four regions where the ratio of EKE to KE was high. In particular, this ratio became extremely high east of the Oki Islands located between the first branch of the TWC (FBTWC) and the second branch of the TWC (SBTWC). Our analysis suggests that the generation of alternate warm and cold eddies east of the Oki Island induced this high ratio. The occurrence of a warm or cold eddy was related to the route—either nearshore or offshore—of the SBTWC. When the offshore route of the SBTWC became dominant, a warm eddy was frequently generated east of the Oki Islands. In contrast, when the nearshore route of the SBTWC became dominant, a cold eddy was more likely to be generated.  相似文献   

Enrichment of alkanes within a phytoplankton bloom during an in situ iron enrichment experiment in the western subarctic Pacific     

Sohiko Kameyama  Urumu Tsunogai  Fumiko Nakagawa  Motoki Sasakawa  Daisuke D. Komatsu  Akira Ijiri  Junko Yamaguchi  Takeo Horiguchi  Hiroshi Kawamura  Aika Yamaguchi  Atsushi Tsuda 《Marine Chemistry》2009,115(1-2):92-101

During the Subarctic Pacific Iron Experiment for Ecosystem Dynamics Study ΙΙ (SEEDS-II), we monitored variations in the concentrations of non-methane hydrocarbons (NMHCs), CH₃Cl, N₂O, and CH₄ within a phytoplankton bloom. Stable isotopic compositions were also determined to evaluate the sources of the variations. Although there was little variation in either the concentrations or the stable isotopic compositions of alkenes, CH₃Cl, N₂O, and CH₄ during the 23-day observation period, alkane concentrations increased substantially as the phytoplankton bloomed. The column-integrated quantities of alkanes increased to 3 times pre-bloom levels for C₂H₆, 5 times for C₃H₈, and 20 times for n-C₄H₁₀. The δ¹³C values of both C₂H₆ and C₃H₈ remained almost constant while concentrations increased, whereas that of n-C₄H₁₀ increased by about 12‰. To evaluate the sources of the alkanes produced during the bloom, we compared their δ¹³C values with those of alkanes produced in axenic phytoplankton cultures in our laboratory. We concluded that during the SEEDS-ΙΙ experiment the major portions of C₂H₆ and C₃H₈ were produced during the autolysis of diatoms cells, whereas n-C₄H₁₀ was produced during autolysis of other phytoplankton cells such as cryptophytes and dinoflagellates.  相似文献   

Characteristics of observed turbulent mixing across the Antarctic Slope Front at 140°E, East Antarctica     

Daisuke Hirano  Yujiro Kitade  Hideki Nagashima  Masaji Matsuyama 《Journal of Oceanography》2010,66(1):95-104

The strength of mixing due to turbulence in the Antarctic Slope Front (ASF) region was investigated using CTD (conductivity-temperature-depth profilers) observations and direct measurements of turbulence conducted off Adélie Land, East Antarctica along 140°E from the 12th–14th February, 2005. The strongest horizontal gradient of the ASF was located below 300 m depth near the 1000 m isobath. The turbulent measurements revealed that the energy dissipation rate frequently exceeded 10^?8 Wkg^?1 on the continental shelf and upper slope regions. Turbulent diffusivities near the shelf break were higher than 10^?3 m²s^?1. Near the ASF the average turbulent heat flux was 5.7 Wm^?2 and 1.1 Wm^?2 across the temperature minimum layer to 250 m and from 300 to 600 m, respectively. The distribution of the high dissipation rate was consistently explained by the characteristic curve of the M2 internal wave emanating from the shelf break and continental slope. The water mass observed in the ASF below 300 m in the continental slope comprised Modified Circumpolar Deep Water and low salinity Shelf Water originating from either the upper layer of the Adélie Depression or the Adélie Bank, and produced by boundary mixing near the shelf break.  相似文献