An optical and near-infrared search for brown dwarfs in the Pleiades cluster     

Chie Nagashima †  Paul D. Dobbie  Takahiro Nagayama  Yasushi Nakajima  Tetsuya Nagata  Motohide Tamura  Tadashi Nakajima  Koji Sugitani  Hidehiko Nakaya  Simon T. Hodgkin  rew J. Pickles  Shuji Sato 《Monthly notices of the Royal Astronomical Society》2003,343(4):1263-1270

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Numerical simulation of impact cratering on granular material   总被引：1，自引：0，他引：1  

Koji Wada  Hiroki Senshu 《Icarus》2006,180(2):528-545

A new numerical code based on the Distinct Element Method (DEM) is developed to study the impact cratering processes on granular material. This code has a potential advantage to simulate the cratering process on granular material, since the movement of discrete particles can be treated. To show the physical plausibility of this code, we conduct 3-D numerical simulations of vertical impact into granular material targets that consist of 384,000 particles, and compare the results with those from experimental studies. It is shown that the excavation stage of cratering derived from experimental studies is represented well by our simulation: the size of the crater cavity, and the ejecta velocity and angle distributions are consistent with those obtained in laboratory experiments. The impact simulation code developed in this study is thus suggested to be useful for the analysis of the impact cratering process on granular material.  相似文献   

OPS mélange: a new term for mélanges of convergent margins of the world     

Koji Wakita 《International Geology Review》2015,57(5-8):529-539

Ocean plate stratigraphy (OPS) is essential to understanding accretionary wedges and complexes along convergent plate margins. Mélanges within accretionary wedges and complexes are the products of fragmentation and mixing processes during and following OPS accretion. A new term, ‘OPS mélange’, is proposed here for mélanges composed mostly of blocks of OPS with an argillaceous matrix, and for a mixture of mélanges of multiple origins with either broken or coherent formations. An OPS mélange results from the fragmentation and disruption of OPS, without admixing of other components. Three major types of OPS mélange can be distinguished on the basis of their components: turbidite type, chert–turbidite type, and limestone–basalt type. These three types potentially form similar mélanges, but they are derived from different parts of the OPS, depending on the level of the decollement surface. The concept of ‘OPS mélange’ can be applied to most of the mélanges in accretionary prisms and complexes worldwide. In addition, this proposal recognizes a distinction between processes of fragmentation and mixing of OPS components, and mixing of ophiolite components, the latter of which results in serpentinite mélanges, not OPS mélanges. Mélanges composed of OPS sequences occur worldwide. The recognition of OPS mélanges is a key aspect of understanding tectonic processes at convergent margins, which result in mélange formation in orogenic belts globally.  相似文献   

Effect of dust event timing on glacier runoff: sensitivity analysis for a Tibetan glacier     

Koji Fujita 《水文研究》2007,21(21):2892-2896

The impact of the timing of dust deposition on glacier runoff was evaluated using a glacier mass‐balance model with a newly improved scheme to track a dusted layer in a snow layer of a glacier. The lowering of surface albedo due to the dusted layer appearing leads to a drastic increase of glacier runoff even under the same meteorological conditions. Calculations of seasonal sensitivity, the relationship between dusted date and resulting runoff, have shown that dust deposition during a melting season might cause a drastic mass outflow from a glacier through changing the surface albedo during the melting season. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

Sediment and carbon storages in the Yahagi River Delta during the Holocene,central Japan     

Kiyoshi Fujimoto  Kumiko Kawase  Shigehiro Ishizuka  Koji Shichi  Akio Ohira  Hiroshi Adachi 《Quaternary Science Reviews》2009,28(15-16):1472-1480

We estimated stored sediment and carbon during the Holocene for each layer of the Yahagi River Delta, central Japan and discussed the provenance of stored carbon. To estimate the bulk density and the carbon content of each layer, we collected two 30 m deep undisturbed cores. The volume of each layer was calculated using ArcView 3D analyst. Although the volume ratio of each layer to the total volume was calculated to be 9.5% for the top mud layer, 34.9% for the upper sand layer, 32.8% for the middle mud layer and 22.9% for the lower sand layer, the mass ratio of each layer to the total mass was calculated to be 8.5, 40.9, 25.2 and 25.4%, respectively, and the stored carbon ratio in each layer to the total stored carbon was 20.4, 4.7, 55.9 and 18.9%, respectively. These results suggest that the top mud and middle mud layers have a significant role as a place for carbon sequestration during postglacial time. Total stored carbon in the study area of only 92.1 km² was estimated at 26 Tg C, which is equivalent to 0.003% of atmospheric carbon. This suggests that deltas on the globe have accumulated a massive amount of carbon during the evolution. The inorganic carbon ratio to total carbon reached more than 45% around the boundary between the middle mud and lower sand layers. The increasing trend in the C_org/N_total ratio accompanied with a decrease in δ¹³C from the bottom to the top horizon in the middle mud layer indicates a gradual increase in terrestrial organic matter contribution. The relative proportion of terrestrially derived materials decreases with increasing distance seaward.  相似文献   

Modeling Local Tidal Constituents Using TOPEX/POSEIDON SSH for the Malaysian Marine Region     

Muhammad Nur J. P. Vella  Sahrum Ses  Majid Kadir  Koji Matsumoto 《Marine Geodesy》2013,36(2):99-106

A new Local Ocean Tide Model, has been produced for the Exclusive Economic Zone (EEZ) of Malaysia, which incorporates some of the latest TOPEX/POSEIDON data for the years 1992 to 1998. Local tide gauge data are used as a comparison, along with another leading Global Ocean Tide Model, Ori96. The leading diurnal and semidiurnal constituents M₂, S₂, N₂, K₁, O₁, P₁ and Q₁ are reproduced using TOPEX/POSEIDON Sea Surface Heights (SSH) in a response analysis type least squares derivation following Munk and Cartwright (1966).  相似文献   

Identification and possible recurrence of an oversized tsunami on the Pacific coast of northern Japan     

Koji Minoura  Shin-ichi Hirano  Tsutomu Yamada 《Natural Hazards》2013,68(2):631-643

Geological evidence of severe tsunami inundation has been discovered in northern Japan. In the dune fields of Shimokita, in northernmost Tohoku, we have found two distinctive sand layers that are tsunami deposits. The run-up height of >20 m and inland inundation of at least 1.4 km are notably larger than any known historical case in Japan. The tsunami-genic earthquake that resulted in these deposits is thought to have taken place in the Kuril Forearc-Trench system nearly 700 years ago. The recurrence interval of major tsunamis originating in the Kuril subduction zone is about 400 years. Given that the most recent unusually large earthquake took place in AD 1611 (corresponding to the Keicho earthquake tsunami), the findings presented here increase the potential and hazard for an outsized tsunami striking the Pacific coast of northern Japan.  相似文献   

The effect of a thin weak layer covering a basalt block on the impact cratering process     

Koji Dohi  Masahiko Arakawa  Chisato Okamoto  Sunao Hasegawa  Minami Yasui 《Icarus》2012,218(2):751-759

To clarify the effect of a surface regolith layer on the formation of craters in bedrock, we conducted impact-cratering experiments on two-layered targets composed of a basalt block covered with a mortar layer. A nylon projectile was impacted on the targets at velocities of 2 and 4 km s^?1, and we investigated the crater size formed on the basalt. The crater size decreased with increased mortar thickness and decreased projectile mass and impact velocity. The normalized crater volume, π_V, of all the data was successfully scaled by the following exponential equation with a reduction length λ₀: ${\pi }_{\text{V}}=b_0{\pi }_{\text{Y}}^{-b_1}\exp (-\lambda /{\lambda }_0)$, where λ is the normalized thickness T/L_p, T and L_p are the mortar thickness and the projectile length, respectively, b₀ and b₁ are fitted parameters obtained for a homogeneous basalt target, 10^?2.7±0.7 and ?1.4 ± 0.3, respectively, and λ₀ is obtained to be 0.38 ± 0.03. This empirical equation showing the effect of the mortar layer was physically explained by an improved non-dimensional scaling parameter, ${\pi }_{\text{Y}}^1$, defined by ${\pi }_{\text{Y}}^1=Y/({\rho }_{\text{t}}{u}_{\text{p}}^2)$, where u_p was the particle velocity of the mortar layer at the boundary between the mortar and the basalt. We performed the impact experiments to obtain the attenuation rate of the particle velocity in the mortar layer and derived the empirical equation of $\frac{u_{\text{p}}}{v_{\text{i}}}=0.50\exp \left(-\frac{\lambda }{1.03}\right)$, where v_i is the impact velocity of the projectile. We propose a simple model for the crater formation on the basalt block that the surface mortar layer with the impact velocity of u_p collides on the surface of the basalt block, and we confirmed that this model could reproduce our empirical equation showing the effect of the surface layer on the crater volume of basalt.  相似文献   

Ecology and restoration techniques for Sargassum beds in the Seto Inland Sea,Japan   总被引：7，自引：0，他引：7  

Terawaki T  Yoshikawa K  Yoshida G  Uchimura M  Iseki K 《Marine pollution bulletin》2003,47(1-6):198-201

Due to the reduction and degradation of coastal areas in Japan by land reclamation and anthropogenic perturbations, from the point of view of conservation of the coastal environment, the restoration of Sargassum beds is essential. Between 1978 and 1991, 6400 ha of seagrass and seaweed beds have been lost along the Japanese coast, of which Sargassum beds were 22%. New techniques for Sargassum bed restoration are summarized based on three coastal engineering techniques. (1) Construction of shallow and gentle sloping bottom substrata have been shown to be effective for the reestablishment of 'management-free seagrass and Sargassum beds' on developed coasts. (2) Seeding or transplanting using artificial substratum for extension of nursery and fishing grounds around natural Sargassum beds. (3) Periodic transplanting of Sargassum plants using artificially produced seedlings is effective to produce niches to allow faunal re-colonization in severely polluted and sparsely vegetated area. However, prior to implementation, the suitability and limitations of these three techniques requires to be ascertained for effective Sargassum bed restoration.  相似文献   

Extended probit mortality model for zooplankton against transient change of PCO(2)     

Sato T  Watanabe Y  Toyota K  Ishizaka J 《Marine pollution bulletin》2005,50(9):975-979

The direct injection of CO(2) in the deep ocean is a promising way to mitigate global warming. One of the uncertainties in this method, however, is its impact on marine organisms in the near field. Since the concentration of CO(2), which organisms experience in the ocean, changes with time, it is required to develop a biological impact model for the organisms against the unsteady change of CO(2) concentration. In general, the LC(50) concept is widely applied for testing a toxic agent for the acute mortality. Here, we regard the probit-transformed mortality as a linear function not only of the concentration of CO(2) but also of exposure time. A simple mathematical transform of the function gives a damage-accumulation mortality model for zooplankton. In this article, this model was validated by the mortality test of Metamphiascopsis hirsutus against the transient change of CO(2) concentration.  相似文献