Asteroids approaching the Earth from directions around the Sun     

Syuzo Isobe  Makoto Yoshikawa 《Earth, Moon, and Planets》1996,72(1-3):263-266

We have estimated close asteroid encounters with the Earth by numerical integrations of a system with the Sun, 9 planets, and 188 near-earth-asteroids during the period 1994–4600. Asteroids approach the Earth from directions within 30? around the Sun in more than 20% of encounters with the closest distance less than 0.01 AU. Since ground-based observations cannot detect these objects, we should develop space-borne and/or lunar observatories in a short time to allow enough warning time before a catastrophic collision.  相似文献   

A new method for estimation of the crustal stress from cored rock samples: Laboratory study in the case of uniaxial compression     

Sumio Yoshikawa  Kiyoo Mogi 《Tectonophysics》1981,74(3-4)

We present a new method by which the maximum previous stress in rocks can be estimated. The Kaiser effect in acoustic emission is often used for an estimation of the stress to which rocks have been subjected. However, there are cases in which the Kaiser effect is not clear. In such cases, estimation of the previous stress is difficult by the conventional method which is based on the acoustic emission activity during one stress cycle. Using a new method of cyclic loading, we can estimate the stress in cases where the conventional method does not work. In the new method, the difference of the acoustic emission activity between the first and second reloading begins to increase markedly at the previous maximum stress. Even if the Kaiser effect is not clear, we can estimate the previous stress by observation of this difference.Our new method may be applicable for estimating the crustal stress. In this paper, we study the influence of water and temperature under uniaxial compression, because rocks in the earth's crust are saturated and subjected to high temperatures and they are disturbed by water and temperature in the sampling process. The present experiment shows that the Kaiser effect is often masked by changes in water content and temperature. However, we can estimate the previous stress by the new method even after the rock has been disturbed by water and temperature. The accuracy in the stress estimation by this method is satisfactory for the ratio of the previously applied stress to the fracture strength of the rock less than 40%.  相似文献   

Evidence and Implications of Recent Climate Change in Northern Alaska and Other Arctic Regions   总被引：19，自引：0，他引：19  

Larry D. Hinzman  Neil D. Bettez  W. Robert Bolton  F. Stuart Chapin  Mark B. Dyurgerov  Chris L. Fastie  Brad Griffith  Robert D. Hollister  Allen Hope  Henry P. Huntington  Anne M. Jensen  Gensuo J. Jia  Torre Jorgenson  Douglas L. Kane  David R. Klein  Gary Kofinas  Amanda H. Lynch  Andrea H. Lloyd  A. David McGuire  Frederick E. Nelson  Walter C. Oechel  Thomas E. Osterkamp  Charles H. Racine  Vladimir E. Romanovsky  Robert S. Stone  Douglas A. Stow  Matthew Sturm  Craig E. Tweedie  George L. Vourlitis  Marilyn D. Walker  Donald A. Walker  Patrick J. Webber  Jeffrey M. Welker  Kevin S. Winker  Kenji Yoshikawa 《Climatic change》2005,72(3):251-298

The Arctic climate is changing. Permafrost is warming, hydrological processes are changing and biological and social systems are also evolving in response to these changing conditions. Knowing how the structure and function of arctic terrestrial ecosystems are responding to recent and persistent climate change is paramount to understanding the future state of the Earth system and how humans will need to adapt. Our holistic review presents a broad array of evidence that illustrates convincingly; the Arctic is undergoing a system-wide response to an altered climatic state. New extreme and seasonal surface climatic conditions are being experienced, a range of biophysical states and processes influenced by the threshold and phase change of freezing point are being altered, hydrological and biogeochemical cycles are shifting, and more regularly human sub-systems are being affected. Importantly, the patterns, magnitude and mechanisms of change have sometimes been unpredictable or difficult to isolate due to compounding factors. In almost every discipline represented, we show how the biocomplexity of the Arctic system has highlighted and challenged a paucity of integrated scientific knowledge, the lack of sustained observational and experimental time series, and the technical and logistic constraints of researching the Arctic environment. This study supports ongoing efforts to strengthen the interdisciplinarity of arctic system science and improve the coupling of large scale experimental manipulation with sustained time series observations by incorporating and integrating novel technologies, remote sensing and modeling.  相似文献   

Reconstruction of a climate record for the past 140 kyr based on diatom valve flux data from Lake Biwa, Japan   总被引：2，自引：1，他引：2  

Michinobu Kuwae  Shusaku Yoshikawa  Narumi Tsugeki  Yoshio Inouchi 《Journal of Paleolimnology》2004,32(1):19-39

To reconstruct the pattern of past climate change in central Japan during the last 140 kyr, total planktonic diatom valve concentrations (valves g^–1) and fluxes (valves cm^–2 year^–1) of total planktonic diatoms flux (PVF) and individual species were examined using a 140-m core taken from Lake Biwa, Shiga Prefecture. Most records had a sample resolution between approximately 150 and 300 yr. Based on characteristics of past and modern diatom responses to possible climate variables, we interpreted changes in Stephanodiscus suzukii flux (SVF) to reflect changes in phosphorus levels, which reflect, in turn, summer precipitation levels; changes in Aulacoseiva nipponica flux (AVF) reflect winter vertical lake-water mixing induced by winter temperatures and snowfall levels. Thus, changes in total planktonic diatom flux reflect a combination of summer precipitation, winter temperature, and snowfall values. During the 140–101 ka interval, changes in S. suzukii productivity at a millennial timescale may correspond to changes in summer rainfall in central Japan. The disappearance of A. nipponica during the same period could indicate weaker vertical mixing, possibly caused by increased temperatures and decreased snowfall levels in winter. During the 101–70 ka interval, the AVF record shows levels near or above those observed in present times, indicating that winter water temperatures fell within the optimal range for A. nipponica to prosper. Generally low AVF values during the 70–7 ka interval indicate weak winter vertical mixing and cold winters. The many intervals with low PVF values during the same period suggest decreased summer precipitation levels. Between 7 and 0 ka, PVF, SVF, and AVF records show levels near or above those of the present, suggesting winter temperatures favorable for A. nipponica growth, and snowfall and summer precipitation levels probably similar to or above those currently recorded.  相似文献   

Paleolimnological investigation of recent acidity changes in Sawanoike Pond, Kyoto, Japan     

Shunnichi Yoshikawa  Shigeo Yamaguchi  Akio Hata 《Journal of Paleolimnology》2000,23(3):285-304

Sawanoike Pond, an artificial reservoir constructed ca. 150 yrs. ago, lies in an area geologically sensitive to acidification and contains water that is presently weakly acidic (pH 5.5; alkalinity 15 eq 1^-1). Sediments from a core obtained from the northern part and a core from the central part of the pond were analyzed for organic matter, total organic carbon, total nitrogen, natural and artificial radionuclides, pollen, diatoms, metals and spheroidal carbonaceous particles. A peaty silt layer at the bottom of the central core was replaced by a silt layer initially rich in humic substances, but became more mineral-rich near the surface. Although most of the dominant diatoms throughout the cores were acidophilous taxa, some acidobiontic species increased moderately in abundance after the middle 1960's. Diatom-inferred pH values for the northern core are relatively low before about 1930, then increase, reaching a maximum level between the 1930's and the late 1940's. They then decrease to the surface and reach minimum level during the middle 1980's. Diatom-inferred pH declined by 0.4-0.5 units in the northern core and 0.2-0.3 units in the central core since ca. 1910, when vegetation around the shore changed to a community similar to that of recent years. An increase in labile Pb, Zn, Cd and spheroidal carbonaceous particles, indicative of atmospheric deposition of fossil fuel combustion by-products, preceded the decline in inferred pH. The inferred pH decline of the pond could be an acidification due to acidic precipitation caused by fossil fuel combustion.  相似文献   

Photosynthesis-irradiance relationship of phytoplankton and primary production in the vicinity of Kuroshio warm core ring in spring     

Ken Furuya  Osamu Hasegawa  Takashi Yoshikawa  Satoru Taguchi 《Journal of Oceanography》1998,54(5):545-552

The light-saturated maximum value (P ^B _max) and initial slope (α) of the photosynthesis-irradiance (P-E) curve were examined in a warm streamer, a cold streamer and a warm core ring off the Sanriku area in the subarctic western North Pacific Ocean during an ADEOS/OCTS Sanriku field campaign in early May 1997. BothP ^B _max and α were within the ranges of temperate populations. A regional difference was apparent inP ^B _max: populations in the warm streamer tended to show higher value ranging between 1.92 and 4.74 mgC (mgChla)⁻¹h⁻¹ than those in the cold streamer and the warm core ring (1.35–2.87 mgC (mgChla)⁻¹h⁻¹). A depth variation was also observed in α in both the warm streamer and the warm core ring: shallow populations tended to have lower α than deep populations. The depth variations in bothP ^B _max and α resulted in a lower light intensity of the light saturation in a deeper population than that of a shallower one. These depth-related variations in the P-E parameters were likely a manifestation of “shade-adaptation” of photosynthesis. Photoinhibition was not observed over in situ surface light intensity varying below ca 1600 μmol photon m⁻²s⁻¹. Water-column primary productivity was biooptically estimated to be 233 to 949 mgC m⁻²d⁻¹ using vertical distributions of the P-E parameters, chlorophylla, phytoplankton light absorption and underwater irradiance. Applicability of surface data sets for estimation of water-column productivity is discussed.  相似文献   

Nitrate-nitrogen isotopic patterns in surface waters of the western and central equatorial pacific     

Chisato Yoshikawa  Yasuhiro Yamanaka  Takeshi Nakatsuka 《Journal of Oceanography》2006,62(4):511-525

To understand the processes transporting nitrate to the surface layer of the western and central equatorial Pacific, we measured the nitrogen isotopic ratio of nitrate (δ ¹⁵NO ₃ ⁻ ), which is a very useful tracer of the source of nitrate, above 200 m depth in this region in December 1999. δ ¹⁵NO ₃ ⁻ is higher (about 13.0‰) in the surface water than in the subsurface water (where it is about 6.5‰) due to isotopic fractionation during nitrate uptake by phytoplankton. The δ ¹⁵NO ₃ ⁻ value has a roughly linear relationship with the natural logarithm of nitrate concentration (ln[NO ₃ ⁻ ]). However, for values above 150 m depth, the intercept of this linear relationship varies with position from east to west. On the other hand, the data at 200 m depth at all observation stations are concentrated around a single point (ln[NO ₃ ⁻ ] = 2.5 and δ ¹⁵NO ₃ ⁻ = 6.5‰) and do not fit the linear relationships for the shallower values. To examine the meaning of the observed distributions of δ ¹⁵NO ₃ ⁻ and nitrate concentration we developed a box model including nitrogen and nitrogen isotopic cycles. By reproducing the observed relationship between δ ¹⁵NO ₃ ⁻ and nitrate concentration using this model we found that most nitrate is transported horizontally from the eastern equatorial Pacific. We also conducted case studies and investigated the effects of differences in pathways of nitrate transport on the distributions of δ ¹⁵NO ₃ ⁻ and nitrate concentration. From these studies we concluded that the observed linear relationships between δ ¹⁵NO ₃ ⁻ and ln[NO ₃ ⁻ ], having a common slope around 6‰ but different intercepts at each station, are evidence of the significant horizontal transport of nitrate to the surface water in this area.  相似文献   

The Experience of Magnetovariational Sounding in the Arctic: the Laptev Sea Region     

Starzhinskii  S. S.  Nikiforov  V. M.  Yoshikawa  A. 《Izvestiya Physics of the Solid Earth》2020,56(2):225-237

Izvestiya, Physics of the Solid Earth - Abstract—We present the results of magnetovariational soundings at two sites (Tiksi Observatory and Kotelny Island in the Laptev Sea region of the...  相似文献   

Meridional movement of northern and southern equatorial ionization anomaly crests in the East-Asian sector during 2002–2003 SSW     

XiaoHua Mo  DongHe Zhang  Larisa Goncharenko  ShunRong Zhang  YongQiang Hao  Zuo Xiao  JiaZheng Pei  Akimasa Yoshikawa  HaDuyen Chau 《中国科学:地球科学(英文版)》2017,60(4):776-785

The present paper investigates the asymmetrical variability of the location of the north and the south equatorial ionization anomaly (EIA) crests in the East-Asian sector, along with their association with simultaneous observations of equatorial electrojet (EEJ) strength, geomagnetic activity index, and solar flux index during the 2002–2003 sudden stratospheric warming (SSW) event. Analysis of these observations indicates the existence of a large-scale quasi 16-day periodic meridional movement in both EIA crests, and also reveals a strong correlation between the quasi 16-day oscillation in geomagnetic latitudes of the EIA crest and EEJ strength. The latitude of the northern/southern EIA crest and the EEJ strength indicate that obvious synchronous periodic oscillations were in-phase in the northern and southern hemisphere when the SSW occurred. In addition, it is also found that both the EIA crest location and amplitude of the periodic movement of the EIA locations exhibit hemispheric asymmetry. The amplitude of the periodic movement of the EIA location in the southern hemisphere is larger than that of the northern hemisphere, and the southern EIA crest is further off from the equator than the north one. Understanding these asymmetries requires a combination of mechanisms that involve at least trans-equator meridional winds and the position of a sub-solar point; however, potential disturbances in neutral winds associated with the SSW may additionally complicate the equatorial ionospheric dynamics.  相似文献   

A nonlinear parametric model based on a power law relationship for predicting the coastal tsunami height     

Yoshikawa  Masashi  Igarashi  Yasuhiko  Murata  Shin  Baba  Toshitaka  Hori  Takane  Okada  Masato 《Marine Geophysical Researches》2019,40(4):467-477

When a subduction-zone earthquake occurs, the tsunami height must be predicted to cope with the damage generated by the tsunami. Therefore, tsunami height prediction methods have been studied using simulation data acquired by large-scale calculations. In this research, we consider the existence of a nonlinear power law relationship between the water pressure gauge data observed by the Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) and the coastal tsunami height. Using this relationship, we propose a nonlinear parametric model and conduct a prediction experiment to compare the accuracy of the proposed method with those of previous methods and implement particular improvements to the extrapolation accuracy.

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