Splitting of the subtropical gyre in the western North Pacific     

Keiichi Hasunuma  Kozo Yoshida 《Journal of Oceanography》1978,34(4):160-172

Examined here is a hypothetical idea of the splitting of the subtropical gyre in the western North Pacific on the basis of two independent sources of data,i.e., the long-term mean geopotential-anomaly data compiled by the Japanese Oceanographic Data Center and the synoptic hydrographic (STD) data taken by the Hakuho Maru in the source region of the Kuroshio and the Subtropical Countercurrent in the period February and March 1974. Both of the synoptic and the long-term mean dynamic-topographic maps reveal three major ridges, which indicate that the western subtropical gyre is split into three subgyres. Each subgyre is made up of the pair of currents, the Kuroshio and the Kuroshio Countercurrent, the Subtropical Countercurrent and a westward flow lying just south of the Countercurrent (18°N–21°N), and the northern part of the North Fquatorial Current and an eastward flow at around 18°N. The subgyres are more or less composed of a train of anticyclonic eddies with meridional scales of between 300 and 600 km, so that the volume transport of the subgyres varies by a factor of two or more from section to section. The upper-water characteristics also support the splitting of the subtropical gyre; the water characteristics are fairly uniform within each subgyre, but markedly different between them. The northern rim of each subgyre appears as a sharp density front accompanied by an eastward flow. The bifurcations of the sharp density fronts across the western boundary current indicate that the major part of the surface waters in the North Equatorial Countercurrent is not brought into the Kuroshio. The western boundary current appears as a continuous feature of high speed, but the waters transported change discontinuously at some places.  相似文献   

Estimation of crustal structure from observed group velocities ofPL-waves in Japan     

Masayuki Fujita  Keiichi Nishimura 《Pure and Applied Geophysics》1991,137(3):233-249

Forward modelling of the crustal structure of the eastern Honshu Island, Japan, was made based on the group velocities ofPL-waves in the period range of 20–30 s. The observed values of group velocity were obtained by appling the multiple filter technique to the seismograms for earthquakes with the epicentral distance ranging from 500 to 1000 km. The theoretical values were calculated using Oliver and Major's method to find the best fit dispersion curve in the least-squares sense. The obtained structural model has considerably high crustal velocities compared to other previous models. It was shown that thePL-wave group velocity in the period range of interest was most sensitive to seismic velocities of the center of the crust. Numerical experiments confirmed the applicability of the approximation methods employed to obtain both observed and theoretical group velocities.  相似文献   

Estimation of crustal structure in central and west Japan from waveform modeling of regionalPL waves     

Masayuki Fujita  Keiichi Nishimura 《Pure and Applied Geophysics》1993,140(4):569-592

An inverse method of modeling the regionalPL waveform with the predominant period of about 20 s was developed to estimate the averageS-velocity structure of the upper crust. Applicability of the waveform modeling was confirmed by the results of the numerical experiments: thePL waveform is most sensitive to theS velocity in the upper crust, whereas it is not affected significantly by errors involved in the focal mechanism solution and focal depth determination when thePL wave is well developed. The method was applied to the observed seismograms recorded in central Japan from the earthquakes with epicentral distances 300–500 km. As a result, distinct regional differences were found in the upper crustalS velocity; in particular, between the southern Shikoku district, west Japan, and the southern Chubu district, central Japan, and between the mountainous and the coastal areas in the southern Chubu district. These differences are in agreement with the general features ofP-velocity structures obtained by explosion experiments and by analyses of natural earthquakes. Our method is effective to the extent that the crustal structure along the propagation path can be assumed a horizontally layered structure; it is not applicable when the sensitivity of thePL waveform to the error in the focal mechanism solution is exceptionally high.  相似文献   

Uranium-series chronology for sediments of Lake Hovsgol, Mongolia, and the 1-Ma records of uranium and thorium isotopes from the HDP-04 drill core     

Aya Sakaguchi  Masayoshi Yamamoto  Junpei Tomita  Kenta Mino  Keiichi Sasaki  Kenji Kashiwaya  Takayoshi Kawai 《Quaternary International》2009,205(1-2):65

Uranium and thorium isotopes in an 81-m long sediment core (HDP-04) of Lake Hovsgol, Mongolia, were measured to investigate their downcore distributions and to explore potential linkage to paleoenvironmental changes. Three-dimensional isochron techniques using isotope-ratio diagrams in ²³⁸U–²³⁴U–²³⁰Th–²³²Th system presented by Ludwig and Titterington were applied to age date the lake sediments at the depths of 11.42, 14.71 and 14.83 m in the HDP-04 section, the estimated ages of these horizons are 66 ± 8, 122 ± 11 and 128 ± 22 ka, respectively. The ²³⁸U concentration throughout the entire section fluctuated by a factor of 12, ranging from 19.9 to 232.1 mBq/g with anomalously high ²³⁸U peak at 23.8 m in depth, while the ²³²Th concentration varied only by a factor of about two between 24.3 and 54.0 mBq/g. The discrimination of the bulk ²³⁸U into authigenic and terrigenous ²³⁸U fractions was attempted, based on the measured ²³²Th as a correction index for terrigenous materials. In the upper 24 m corresponding to the last 250 ka, the authigenic ²³⁸U was higher in interglacials and lower in glacials. This depth profile of authigenic ²³⁸U contents was almost identical pattern with that found in a sediment core (VER98-1-6) from the Academician Ridge, Lake Baikal. Further, this profile can be correlated well with that of photosynthetic pigment contents, one of proxies of paleoproductivity, suggesting that the variation of authigenic ²³⁸U contents were associated with the environmental change around Lake Hovsgol.  相似文献